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Garlic

Garlic (Allium sativum) is a food product that can improve immunity and cardiovascular health.

Our evidence-based analysis on garlic features 669 unique references to scientific papers.

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Research Breakdown on Garlic


1Sources and Composition

1.1Sources

Allium sativum (of the Allioideae subfamily) is a food product known as Garlic in the same genus as onion plants (allium) alongside some other common food products such as onion, garlic, chive, leek, and rakkyo. The genera contains above 500 different plants, and while in the past it was placed in the Liliaceae family it now resides as the largest genera in the Amaryllidaceae family.[7][8][9] There are some other vegetables that bear the name of garlic but are different species, and this includes Tulbaghia violacea (Sweet Garlic) and Allium ursinum (Wild Garlic).[10]

There are two main varieties (variants or var.) of garlic named either ophioscorodon (full designation is Allium sativum var. ophioscorodon), which is also called hardnecked garlic or purple striped garlic, and sativum (literally Allium sativum var. sativum), which is also called creole garlic or artichoke garlic.[7][11] Other variants include Voghiera (large bulbed Italian variety),[12] Spring Garlic (pinkish and with a milder taste),[13]

Garlic is a plant known as Allium sativum, which is a plant related to both onions and leeks, and it can be found in a wide variety of 'variants' (variations of the typical vegetable), which are specifically cultivated for their taste properties or yield

Garlic appears to have historical usage for being a medicinal food product as the Babylonians, Egyptians, Phoenicians, Vikings, Chinese, Greeks, and Romans have reported usage of garlic[14][15] for intestinal disorders, flatulence, worms, respiratory infections, skin diseases, wounds, symptoms of aging, and a variety of other purposes (as a general prophylactic).[14] It can be traced back about 6,000 years with known cultivation 5,000 years ago in India for medicinal purposes and recorded usage in China (3,000 years ago) and Egypt (1,550 BC)[16][17] and appears to have been given to slaves to increase their ability to do more labour[16] and was given to Grecian athletes and is sometimes referred to as the first performance enhancing supplement.[17]

It also appears to be quite a popular supplement, with survey research in 2002 (USA) suggesting that 3.76% of the population used garlic supplements[18] and in Australia (2007) this number was 10.7%, with 29.8% of those using garlic supplements in Australia (3.18% of the population) using it for the treatment of colds.[19]

Traditional usage of garlic tends to be catered towards intestinal health and longevity, but it has a fair bit of traditional usage in bolstering the defenses of the body (this can be interpreted as either the immune system, anti-infective properties, or actual physical enhancement) and there are limited reports of its cardiovascular benefits

1.2Composition

When looking at the garlic bulb itself (overall vegetable product) it tends to contain:

  • A water content of around 65%[16]

  • A carbohydrate content of around 28% (which is mostly fructans)[16]

  • Protein at around 2% (mostly alliinase and glycoproteins) and 1.2% free amino acids[16]

  • 2.3% organosulfur compounds (commonly seen as the main bioactives)[16]

  • 1.5% dietary fiber[16]

Garlic bioactives are somewhat unique in the vegetable, as there are two main groups of molecules that exist in the actual clove prior to processing; alliin (S-allylcysteine sulfoxide) and the Glutamyl-S-allylcysteine molecules. These two classes are some of the organosulfur compounds mentioned above, and aside from being relatively balanced (unless otherwise processed) they make up the majority of the organosulfurs.[20]

When the clove is mechanically disturbed (chewing, slicing, crushing) then alliin turns into allicin via alliinase and then allicin spontaneously creates all manner of bioactives and gives off some hydrogen sulfide (H2S) in the process. When the clove ages, the Glutamyl-S-allylcystiene molecules slowly lose their glutamyl moieties and it increases levels of S-allylcysteine (SAC) among some other similar cysteine prodrugs.

Garlic contains two main classes of molecules, which spontaneously form a wide variety of bioactives. This includes alliin (main sulfur containing compound in fresh garlic), which converts into allicin via the above pathway, and the glutamyl-S-allylcysteine, which gradually form S-allylcysteine during the aging process (which can then form SAMC and SMC)

The known bioactives of garlic are:

  • Alliin (S-allylcysteine sulfoxide) as one a pool of bioactives at 10mg/g fresh weight and 30mg/g dry weight of raw garlic[20] and of which 70-80% degrades into Allicin (Diallyl thiosulfinate; not present in the garlic initially[14][21]) and is then further degraded into the Diallyl sulfides, the Ajoene molecule[22] and the dithiin class of cyclic molecules[23]

  • The Glutamyl-S-allyl-L-cysteine class of molecules (second initial pool of bioactives) including γ-glutamyl-S-allyl-L-cysteine and γ-glutamyl-S-(trans-1-propenyl)-L-cysteine, two molecules present in garlic in high levels alongside low levels of γ-glutamyl-S-allyl-mercapto-L-cysteine and S-allylcystiene (SAC);[24] SAC content increases during aging of garlic from 200µg/g to 7,200µg/g[25] and is seen as the main bioactive of 'aged' garlic[26]

  • The diallyl sulfides, which include; diallyl sulfide, diallyl disulfide (DADS), diallyl trisulfide (DATS or Allitridi), and diallyl tetrasulfide (DATTS),[27][28][29] which are seen as the main bioactives of garlic oil and main derivatives of allicin;[28] letting allicin sit for 20 hours results in DADS (66.7%), DATS (14.6%), DAS (13.3%) and diallyl tetrasulfide (5.4%)[30] with higher polysulfides being of sparse quantities;[31] diallyl sulfides are seen as the main metabolite(s) of allicin

  • A cyclical form of alliin known as cycloalliin[32] and the fat soluble[22] cyclical derivatives of allicin[23] known as vinyldithiins such as 1,2-vinyldithiin;[33] these derivates are more sparse than the diallyl sulfides[34]

  • Ajoene ((E,Z)-4,5,9-trithiadodeca-1,6,11-triene 9-oxide), one of the stable end products of alliin degradation[35] and made from allicin S-thiolation and 2-propenesulfenic acid addition and again less prominent than diallyl sulfides[34]

  • Allylmercaptane (AM) and allylmethyl sulfide (AMS), which are produced from DADS after oral ingestion[36] and oxidized into allylmethyl sulfoxide (AMSO) and then allylmethyl sulfone (AMSO2)[35]

  • S-methylcysteine sulfoxide (Methiin)[34]

  • Dimethyl sulfides as well as allyl methyl sulfides, also present in garlic oil and similar to the diallyl sulfides are derivatives of allicin[28]

  • Thiacremonone (2,4-dihydroxy-2,5-dimethylthiophene-3-one), a cyclic sulfur bearing compound[37]

  • Garlicnins A1, B1-4, C1-3, and D1 (Cyclic sulfoxides);[38][39][40] thought to be formed sponaneously from allicin

  • Allixin, a cyclic non-sulfur component that accumulates in the necrotic areas of garlic bulb[41][42] that may reach up to 1% of the bulb's dry weight after a year[41]

  • Sodium 2-propenyl thiosulfate[43]

Finally, the protein fragment of garlic itself contains some bioactives. There is also a 14kDa glycoprotein known to be involved in the induction of natural killer (NK) cells[44] and thought to be involved in altering T-cell cytokine production, since the extract of garlic where the glycoprotein is found alters T-cells.[45]

In regards to the sulfur containing compounds, there are two main classes of traditional bioactives, which are pretty much either made from allicin spontaneously reconfiguring itself (this makes diallyl sulfide molecules, Ajoene, and vinyldithiins) or from the Glutamyl-S-allylcysteine being aged (this makes S-allylcysteine mostly). There is a protein fragment involved in immune health, and some cyclical bioactives with unknown origins

With some non-sulfur based bioactives including (note, some variants of garlic will be specified):

  • ACE inhibiting dipeptides (Ser-Tyr, Gly-Tyr, Phe-Tyr, Asn-Tyr, Ser-Phe, Gly-Phe, and Asn-Phe)[46]

  • β-Chlorogenin[32]

  • Nitrate at 183mg/kg (34–455mg/kg)[47]

  • The lignans Matairesinol (37.4µg/100g raw weight) and Secoisolariciresinol (26.6µg/100g raw weight)[48] thus able to produce endogenous enterodiol[49]

  • Quercetin at 47[50] to 80.6mg/kg[51] but usually not detectable[51][52][53]

  • Apigenin once reported to be 217mg/kg dry weight[54] but otherwise not detectable[52][53]

  • Myricetin once reported to be at 693mg/kg[50] but otherwise not detectable[53]

  • Kaempferol once reported at 1mg/kg[50] and usually not detectable[52][51][53]

  • Molybdenum at 2.80µg/100g[55]

  • Luteolin at 1.4% of a methanolic extract of the peels[56] but otherwise at low to undetectable levels in the bulb[50]

  • L-Methionine[32]

  • L-cysteine and related amino acids L-methylcysteine and L-ethylcysteine[32]

  • L-Arginine and related structures such as fructosyl-arginine[32]

  • Glutathione itself (93.5mg/100g[57])

  • Selenium containing molecules including selenate, Dimethylselenide, selenomethionine, selenocysteine, S-methyl selenocysteine and γ-glutamyl-S-methylselenocysteine;[58] structurally, the selenium replaces the sulfur of some garlic bioactives and of sulfur containing amino acids

  • Vitamin C (5.3mg/100g[57])

  • Caffeic acid (2.9mg/kg dry mass[53]) and ferulic acid (2.6mg/kg dry mass[53])

  • A dietary mannose-containing lectin[59] that acts on the insulin receptor[60] and does not agglutinate red blood cells[61] at 30.3ng/g fresh weight[60]

  • Furostanol saponins Voghieroside (A at 5.7mg/kg, B at 9.1mg/kg, C at 10.6mg/kg, D at 0.4mg/kg, and E at 0.3mg/kg; all isomer pairs) from the variant Voghiera[12]

  • Agigenin (not to be confused with Apigenin) 3-O-trisaccharide from the variant Voghiera[12]

  • Gitogenin 3-O-tetrasaccharide from the variant Voghiera[12]

  • Eugenol diglucoside (0.7mg/kg) and rutinoside (6.1mg/kg) in the variant Voghiera[12]

The bioactives in garlic that are not the unique sulfur containing ones are not present in very high levels, and it is likely that the things in this list are ingested at much too low of a dose to be bioactive

When looking at overall groups of molecules, an average garlic bulb is about 0.9% γ-glutamylcysteines and up to 1.8% alliin.[14]

Total flavonoids have ranged from 6.36-9.00mg gallic acid equivalents (GAE) per kilogram dry mass[62] and elsewhere 6.99–8.70mg,[54] 19.4mg/kg,[50] and 0.075–0.12mg/kg.[52] Garlic skin (the coating of the buds) has a poor content of sulfur containing bioactives.[63]

One major molecule is Alliin, which is rapidly converted to Allicin (probably not bioactive in the body) and then allicin is further degraded into either Ajoene (which is one major bioactive) or the the vinyldithiin and dithiin molecules (also bioactive), although S-allylcysteine also may play a role

When analyzing commerical garlic products, the allicin content appears to be less than 1ppm (less than 1µg per gram).[21]

1.3Physicochemical Properties

Crushing or cutting a garlic clove to expose it to air (chewing also qualifies for damage, but there is less air exposure) the enzyme known as alliinase is activated and rapidly lyses the cysteine sulfoxides (alliin) to form the alkyl alkane-thiosulfinates of which allicin temporarily forms 70-80% of them.[14] This rapid conversion of alliin to allicin causes the characteristic garlic odour when initially cut, but due to allicin itself being instable it is then further degraded into ajoene and dithiins.[35][14]

Despite being the most prominent bioactive in garlic, allicin is highly instable to both heat and oxidation[35] and is broken down into the more stable vinyldithiins[23] or ajoene.[35][64] Even under best case scenarios, allicin in solution can reach a half-life of up to 12 days in 20-50% ethanolic extracts.[35]

Allicin has poor stability in processed products or in liquid solution, suggesting that products containing allicin may contain the catabolites of allicin instead (due to decomposition)

When looking at S-allylcysteine, it has a melting point of 223.3–223.7°C and is seen as a white crystalline powder (with minor yellowing after prolonged storage);[65] as the name suggests, it is structurally related to L-cysteine and unlike allicin or its parent compound γ-glutamyl-S-allylcysteine, S-allylcysteine appears to be quite stable and can remain unaltered in garlic for up to two years.[25] If S-allylcysteine is in an alkaline medium, within six days there may be some detectable catabolites (allylmercaptan and allylsulfide) although these changes do not occur in acidic medium.[65]

Relative to other compounds in garlic, S-allylcysteine is pretty stable and is commonly seen as an end-point of garlic metabolites; it can still degrade further into some catabolites if placed in an alkaline solution

In general, the oil soluble sulfur components of garlic (arising from the oil fragment) are known to carry the aromatic properties of garlic,[14] which mainly consist of the main diallyls (sulfide, disulfide, and trisuldie) as well as methyl allyl disulfide (and trisulfide), 2-vinyl-1,3-dithiin, 3-vinyl-1,2-dithiin, and Ajoene.

1.4Formulations and Variants

The most basic variant of garlic supplement is the basic garlic powder, which is simply dehydrating the bulb and crushing it into a powder; it contains some alliin although usually not allicin since whatever allicin was form spontaneously reconfigues into its derivatives.[34]

Due to the dehydration (and assuming a 65% water content of the garlic cloves), one gram of the basic garlic powder is equivalent to about 2,850mg of raw garlic.

Garlic powder (when 'aging' is not mentioned) is simply a concentrated form of garlic supplement that is pretty much bioequivalent to consuming raw garlic bulbs, assuming the dose is corrected (with one gram of powder being equivalent to 2,850mg of the raw bulb)

Aging garlic for 20 months (at low temperatures and dissolved in 15-20% ethanol) produces 'Aged Garlic Extract', which is usually sold in 10% ethanol.[25][66][20] The major brand name for aged garlic extract is 'Kyolic', and the process of aging has the benefit of making the garlic supplement odorless.[25]

The molecular differences in aged garlic extract involves the Glutamyl-S-allylcysteine molecules. During the aging process, the parent molecules (γ-Glutamyl-S-allylcysteine and γ-Glutamyl-S-1-propenylcysteine) passively degrade over time to increase levels of S-Allylmercaptocysteine (SAMC), S-allylcysteine (SAC), and S-1-Propenylcysteine.[25] Kyolic as a product tends to be standardized to SAC (since it is seen as the major bioactive), and during the aging process SAC can increase from 200µg/g of the dry extract to 7,200µg/g dry extract (associated with full depletion of the parent compounds)[25] although to be sold on the market as 'Aged Garlic Extract' you only need a minimum of 0.05% (500µg/g) S-allylcysteine. Kyolic was specifically mentioned due to being a product used in many studies (either in humans or in vitro) and while some are partly funded by the producer of the supplement (Wakunaga Pharmaceutical)[67] there some independent trials that also return positive results.[26][68]

The aging process does produce some other unique bioactives as well as the above. This includes the tetrahydro-β-carbolines[69][70] and Nα-(1-deoxy-D-fructos-1-yl)-L-arginine[71] as well as other fructans.[72] Some molecules in these categories show direct antioxidant properties[73] and total fructans may be up to 0.22% of total dry weight.[72] Due to these unique bioactives and the increased level of Glutamyl-S-Allylcysteine derivatives, aged garlic extract has more antioxidative potential than does standard (fresh) garlic extracts.

Aging garlic tends to encourage the degradation of Glutamyl-S-allylcysteine parent molecules to lose their glutamyl groups, and this increases the levels of its derivatives such as S-allylcysteine. This variant of garlic supplement is also odorless

Garlic oil is a product of garlic manufacturing (usually by steam distillation),[74][29] which is concentrated for the fat soluble sulfur containing molecules; this mostly refers to diallyl sulfides (end product of ex vivo allicin breakdown) and usualy excludes the glutamyl-S-allylcysteines seen in Aged garlic extract.

Garlic oil tends to contain polysulfides (many sulfur groups) mostly of the diallyl class, and a standard breakdown of bioactives in garlic oil is as follows:[28]

  • Diallyl disulfide (26%)

  • Diallyl trisulfide (19%)

  • Diallyl tetrasulfide (8%)

  • Allyl methyl disulfide (13%)

  • Allyl methyl trisulfide (15%)

  • Allyl methyl tetrasulfide (6%)

  • Dimethyl trisulfide (3%)

  • Pentasulfide (4%)

  • Hexasulfide (1%)

At times, you can have an oil-macerated garlic oil, which is the same as above but contains Vinyl-Dithiins and Ajoene or an ether-extracted garlic oil, which has a higher content of vinyl dithiins (5.7mg/g) and ajoene (0.4mg/g) relative to garlic, and may have a total allyl sulfide content of 1.4mg/g[28] The macerated garlic oil contains alliin but not allicin, whereas the garlic oil contains neither.[34]

Garlic oil is a processed form of garlic catering towards the sulfur donating molecules, particularly diallyl disulfide (DADS). Unlike aged garlic, garlic oil is highly aromatic

Boiling garlic bulbs is known to inactivate the alliinase enzyme which degrades alliin into allicin (which then spontaneously begins producing other bioactives) and due to this boiling garlic bulbs (or, technically speaking, any high level of heat without cutting the garlic bulb first) would cause supplementation of alliin without necessarily forming any of the bioactives made from alliin since alliin does not appear to be metabolized.[34]

Boiling garlic has been noted to prevent properties of garlic from occurring including vasorelaxation,[75] anticancer effects,[76] and the increase in nitric oxide and IFN-α.[77] Conversely, it has been noted to augment some properties such as memory enhancement (diabetics rats).[78] Boiling does not modify its antioxidant properties relative to raw garlic when measured in vivo_[79] or when testing the ability to prevent LDL oxidation[80] and the alterations in triglycerides and cholesterol seen with raw garlic are not affected by boiling.[79]

The above 'heating' applies to 60s of microwave cooking and 45 minutes of baking in the oven,[76] but 30s of microwave cooking is fine.[81] Crushing the garlic and letting it stand for 10 minutes preserves the bioactivity of garlic.[81]

Boiling garlic, or any heat treatment at high levels without first destroying the structure of the clove, will prevent alliin form converting into its catabolites (diallyl sulfides, vinyl-dithiins, and Ajoene) and prevent their activity in the body. This will prevent the blood flow enhancement and anticancer properties specifically, although the cognitive protective, lipid lowering, and antioxidant effecs seem mostly unaffected

2Molecular Targets

2.1H2S

Hydrogen Sulfide (H2S) is an small gaseous signalling molecule akin to nitric oxide, informally known as a gasotransmitter due to its state[82] and produced by one of three enzymes known as cystathionine γ-lyase, cystathionine β-synthase, or 3-mercatopyruvate sulfurtransferase.[83][84] It seems that garlic itself, when incubated in red blood cells, produces H2S from garlic bioactives[85] and that H2S can relax blood vessels.[86] H2S is mostly produced in the blood vessels from cystathionine γ-lyase[87] although it may also be produced nonenzymatically.[88][85]

The main mechanism of action for garlic, as it pertains to the blood pressure reducing and blood flow enhancing properties, is merely acting as a reservoir of sulfur, which can be used to make the signalling molecule known as hydrogen sulfide (H2S), which can either directly relax vessels (via ion channels) or indirectly accelerate nitric oxide signaling

There is also some crosstalk with the H2S signalling pathway and the nitric oxide pathway, since H2S can degrade the molecule known as S-nitrosoglutathione,[89] which serves as an intracellular (and to a degree, extracellular[90]) reservoir of nitric oxide that garlic is known to stimulate.[91] Due to this, the influence of garlic on blood pressure and blood flow is in part due to nitric oxide signalling per se (the vasorelaxing effect of opening potassium channels to be discussed in the ion channels section[87]) and in part an influence of the hydrogen sulfide system unto the nitric oxide system.

Hydrogen sulfide production may also underlie the interactions of garlic bioactives and both TRPV1[92] and TRPA1[93] as well as the interactions between garlic bioactives and AMPK activation.[94]

The production of the gasotransmitter hydrogen sulfide (H2S) seems to underlie pretty much all other mechanisms of action that are attributed to garlic supplementation, which supports the hypothesis that production of hydrogen sulfide is the main mechanism of action of garlic supplementation

2.2Ion Channels

Garlic and its bioactives appear to open potassium channels in vitro, and when measuring membrane hyperpolarization and relaxation (maximal efficacy reaching 20% of baseline) it appears that a water extract has been noted to have an EC50 value of 1.15µg/mL and isolated bioactives having EC50 values of 6.2nM (allicin) and 9.9nM (ajoene).[95] Since garlic is known to spontaneously form H2S in vitro and H2S itself is a potent potassium channel opener,[87] it is thought that the potassium channel opening properties of garlic are secondary to H2S.

Garlic components appear to be quite potent potassium channel openers in endothelial cells, and this is thought to be due to the production of hydrogen sulfide from these components (with more contribution of sulfur causing more vasorelaxation)

2.3TRPs

Diallyl sulfide activates TRPA1 (a receptor that responds to noxious cold) with an EC50 of 254μM and reaching maximal stimulation of 90.8%, which is lesser than diallyl disulfide (EC50 of 7.55μM and reaching maximal stimulation of 76.6%) as well as diallyl trisulfide which activates TRPA1 with an EC50 of 0.49μM (490nM) and reaches maximal stimulation of 83.6%;[27] the efficacy of diallyl trisulfide is greater than the bioactive of mustard oil (allyl isothiocyanate at an EC50 of 1.47μM)[27] and Allicin itself has a fairly potent EC50 value of 1.32-1.91μM while its precursor (alliin) is ineffective.[96] This increasing potency correlates with increasing sulfur groups, which suggests that the activation of TRPA1 from hydrogen sulfide[93] underlies the influence of garlic bioactives on this channel.

Garlic[97] and mustard oil[98] also activate TRPV1 and the diallyl sulfides, with EC50 values of 151μM (diallyl sulfide), 36.7μM (diallyl disulfide), and 43.7μM (diallyl trisulfide) all of which are a potency significantly lesser than the reference compound Capsaicin (EC50 of 1.47μM).[27] Allicin is slightly effective (EC50 of 51.22μM) and its precursor (alliin) is ineffective[96] and there is a possibility that this is also due to hydrogen sulfide, which is known to activate TRPV1 directly.[92]

Interestingly, raw garlic has been found to activate TRPA1 and TRPV1 while baking the garlic (400°F for an hour in an oven) has failed to have the same effect[96] and TRPM8 (a receptor that response to both innocuous cold sensation and menthol from peppermint[99][100]) has not been activated by either form of garlic.[96]

Components of garlic appear to stimulate both TRPA1 and TRPV1, and while the efficacy on TRPA1 seems quite potent it is less potent than capsaicin (hot pepper extract) on TRPV1 activation. The activation of these channels is probably secondary to hydrogen sulfide production

2.4AMPK

Numerous components of garlic have been noted to activate AMPK including Ajoene (LKB1 and SIRT1 dependent signalling, confirmed to not alter ATP concentrations[101]), S-allylcysteine (SIRT1 and CaMKK dependent signalling[102]), S-allylmercaptocysteine (LKB1 dependent[103]), Allicin,[104] and Thiacremenone.[105] Noradrenaline secretion is also able to activate AMPK in white (but not brown) adipose tissue[106] and in this case the molecules of garlic that induce secretion of noradrenaline (S-allylcysteine, allicin, and the Diallyl sulfides[107]) may indirectly stimulate AMPK.

Seven weeks ingestion of 2-5% of the diet as garlic to rats has been confirmed to increase AMPK in brown (20-80% higher than control) and white (50-100%) adipose tissue as well as the skeletal muscle (10-30%) and liver (10-40%); acting in a dose-dependent manner,[108] confirming the relevance of this pathway to oral garlic supplementation.

Similar to the other mechanisms of action, there is the possibility that garlic bioactives work secondary to producing hydrogen sulfide as H2S itself has been noted to activate AMPK[109] and in the same manner outlined above (dependent on LKB1 and CaMKK[94]); the connection between garlic per se and AMPK has not yet been confirmed.

Almost all major sulfur-containing bioactives in garlic are implicated in increasing AMPK activation either directly (usually via a LKB1/SIRT1 dependent mean like resveratrol) possibly related to hydrogen sulfide, or indirectly in white adipose tissue via increasing noradrenaline

AMPK activation is thought to underlie the antihepatosteatotic effects of garlic (seen with 10-30mg/kg ajoene daily in mice)[101] and the antiadipogenic effects of Thiacremonone seem in vitro.[105]

The pathways related to a reduction in lipid synthesis (fatty liver and obesity) are usually tied back into the AMPK pathway

2.5NF-kB

NF-kB is an inflammatory signalling molecule which is a dimer (pair) of proteins that, when they interact, cross the nucleus to cause a genomic inflammatory response; the most common variant is a heterodimer (different pair) comprised of the p50 and p65 protein[110] while p50:p50 and p65:p65 homodimers (same pair) have also been reported[111] where the p50 pairings are thought to be more relevant to inflammatory responses[112][111] and p65 pairings more relevant to cancer metabolism.[113] Sulfur bearing compounds tend to interact with the cysteine residues on NF-kB[114][115] which seems to apply to garlic bioactives.

Thiacremenone (2.5-10μg/mL in macrophages with an IC50 of 8μM) appears to directly interact with NF-kB resulting in its inhibition. This is secondary to oxidizing the sulfhydryl residue on the p50 subunit[37] which is critical for interaction between p50 subunits and is inhibited by coincubation of reducing agents.[37] Elsewhere in cancer cells, p65 is thought to be the target of thiacremenone[113] and the reason for this differential response is not known but likely due to the different cells used.

Thiacremenone appears to directly interact with the protein subunits of nF-kb (either p50 or p65) resulting in its inhibition which is thought to be since it is a sulfur bearing molecule, and sulfur bearing molecules tend to interact with the cysteine residue on these proteins that is needed for their activity

Diallyl disulfide and allylmercaptane have both failed to inhibit TNF-α induced NF-kB activation in isolated endothelial cells at concentrations up to 100µM.[116]

2.6HDACs

Histone deacetylases (HDACs) are enzymes that remove acetyl group from histone proteins, and said deacetylation will modify the function of these histones and genomic transcription. As a general statement, deacetylation silences some epigenetic genese in cancer cells and their reacetylation is met with reactivation.[117][118] Thus, HDAC inhibition appears to be useful for chemotherapy yet limited natural products have this ability (garlic organosulfurs, sulforaphane, and butryic acid from colonic fermentation[119] as well as allyl isothiocyanates from wasabi and onion[120]).

Histone deacetylation modifies the genes of a cancer cell to repress or 'silence' a subset of genes, and reactivating these genes by acetylating them is associated with anticancer properties; inhibiting the HDAC enzymes is the main mechanism by which histone acetylation occurs

Diallyl Disulfide (DADS) at 200µM[121] and allyl mercaptan (AM) at the same concentration[122] both inhibit HDAC resulting in an enhancement of Sp3 binding to the p21/WAF1 promoter, although AM (92%) is much more potent than DADS (29%)[121] and when compared to another HDAC inhibitor from garlic (S-allylmercaptocysteine or SAMC) AM remains more potent;[123] since AM is known to be a direct competitive inhibitor of HDAC[121][122] and a metabolite of both DADS[36] and SAMC[123] it is thought that DADS and SAMC are acting as prodrugs for AM. When comparing the potency of AM against other HDAC inhibitors, it appears to be less potent than the refrence drug Trichostatin A (TCA).[121]

Of these, oral ingestion of diallyl sulfides (single dose of 200mg/kg; 80% DADS) has been confirmed to inhibit HDAC in rat colonic cancer cells within six hours (3-fold for histone H4 and between 1.4 and 2.5-fold for histone H3) but no longer persisting after 17 hours.[124] Unpublished research (mentioned here[119]) suggest this occurs in the liver of mice as well.

Two bioactives in garlic from both the alliin derivatives (DADS) and from the glutamyl-S-allylcysteine derivatives (SAMC) can be seen as molecules that donate an allyl mercaptan (AM) molecule to a cell; also known as allyl mercaptan prodrugs. Allyl mercaptan itself appears to be a relatively potent HDAC inhibitor

3Pharmacology

3.1Serum

Oral ingestion of 500mg of aged garlic extract (Kyolic brand) has been noted to increase plasma S-allylcysteine concentrations 10μg/L (100ng/mL) or more in most (78%) subjects[125] and elsewhere 2,560mg of this product increased blood levels of S-allylcysteine to 20μg/L (200ng/mL);[126] both studies have noted a basal S-allylcysteine concentration in the control groups, suggesting that it is an endogenous compound.

Oral ingestion of aged garlic extracts has been confirmed to increase circulating S-allylcysteine (SAC) concentrations to the low nanomolar range

3.2Cellular Kinetics

Garlic bioactives (demonstrated with allicin) are thought to passively cross the cellular membrane (without inducing leakage, fusion, nor aggregation of membranes)[127] and some studies attempting to block exofacial membrane thiols fail to abolish the effects of garlic;[85] concluding that garlic can exert its main mechanism of action (production of H2S via sulfur donation) in any thiol containing cell via passive diffusion.[85]

It seems that no particular transporter is required for the investigated sulfur bearing bioactives, and that they passively diffuse through the cellular membrane without causing damage. If H2S is produced extracellularly, it may also passively diffuse (like all gasotransmitters)

3.3Metabolism

Allicin is known to be metabolized into diallyl disulfide in the liver[128] but is inherently instable even in plasma (37°C) and can spontaneously be fully degraded within five minutes;[21] and the instability limits the bioactivities of allicin per se.[129][34] Diallyl disulfide (DADS) and Ajoene are also somewhat unstable,[129][34] but allyl mercaptane (AM) seems to be stable.[129]

Diallyl sulfide (DAS) can also be metabolized in liver cells to form allyl mercaptane (AM) and allyl methyl sulfide (AMS) with significantly more production of AM over AMS.[36] This is confirmed in breath tests where AM is eliminated in high levels initially, and later AMS seems to be the prevalent metabolite (DAS and DADS both in low levels at both times),[130][131] which is thought to be due to a converion of AM into AMS via a methyl donation from S-adenosyl methionine, and AMS is thought to be the primary metabolic end product of allicin metabolism via the breath.[34] When measuring the urine, oxidized products of allylmethyl sulfoxide (AMSO) and allylmethyl sulfone (AMSO2) have been detected in rats.[132]

An alternate pathway involves DAS being metabolized by the CYP2E1 enzyme to produce diallyl sulfoxide, and then being again metabolized by the same enzyme to produce diallyl sulfone.[133]

When alliin is ingested (cooking garlic prior to activating the alliinase enzyme), the end products of allicin metabolism are not detected in the breath.[130]

Allicin is highly unstable, and is spontaneously eliminated in plasma rapidly (so no 'effects of garlic' are likely to be due to allicin). The diallyl sulfides are stable outside of the body, but tend to be metabolized into smaller sulfur bearing molecules such as allylmercaptan (AM) and allylmethyl sulfide (AMS) which are thought to underlie most benefits of alicin metabolites

When looking at the glutamyl-S-allylcysteine metabolites, oral ingestion of S-allylcysteine (SAC) results in a serum level of SAC[134] and some urinary excretion of SAC although the major urinary metabolites seem to be N-acetyl-S-allylcysteine and N-acetyl-S-(2-carboxypropyl) cysteine.[135][136] S-allylcysteine (SAC) seems stable, although S-allylmercaptocysteine (SAMC) is a little bit unstable.[34]

S-allylcysteine (major bioactive of aged garlic extract) seems more stable, and oral ingestion of SAC seems to result in serum levels of SAC

3.4Mineral Detoxification

Garlic has been noted to have iron chelating properties in vitro comparable with parsley (yet lower than rosemary, sage, and onions)[137] A rat study has confirmed increased fecal, but not urinary, excretion of cadmium in rats given raw garlic.[57]

Supplementation of garlic or isolated allicin is well known to reduce lead accumulation in fish,[138] sheep,[139] mice,[140] and rats[141] with no significant difference between allicin supplements and an equal dose of allicin via fresh garlic[140] and most efficacy at a dosage of around 6.7-7% of the rat diet (more potent than 3.35%, and 1.70% was ineffective[57]) and when it is taken concomitantly alongside heavy metal exposure.[142] Beyond lead, garlic at these relatively high dietary levels has been noted to reduce cadmium[142] and mercury[142][57] bioaccumulation and toxicity with somewhat equal potency.

The studies that measure organ damage note that the reduced mineral accumulation is associated with less organ damage, and the protective effects of 6.7% dietary garlic in rats seems equivalent to N-acetyl-D,L-penacillamine (60mg/kg) and 2,3-dimercaptosuccinic acid (60mg/kg), two reference drugs.[57] The main bioactive seems to be diallyl disulfide or trisulfide,[57] although S-allylcysteine has not been tested in vivo (in vitro, it was noted that a heated extract containing S-allylcysteine but not allicin metabolites failed to chelate copper[143] suggesting it is inactive)

Garlic has been confirmed to, at higher than normal doses, protect the body from the damage associated with heavy metals which is associated with less organ accumulation of these minerals. The sulfur compounds (diallyl disulfide and diallyl trisulfide) seem to be implicated as the active ingredients

When tested in humans, consumption of garlic thrice daily (each dose equivalent to 1,200µg allicin) for four weeks in workers in a car battery plant (excessive exposure to lead) was able to reduce various clinical signs of toxicity including headache, irritability, and the reduced blood pressure and muscle reflexes;[144] the reference drug of D-penicillamine (250mg thrice daily) failed to reduce symptoms, although garlic and D-penicillamine were equivalent in reducing lead levels in the blood (19% and 24% reduction, respectively).[144]

Appears to be comparable to D-penicillamine in reducing lead levels in the blood with standard oral doses of garlic, attainable via food consumption

3.5Phase I Enzyme Interactions

Garlic is thought to interact with various enzymes of the P450 class.[145][133] Of these, CYP2B1 has been noted to be induced with garlic and its prototypical sulfur containing compounds at high doses (200mg/kg diallyl sulfide).[146][147] The compounds that inhibit CYP2E1 are also known to cause an increase in CYP1A mRNA levels acutely and protein content with daily administration[133] and all bioactives tested are known to cause an increase in CYP3A2 levels.[133]

CYP2B1 may be increased with very high doses of sulfur compounds from garlic

CYP2E1 has been reported to be slightly increased in one study with DADS (14 days of 100μM/kg)[148] although the majority of studies have noted inhibitory effects; inhibition of CYP2E1 has underlied hepatoprotective properties from acetominophen[149] due to diallyl sulfide and its metabolite diallyl sulfone[150][151] and later both S-allylmercaptocysteine (AMS)[152][153] and diallyl disulfide (DADS).[133] S-allylcysteine has failed to interact with CYP2E1, and propyl sulfides are similarly ineffective.[133]

CYP2E1 is inhibited by garlic bioactives (diallyl sulfide, diallyl disulfide, and AMS), which may underlie some health effects but also predisposes garlic to some potential drug-drug interactions. These components are found in the garlic oil, so this effect may not be seen with water extracts or aged garlic supplements

Studies assessing CYP2C9 activity have faile to find an influence of garlic (2,000mg of fresh bulb conferring 3.71 mg allicin twice daily for two weeks prior to warfarin).[154]

No known interaction with CYP2C9

3.6Phase II Enzyme Interactions

Glutathione S-Transferase (GST) is an enzyme that uses NADPH and a proton to convert glutathione disulfide (GSSG or 'reduced glutathione') into active glutathoine, and serves as a glutathione recycling enzyme.

Injections of DATS (10µM/kg) and DADS (100µM/kg) for two weeks have increased GST activity in rats by 43-54% while DAS is ineffective.[148]

DAS has been noted to increase GST activity at higher doses (100-500mg/kg) with a potency nonsignificantly greater than the reference drug N-acetylcysteine (500mg/kg) yet was more effective at improving the GSH:GSSG ratio at the higher dose.[155]

200μM of E-Ajoene has been noted to partially (50%) interfere with GST's reduction of GSSG while enhancing the oxidation of NADPH, resulting in prooxidation.[156]

Glutathione S-Transferase (GST) is normally benefitted by supplementation of garlic, although concentrations of garlic bioactives that are in the toxic level have the opposite effect

Quinone Reductase (QR) refers to enzymes that use NADPH and a proton to convert a quinone into a semiquinone via the process of reduction, and they are antioxidant enzymes known to reduce toxicity associated with the carcinogenic benzo{a}pyrene.[157] An induction of QR is thought to be protective against any quinone based carcinogen, or other carcinogens that happen to be inactivated by QR. A relatively important QR is NADPH quinone oxidoreductase 1, or referred to as NQO1.

QR is noted to be increased with DADS ingestion (more potent than DAS and DATS although both are effective[157]), and the increase is relatively larger than that seen with GST.[158] It has been noted in most organs (Liver, heart, kidney, spleen, bladder, lungs) at doses as low as 300µg/kg in rats (intestinal tract only) and peaking at 15-30mg/kg in the stomach.[158] DADS (100μM) induced QR induction to 3.2-fold of control is associated with Nrf2 translocation[159] and at least when looking at DAS it is thought that Nrf2 is activated secondary to activating MAPKs (ERK/p38, but not JNK).[155]

Aged garlic extract is also known to induce QR secondary to Nrf2 at 100μg/mL by 2.1-fold (less potency than 100μM DADS at 3.2-fold)[159] although heated garlic juice at 100mg/kg oral intake to rats for four weeks has failed to increase NOQ1.[160] Isolated DAS (100-500mg/kg to rats) activates NQO1 to a level equal to 500mg/kg N-acetylcysteine,[155] and low dose injections of both (10µM/kg) and DADS (100µM/kg) have increased QR activity by 41-91% (while 100µM/kg DAS was inactive).[148]

Quinone Reductase is an antioxidative enzymes that reduces quinone structures (and plays a role in cancer prevention), and it appears that QR is increased following oral intake of garlic extracts. Several bioactives are known to increase it, with DADS and DATS being active in the lowest doses and likely relevant to garlic ingestion

Diallyl sulfide (DAS) is known to induce transcription of SULT1E1 to 250-fold of baseline secondary to causing nuclear accumulation of CAR, although no perturbations of basal estrogen sulfation were noted in mice treated orally with DAS at 800mg/kg.[161]

DAS is associated with a relatively potent induction of SULT1E1 of unknown practical significance

Garlic has failed to show inhibitory efects on UGT1A1 using an 80% methanolic extract (3.25% alliin or more) up to 500µg/mL, while EGCG from green tea catechins was most potent (IC50 7.6+/-0.7µg/mL).[162] When looking at other enzymes of glucuronidation, this same extract failed to inhibit UGT1A4, UGT1A6, and UGT1A9[163] while elsewhere isolated diallyl sulfides (1mmol/kg oral ingestion) to rats noted that all tested compounds increased the activity of UDP-glucosyltransferase in the liver and kidney while only DADS increased it in the intestines and lung (and was more effective in the liver and kidneys).[164]

It is possible that oral ingestion of the diallyl sulfides can increase the activity of UGP-glucosyltransferase in rats, and there is no current human evidence

3.7Known Drug Interactions

Garlic (2,000mg of fresh bulb conferring 3.71 mg allicin) twice daily for two weeks before a dose of Warfarin has failed to alter its pharmacokinetics or blood thinning abilities overall,[154] although in subjects with a wild type VKORC1 gene there was an increase in the S-warfarin EC50 (22%) with garlic.[154] While this evidence suggests no adverse interaction and low doses of garlic (4.2g of the raw cloves) do not necessarily interact with platelets[165] there have been reported case studies of high garlic intakes causing cerebral hemhorraging when coingested with Warfarin.[166]

Garlic does not appear to alter the blood thinning abilities of Warfarin supplements when taken in normal doses, although there is some limited evidence of adverse interactions with higher doses. Coingestion is possible but must be approached cautiously with the oversight of a medical professional

It appears that garlic can reduce overall exposure (AUC) of the drug saquinavir (a protease inhibitor used in HIV therapy) by 51% associated with intake of 8g raw garlic[167] which is mostly due to a rapid reduction in Cmax (54%) and is mostly normalized after 10 days of washout.[168] Another antiviral known as ritonavir has its efflux hindered in vitro with garlic due to P-glycoprotein inhibition[169] but this does not seem to occur in humans following consumption of 10mg of a garlic concentration (1g raw garlic equivalent).[170]

Garlic ingestion at levels of which it is concievable to eat in the diet (two cloves weight 8g) is able to half the bodily exposure to the anit-HIV drug saquinavir

4Longevity

4.1Rationale

Garlic appears to be touted for, amongst other things, longevity promotion[171] which appears to be a traditional usage of garlic as well.[14][15] It seems to currently be thought that this 'antiaging' property is due to an increase in its cardiovascular and cerebral benefits improving vitality in older age, although other benefits of garlic are known to treat 'comorbidities' of aging.[172]

There appears to be traditional usage of garlic for the purpose of 'longevity' and 'anti-aging', and the best evidence at this point in time (mostly reviews) tend to suggest that this is an increase in functionality and vitality during the aging process causing the perception of youth

4.2Mechanisms

When assessing isolated human fibroblast cells via the Hayflick system (seeing how many times a cell can divide until it cannot anymore, and an increased Hayflick limit being thought to demonstrate increased cellular antiaging) has noted some potential with garlic extract.[173]

There has been one study investigating a dietary lectin from garlic (as well as bananas) that physically interacted with the pro-insulin receptor and secondary to siganlling through this receptor and ERK it reduced oxidative stress in stem cells;[60] this was observed in mice following weekly ingestion of 10ng of the lectin (330mg of raw garlic bulb equivalent) for 6-8 weeks, suggesting it is relevant to oral ingestion.[60]

4.3Interventions

Diallyl trisulfate (DATS) appears to promote longevity in C. elegans[174] via activating transcription of skn-1, which is the worm homologue of the human gene Nrf2.[175] Concentrations of 5-10μM (20μM appeared less effective and higher doses toxic) increased lifespan by 11.7-12.6%, although this was reduced to 6.8-9% when the possible ability of DATS reducing bacterial contamination was accounted for.[174] The daf-2 and daf-16 pathways were not involved, and the worm model for caloric restriction (eat-2) did not experience benefits.[174]

DATS appears to have very minor longevity promoting benefits in worms (C. Elegans) that may not benefit those undergoing a calorically restricted diet

There are a few studies in senescence accelerated mice (SAMP8) as a model for aging which note improvements in cognitive performance which is usually impaired (and thought to reflect the cognitive impairment of aging)[176][177][178] and garlic is associated with a reduction in brain atrophy associated with aging.[178][179] S-allylcysteine itself has been noted to improve cognition associated with aging in SAMP10 mice at 40mg/kg of the diet,[180] which was similar to the aforementioned studies using 2g of aged garlic extract per 100g of food.

When looking at rodent studies, in senescence accelerated mice (SAMP8) given aged garlic extract at 2% of the diet (0.1% total dietary S-allylcysteine) after two months of life it was noted their lifespan was normalized to senescence resistant mice (SAMR1)[176] and elsewhere SAMR1 mice have failed to experience an increase in lifespan with aged garlic extract at 2% of the diet.[177]

The prolongation of lifespan seen in SAMP8 is not seen in another strain of senescence accelerated mice (SAMP10)[178] but the attenuation of brain atrophy seen during aging also applies to not only SAMP10 but also the senescence resistant SAMR1 mice.[179]

There may be a longevity promoting effect of garlic ingestion, but this seems to only occur in the mice which are prone to accelerated aging. The mice who are genetically resistant to aging recieve no apparent extension of lifespan with garlic, but all groups seem to have improved cognitive performance during aging associated with reduced brain atrophy

5Neurology

5.1Kinetics

Under physiological conditions, the concentration of hydrogen sulfide in the brain has been reported to be in the range of 50-160μM which is thought to be due to the enzyme cystathionine β-synthase, since its inhibition lowers brain concentrations of H2S.[181]

5.2Adenosinergic Neurotransmission

The inhibition of adenosine deaminase (the enzyme that degrades the adenosine neurotransmitter into inosine and ammonia[182]) by methylmercury is inhibited by garlic extract at 100µg/mL[183] which is simply thought to be due to the interaction between thiol groups (known to sequester toxic minerals) and mercury, since it was also seen with glutathione (50µM).[183]

5.3Adrenergic Neurotransmission

Garlic ingestion at 25-100mg/kg (20% ethanolic extract) to mice over two weeks has been noted to decrease levels of both MAOA and MAOB in the brain to a level comparable to 15mg/kg Imipramine.[184]

The antidepressant effects of garlic have been noted to be partially blocked by coadministration with prazosin, the α1-adrenergic receptor blocker.[184]

5.4GABAergic Neurotransmission

25-100mg/kg of a 20% ethanolic extract of garlic to mice for two weeks prior to antidepressant testing (forced swim and tail suspension tests) in a manner that is partially blocked by baclofen, a GABAB agonist.[184]

5.5Serotonergic Neurotransmission

Garlic ingestion at 25-100mg/kg (20% ethanolic extract) to mice over two weeks has been noted to decrease levels of both MAOA and MAOB in the brain to a level comparable to 15mg/kg Imipramine.[184]

The antidepressant effects of garlic appear to be partially inhibited by coadministration of p-CPA (Fenclonine), a serotonin synthesis inhibitor.[184]

Injections of 300mg/kg S-allylcysteine to mice over eight weeks appears to increase then levels of the 5-HT1A receptor in the mouse dentate gyrus to 223.4% of control.[185]

One study in adult rats given 250mg/kg of fresh garlic homogenate for 21 days, which say improvements in cognition, noted increases in plasma and brain concentrations of free tryptophan as well as similar increases in serotonin and the metabolite 5-HIAA.;[186] alongside the increase in free tryptophan in plasma was a decrease in total tryptophan,[186] which suggests that the total tryptophan (90% in bound form to proteins in serum) is being freed up which is the main determinant of increased serotonin synthesis.[187][188]

5.6Dopaminergic Neurotransmission

The antidepressant effects of garlic appear to be partially inhibited by blockade of the D2 dopamine receptor by sulpiride.[184]

5.7Neurogenesis

In NGF-treated PC12 cells, aged garlic extract (1-2.5mg/mL) and isolated S-Allylmercaptocysteine (5-25µg/mL, no concentration dependence) augmented the neuronal growth induced by NGF; there was no effect on growth without NGF and S-allylcysteine was ineffective up to 250µg/mL.[189] High doses (300mg/kg) of injected S-allylcysteine have been noted induce neurogenesis in mice[185] which were correlated with an increase in 5-HT1A receptor expression (known to induce neurogenesis in the dentate gyrus[190][191]).

The components of aged garlic extract have been noted to increase neurogenesis, and the mechanisms of this are not well understood. There are no practical studies on garlic for this purpose so it isn't known how relevant the above information is for supplementation

Oral administration of 10mg/kg diallyl disulfide (DADS), but not 1mg/kg, to rat pups is able to suppress neuronal growth without inducing inflammation.[192] This is thought to be due to suppressing neuronal proliferation (with no influence on cell viability) which was seen to a mild degree at 10µM DADS (0.1-1µM ineffective) while both S-allylcysteine and S-allylmercaptocysteine were inactive,[192] and it was thought that DADS reduced BDNF concentrations and thus reduced ERK phosphorylation.[192]

The components of garlic oil appear to suppress neurogenesis in young rats, with no evidence in adult rats. Although a higher dose was used it was still reasonable, and due to this it may be prudent to avoid giving youth higher than normal doses of garlic oil supplements

5.8Memory and Learning

A dose of diallyl disulfide to rat pups that is able to reduce neurogenesis (10mg/kg for six weeks) seems to also cause impaired performance in learning as assessed by cross-over latency.[192]

When diallyl disulfide impairs neurogenesis, it can result in less than optimal memory performance in young rats

S-allylcysteine, at a dosage that attenuates cognitive deficits associated with Alzheimer's has failed to outperform control in otherwise healthy rats.[193]

250mg/kg of fresh garlic homogenate fed to otherwise healthy adult rats over 21 days is able to improve cognitive performance as assessed by a step-through passive avoidance test (near doubling in latency)[186] which was thought to be related to the increase in serotonin synthesis seen as serotonin itself has cognitive promoting effects.[194] Raw garlic has been used elsewhere, where 1,000mg/kg (but not 2,000mg/kg) to rats improved performance in physical memory tests (rope climbing and rotarod treadmill test) but was not significantly effective in a learning maze test.[195]

A larger dose of 1,000mg/kg garlic extract (fresh, aged, and boiled) to diabetic rats for four weeks noted that while cognitive function could be preserved relative to control, it only occurred with boiled garlic.[78]

Mixed evidence as to whether garlic supplements can improve cognition in otherwise healthy rodents, with the only hint being that lower doses of raw garlic appear to be more effective than higher doses (250mg/kg in rats is about 40mg/kg for humans and 1,000mg/kg is 160mg/kg)

5.9Depression

25-100mg/kg of a 20% ethanolic extract of garlic to mice for two weeks prior to antidepressant testing (forced swim and tail suspension tests) appeared to exert antidepressant effects in a dose-dependent manner, with 100mg/kg being as potent as the reference drugs (imipramine at 15mg/kg and fluoxetine at 20mg/kg)[184] although garlics benefits were partially blocked by all tested antagonists (sulpiride, p-CPA, prazosin, and baclofen).[184]

5.10Alziemer's Disease

Garlic is being investigated for Alzhiemer's disease in part because S-allylcysteine does show promise itself[196] and there were a lot of studies conducted on senescence accelerated mice (SAMP8) which showed improved cognition during the aging process[176][177][178] which were used initially to explain antiaging properties of garlic, but SAMP8 is also the mouse model for Alzheimer's disease.[197]

The mouse model of aging that has been used is also the mouse model for Alzheimer's, so with the success (somewhat, see the longevity section for clarification) in this mouse line previously the research was expanded into assessing Alzheimer's disease

Garlic is known to suppress amyloidogenesis (production of amyloid proteins) in vitro which is seen with fresh garlic but not boiled.[198] This is thought to be due to the compounds mostly in the aged garlic extract, since thiacremonone has been found to inhibit LPS induced amyloidogenesis in vitro (2-5µg/mL being as potent as 2µg/mL thiacremenone) and in vivo (21 days of 1-10mg/kg oral intake)[199] and with isolated S-allylcysteine.[200] S-allylcysteine has also been demonstrated to induce destabilization of amyloid in vitro[200] and when tested in transgenic mice who develop Alzheimer's disease-like pathology, oral ingestion of 2% aged garlic extract in the diet appeared to attenuate accumulation of both water soluble amyloids (sAβ40 by 39.6% and sAβ42 by 33.3%) and detergent resistant amyloids (fAβ40 by 46.5% and fAβ42 by 39.3%) in both Tg2576 and TgCRND8 strains of mice.[201]

Beyond reductions in amyloid proteins directly, S-allylcysteine is known to reduce apoptosis in neurons (PC12) caused by said amyloid proteins[202] and also H2O2 and reactive oxygen species (ROS).[203] A protein known as SNAP25 is also known to be reduced in Alzheimer's rodent models[203][204] which, due to its importance in building synaptic connections,[205] is thought to contribute to the pathology of the condition (since Alzheimer's is associated with poor synaptic connections[206]); aged garlic extract (2% of the mouse diet) or S-allylcysteine in vitro can preserve up to 70% of SNAP25 levels[203] which is thought to be secondary to the antioxidative effect. Diallyl disulfide was confirmed to not be active.[203]

Components in garlic appear to exert a reducing effect on amyloid protein content (reducing its synthesis and inducing its degradation) as well as protecting cells from the toxic effects of amyloid signalling. This is noted with the bioactives of aged garlic extract mostly, and appears to occur following oral ingestion of the standard aged garlic extract dose

The learning deficits that occur alongside LPS injections (which are associated with amyloid production) are inhibited by thiacremonone when it inhibits amyloid production.[199] S-allylcysteine, known to be active in isolation in SAMP8 mice in attenuating learning deficits[180] has been shown to also attenuate learning deficits in the streptozotocin model of Alzheimer's disease[207] associated with partially reversing oxidative and genomic damage as well as apoptosis of cells.[193]

As mentioned previously, supplementation of 2% of the diet as aged garlic extract (usually 0.1% S-allylcysteine) is known to improve cognition in SAMP8 mice when taken as a daily supplement throughout their lifetimes[176][177][178] and other types of mice (TgCRND8) also note improvements in cognitive function in later life with this same dose.[201]

In animals that have Alzheimer's (various models of Alzheimer's) and are treated with garlic, there are improvements in cognition relative to the Alzheimer's control. This improvement is significant, but not to the level of normalizing cognition relative to nondiseased control

6Cardiovascular Health

6.1Cardiac Tissue

In rats supplemented with garlic, the increase in cardiac size seen with hypertension when given at 0.5% of the diet.[208]

The increase in diabetes induce cardiac hypertrophy is reduced in rats with oral intake of 10-100mg/kg of garlic oil (every other day)[209] and 100mg/kg of the water extract of garlic for eight weeks (intraperitoneal administration to diabetic rats) was able to attenute the increase in contractility seen with diabetes[210] which is a benefit seen in the periphery as well as the heart.[211]

When fed to rats in instances where the heart tissue would normally enlarge, standard doses of garlic appear to attenuate the increase somewhat

In cardiomyocytes, while garlic extract alone did not alter cardiomyocytes the influence of noradrenaline (hypertrophy) is prevented in a manner that is partially prevented by inhibiting either nitric oxide signalling or hydrogen sulfide signalling.[63] The reduction in oxidative stress seen with garlic was also partially inhibited by preventing the actions of the gasotransmitters[63] and this has been seen in rodents given 2-4mL/kg of the juice or 2-4mg/kg of the oil before being treated with isoprenaline (cardiotoxin via the same receptors as adrenaline).[212]

Inhibiting MAPK activation (p38, ERK, JNK) has also been implicated in rats fed 10-100mg/kg of garlic oil, as has suppression or ERK5/MEK5.[209] Both the three standard MAPKs[213][214] as well as ERK5/MEK5[215] are known to be involved in cardiac hypertrophy, suggesting that this inhibition is relevant to the actions of garlic on heart tissue.

Protective effects are, at least in part, mediated by the two gasotransmitters nitric oxide and hydrogen sulfide although other mechanisms are involved. The protection from noradrenaline is interesting, since garlic itself increases noradrenaline and may circumvent its own harm

6.2Red Blood Cells

As demonstrated by allicin in vitro, the sulfur containing compounds are thought to be able to passively diffuse into red blood cells without disrupting membrane function.[127]

Red blood cells are capable of reducing sulfur into H2S at a steady rate when given substrate and in a manner not inhibited by antioxidants (ie. reducing agents);[88] it can be stimulated with glucose[88] and red blood cells incubated with garlic the production of H2S is accelerated.[85] Red blood cells seem account for 75% of garlic derived H2S,[85] whereas the rest is due to spontaneous formation of H2S extracellularly from any thiol component (including N-acetylcysteine and homocysteine).[85]

Red blood cells are able to accept garlic bioactives in a passive manner (not requiring a membrane receptor) and when inside the cell it facilitates conversion of garlic bioactives into hydrogen sulfide

It has been noted that garlic, at 4% of the mouse diet over 22 weeks, stimulated red blood cell (RBC) production and turnoever without increasing plasma erythropoietin nor spenic Hif1α.[216] There was actually a mild reduction in plasma erythropoietin, but no mouse was anemic.[216]

The authors detected increase bilirubin and iron (breakdown products of heme via the enzyme heme oxygenase 1 or HO-1[217][218]) within 5-10 days, and since carbon monoxide (CO) is a gasotransmitter produced by this reaction[218] they investigated the pathway and noted that the CO signalling pathway (MAPK p38β activation, resultin in Hsp70 upregulation and Hsp110 stabilization, which then stabilizes the major transcription factor Gata-1[219][220]) was upregulated and replicated with carbon monoxide exposure.[216]

The authors suspected that an induction of heme oxygenase 1 (HO-1) to 2.5-fold of control accelerated CO production within the cell and stimulated the above pathway; there was no influence on HO-2.[216] CO is known to stimulate RBC production independent of erhythropoitein[221] as it is the third major gasotransmitter.[222]

Garlic appears to also stimulate the third major gasotransmitter, carbon monoxide, which underlies a possible increase in red blood cell production and turnover seen in mice given the standard doses of garlic

6.3Atherosclerosis

The deposition of calcium in the arterial wall (a process known as arterial calcification, and commonly referred to as 'arterial stiffening') is an early predictor of atherosclerosis[223] and is highly correlated with overall plaque volume post mortem[224][225] and risk of death from cardiovascular disease;[226] similar to Vitamin K supplementation, garlic is said to reduce calcification[227] and persons who routinely consume garlic supplementation[228] or fighterfighters who take garlic (alongside CoQ10[6] seem to have less biomarkers of stiffness as assessed by PWV, a measure of aortic stiffness.[229]

One study with garlic (1,200mg aged garlic extract) in persons at high risk for cardiovascular disease who were already on aspirin and statin therapy but further given garlic for one year noted that the 22.2% increase in coronary calcium seen in placebo was attenuated to 7.5%.[230]

White adipose tissue appears to be highly correlated to coronary artery calcium levels[231] and an aged garlic extract (250mg) alongside some other confounds (100μg of B12, 300μg of folic acid, 12.5mg of B6, and 100mg Arginine) in humans for one year appears to prevent the progression of coronary calcium buildup, associated with a relative (to placebo) reduction in homocysteine and less white adipose generation.[231]

Mechanisms that underlie the interactions between garlic and white/brown adipose tissue may also underlie coronary calcification, a phenomena that contributes to arterial stiffness (and therapeutic effect of Vitamin K)

In endothelial cells NF-kB activation is implicated in promoting immune cell recruitment and proliferating smooth muscle cells, all of which contribute to atherogenesis and inhibiting its activation preventing plaque formation to a degree.[232][233] NF-kB can be activated in these cells by oxidants such as H2O2,[234] oxidized LDL cholesterol (oLDL; via macrophage activation into foam cells),[235] or inflammatory factors such as TNF-α, IL-1, and IFN-γ.[236]

S-allylcysteine can reduce the activation of NF-kB in response to oxidized LDL cholesterol, TNF-α, and H2O2 at concentrations of 2.5mM or higher[237] and elsewhere both aged garlic extract (1-5mg/mL) and S-allylcysteine (1-20mM) reduced activation in a concentration dependent manner associated with buffering intracellular glutathione.[238] When looking at lower concentrations, 100µM has twice failed to significantly reduce NF-kB activation in response to these stressors.[237][238]

One study has noted that equivalent levels of aged garlic extract (2mg/mL) and S-allylcysteine favored the former in protecting the cells viability (assessed by LDH leakage)[239] which implies other bioactives. That being said, diallyl disulfide (DADS) and allylmercaptane (AM) have failed to inhibit TNF-α induced NF-kB activation in endothelial cells (1-100µM).[116]

When looking at the level of the endothelium (vessel wall exposed to the blood) S-allylcysteine and aged garlic extract appear to be effective in reducing inflammatory activation, which is one of the first events in atherosclerotic pathology. However, this happens at higher than normal doses and may not be too relevant following supplementation

When looking at LDL oxidation rates, S-allylcysteine has weak protective effects at 100µM and stronger effects at 1-10mM in vitro in response to copper induced oxidation;[237][240] and due to S-allylcysteine not being a metabolite of allicin heat treatment does not affect this capacity.[143] The mechanism of action for S-allylcysteine in these models does not appear to be related to chelating copper ions,[143] but may be more related to direct antioxidant effects.

Diallyl sulfide and disulfide have been noted to inhibit LDL oxidation in vitro,[241][242] which is currently thought to be related to their copper chelating properties.[143] In fact, one study in mice given 9mg/kg allicin only noted that, when their LDL was extracted, it exhibited reduced binding to copper stimulation[243] and a water garlic extract has shown inhibition of calcium binding to proteoheparin sulfate,[244] both of which form a complex with LDL to initiate plaque formation.[244][245]

In human studies, 6 months supplementation of 7,200mg of aged garlic extract has shown a trend to reduce oLDL in hypercholesterolemic men that failed to reach significance, but TBARS (a biomarker of lipid peroxidation) was reduced.[246] Another study in this population (900mg garlic powder for 12 weeks) has failed to find an influence on LDL oxidation.[247]

Two weeks supplementation of 600mg garlic powder to otherwise healthy persons resulted in a 34% reduction in LDL oxidation[248] and a review mentioning a small (unpublished) pilot study noted that 3,600mg aged garlic extract in older individuals (no mention of hypercholesterolemia) attenuated the rate of LDL oxidation after six weeks.[249]

There appears to be mild protective effects against LDL oxidation associated with the antioxidants in garlic, but the potency is not to a remarkable magnitude (significantly weaker than olive leaf extract, the supplemental reference for this category) and human studies fail to find an effect in persons who are at risk for cardiovascular disease

Oxidized LDL cholesterol (oLDL) is known to activate immune cells via the oLDL receptor, a property not seen with normal LDL cholesterol.[250] Activation of the oLDL receptor on macrophages accelerates their conversion to foam cells which are a major determinate of atherosclerosis,[249][251] and interventions that could reduce this process include immunosuppressive agents (prevent the already made foam cell from adhereing to the artery), reducing oxidized LDL so less can activate the receptor, or reducing receptor expression itself (less activation independent of oLDL concentrations).

In vitro, activation of immune cells is known to increase CD36 receptor expression inherently (contributes to atherosclerosis[252]) and this process is increased in the precense of homocysteine (risk factor for atherosclerosis[253][254]) yet coincubation with aged garlic extract at 5mg/mL is able to suppress the increased expression by 48.6-61.8% (with and without homocysteine, respectively).[255][256] This appears to be related to the PPARγ pathway (known to increase CD36 expression[257]) since it blocked the increase by a PPARγ agonist.[258]

Uptake of oxidized LDL into THP-1 macrophages has been noted to be reduced at 5mg/mL aged garlic extract by 85.6+/-2.8%[255] and replicated in this cell line at 61.8+/-7.4%;[258] primary macrophages also note inhibition at the degree of 50.5+/-7.1%.[258] One study in mice given 9mg/kg allicin that confirmed a reduction in atherosclerotic plaque noted less oLDL uptake from macrophages,[243] suggesting biological relevance.

The expression of the LDL receptor on leukocytes has been noted to not be modified by 900mg garlic powder supplementation over 12 weeks in hypercholesterolemic men;[247] this study also failed to find reductions in cholesterol overall.

Garlic extract does appear to, in vitro, reduce the ability of macrophages to take up already oxidized LDL cholesterol secondary to suppressing the signalling in the PPARγ pathway. That being said, the in vitro studies use a high dose and while this is thought to apply to supplementation of garlic in humans it has not been confirmed

One study in adults (aged 45-75) lasting for 48 months noted that supplementation of garlic daily (900mg of the powder) was associated with a stasis of plaque in the main carotid arteries whereas placebo experienced an increase in plaque.[259] This study later was statistically evaluated, and while it was elaborated that the increase of plaque seen in placebo (74.9% increase over four years relative to their own baseline) was more than garlic (14.2%) overall, but it was only statistically significant in women but not men.[260] Later, this was replicated with aged garlic extract (250mg) in persons at intermediate risk for cardiovascular disease where arterial calcium progression was significantly attenuated; this study did have nutrient confounds though.[2]

Limited studies have measured plaque levels in humans or their progression over time, and while the protective effect seems moderate and shows gender differences (more protective in women) there does appear to be a mild protective effect associated with daily garlic ingestion

6.4Vasorelaxation

Garlic intake in the diet has been noted to actually be an independent predictor of endothelial function, with a higher intake being associated with better flow mediated vasodilation (FMD; indicative of blood flow).[261]

The enzyme cystathionine γ-lyase has been detected directly in the endothelial tissue of rats, although the other H2S producing enzyme cystathionine β-synthase is not present.[87] It is thought that the former enzyme (cystathionine γ-lyase) mostly underlies the fasting levels of hydrogen sulfide, which were detected at 46μM in rat serum.[87]

The enzymes that produce hydrogen sulfide are present in the endothelium

Incubation of blood vessels with 50-500μg/mL garlic is able to reduce blood vessels in a concentration dependent manner that correlates with the production of H2S, and the molecules that confer more H2S (diallyl disulfide and trisulfide) seem to induce relaxation to a larger degree.[85] H2S can directly induce vasorelaxation via acting as a potent potassium channel opener (independent of cGMP production)[87][262] or acts indirectly through nitric oxide signalling and cGMP.[262] Studies that cause vessel relaxation with hydrogen sulfide note that the benefits are partially prevented by both blocking potassium channels (blocking the effects of H2S) or blocking cGMP (blocking nitric oxide signalling), supporting the two independent mechanisms.[262]

The hypertension induced by blocking H2S production (via abolishing the cystathionine γ-lyase enzyme) is circumvented by providing donation of H2S,[263] supporting the involvement of H2S directly rather than another property of the enzyme.

Garlic is known to produce hydrogen sulfide, and hydrogen sulfide is known to reduce blood pressure secondary to positively influencing the nitric oxide signalling system (to be discussed); prior to that, however, hydrogen sulfide has some direct properties in reducing blood pressure via opening potassium channels

Garlic has been noted to increase the activity of nitric oxide synthase (NOS) enzymes in vitro[264] and oral intake of 2.86g/kg aged garlic extract in rats caused an increase in nitric oxide (30-40%) within 15-60 minutes after oral ingestion in a manner that was blocked by inhibiting constitutive NOS enzymes (eNOS and nNOS)[265] suggesting its relevance in vivo.

Homocysteine is known to inhibit NOS enzymes via oxidative modification of a part of the NOS enzyme called tetrahydrobiopterin.[254] While aged garlic extract does not appear to alter homocysteine concentrations in vitro but prevents homocysteine from suppressing NOS enzymes[266] and the ability of superoxide (O2-) to inhibit NOS via tetrahydrobiopterin[267] may also be negatively affected since superoxide levels in hypertensive perosns supplementing garlic (250mg of garlic pearls) has been noted.[268] Diallyl disulfide and trisulfide have also been noted to reduce the inhibitory effects of oxidized LDL on NOS activity,[269] which was thought to be secondary to Akt signalling.[269]

There are a few compounds that are elevated in instances of cardiovascular disease which then suppress the nitric oxide synthase (NOS) enzyme and reduce nitric oxide levels; garlic appears to reduce the inhibitory effects of many of these stressors to indirectly preserve nitric oxide signalling

S-Nitrosoglutathione is one of a few molecules in the S-nitrosothiol class (alongside S-nitrosocysteine and S-nitrosocysteinylglycine) that is produced when nitric oxide reacts with the thiol, and serves as an intracellular reservoir of nitric oxide. The nitric oxide is either released enzymatically[270] or decomposition is induced by hydrogen disulfide (H2S),[89] and despite mostly being intracellular it can be found in plasma in the low nanomolar range.[90] 

Despite being a reservoir of nitric oxide and nitric oxide relaxing blood vells, the blood vessel relaxing properties of S-nitrosothiols is not mediated by nitric oxide at the physiological level.[271][272] Garlic (in vitro at 45µg/mL) has been noted to stimulate the release of nitric oxide from S-nitrosothiols (physiological relevance uncertain) but to also prolong the relaxation associated with S-nitrosoglutathione.[91]

In isolated HUVEC cells, S-allylcysteine increased cGMP production in a concentration dependent manner (20-40μM) which does imply nitric oxide signalling, but blocking the NOS enzymes have failed to abolish the effects.[273]

The S-nitrosothiol class of molecules reduce blood pressure and serve as a donor of nitric oxide, but oddly at a physiological level the blood pressure reduction is not related to nitric oxide; garlic appears to stimulate the release of nitric oxide and prolong relaxation from S-nitrosoglutathione though, suggesting a possible benefit to blood pressure by increasing the activity of a normally redundant pathway (in regards to blood pressure)

Finally, one study has noted that a plasma increase in nitric oxide (by 224%) in humans correlated with an increase in interferon alpha (IFN-α) by 386% following ingestion of 2g of raw garlic, and this increase was noted for at least seven days (length of the study).[77] The authors suggested that the increase in nitric oxide preceding the increase in IFN-α although IFN is known to also directly stimulate NOS activity.[274][275]

Although more research is needed to figure of what is doing what and how, it seems that an increase in interferon alpha (IFN-α) is associated with the increase in nitric oxide seen with raw garlic

6.5Blood Pressure

Mechanistically, phenolic rich garlic extracts are able to inhibit the ACE enzyme in the 2-10μg/mL,[276] with the 'bound' phenolics (ie. glycosides) having an IC50 of 3.48μg/mL and the 'free' phenolics (aglycone) less potent at an IC50 of 14.81μg/mL.[276] Elsewhere, there has been a noted ACE inhibitory activity of leaf extracts of garlic (thought to be related to ajoenes and thiosulfates)[277] and dipeptides in the water soluble fraction of garlic which are less potent than captopril.[46]

One study in rats using fresh garlic juice (100mg/g intraperitoneal injections) noted that, after eight weeks of treatment, that there was a highly significant reduction in serum ACE activity that extended to both diabetic rats (the ones being investigated) as well as control rats.[278] This parallels previous research in hypertensive rats (oral ingestion of a basic extract)[279] and in rats undergoing ischemia/reperfusion given a different species of garlic with similar bioactives (Allium ursinum),[10] and oddly the magnitude of reduction was less in the diabetic group (of which it was normalized to nondiabetic control) than it was in nondiabetic rats (nearly abolished the activity).[278]

There are several compounds in garlics that demonstrate the ability to directly reduce ACE activity which should theoretically reduce blood pressure, and it appears that prolonged intake of garlic is able to reduce ACE in rats. This has been confirmed in both diseased models (diabetes, hypertension) and at least one study noted a potent reduction in otherwise healthy control rats

1,200mg of aged garlic supplementation for 12 weeks in persons with metabolic syndrome (nonhypertensive) failed to significantly reduce blood pressure or blood flow relative to placebo[67] and one study with 250mg of garlic pearls for two months in normotensive controls failed to find a reduction in blood pressure.[268] One study, however, found a small reduction of 2.4-4.2% blood pressure in hypercholesterolemic persons who were also normotensive due ot 900mg garlic supplementation thrice daily[280] and 7.2g of aged garlic has shown a similar effect (5.5%).[281]

In otherwise normotensive persons, it seems that garlic is quite unreliable in its effects and is either wholly ineffective or causes a mild reduction in blood pressure

Supplementation of garlic (240-960mg conferring 0.6-2.4mg S-allylcysteine) for twelve weeks in hypertensive persons noted that only the middle dose (480mg of garlic conferring 1.2mg S-allylcysteine) was able to reduce systolic blood pressure significantly (11.8+/-5.4mmHg) as the higher dose trended to reduce blood pressure by 7.4+/-4.1mmHg.[26] Reductions in blood pressure have been noted elsewhere in hypertensives to the tune of 6% or 8mmHg (systolic) and 10% or 9mmHg (diastolic)[268] and in persons with high cholesterol (appeared to be hypertensive) by 4.7% systolic and 5.7% diastolic (1,200mg garlic for 12 weeks).[282]

When looking at persons who are being treated for their elevated blood pressure yet are still hypertensive (140mmHg or greater), supplementation of 900mg aged garlic extract (2.4mg S-allylcysteine) daily for 12 weeks is associated with a 10.2+/-4.3mmHg reduction in systolic blood pressure relative to placebo with no influence on diastolic.[283] In this same population (hypertensives), garlic as aged garlic extract at doses of 600-1,500mg in divided doses was able to reduce blood pressure in the range of 3-6.3% (4.3-7.6mmHg) systolic and 3.7-5.4% (4-5mmHg) diastolic; performing equally to the reference drug atenolol after 24 weeks, although only 900-1,500mg was effective at 12 weeks and 300mg was not as effective as the reference drug (2% reduction after 24 weeks).[284]

One study has noted large variability, with one subject noting a 40mmHg reduction yet 33.8% of the subjects reporting less than 5mmHg changes.[26]

Although the magnitude of reduction is somewhat variable in hypertensive persons, it seems that a reduction at around 5-10% is almost always seen with supplementation of garlic suggesting an antihypertensive effect. In hypertensive persons, this has once performed equally to the reference drug atenolol

A meta-analysis of hypertensive patients in nine double-blind trials with either placebo or another active treatment ranging from 3 to 26 weeks in length found a significant reduction in both systolic (-9.1mmHg, 95% CI −12.7 to −5.4mmHg) and diastolic blood pressure (-3.8mmHg, 95% CI −6.7 to −1mmHg) with a high heterogenity between trials; restricting the analysis to high-quality trials yielded a significant lowering in only systolic blood pressure (–5mmHg, 95% CI –8.7 to –1.2mmHg), with no significant change in diastolic levels.[285]

A meta-analysis found that garlic significantly reduces systolic blood pressure and possibly diastolic blood pressure (depending on the quality of the trials included). However, trials past 26 weeks have not been conducted to see if garlic can affect blood pressure in the long-term.

6.6Platelets

When looking at components that influence platelet function, numerous compounds have been noted to exert inhibitory effects on aggregation including ajoene (50-80% inhibition at 300-600µM),[22] sodium 2-propenyl thiosulfate,[43] most cysteine related amino acids,[32]

Components that seem to work in the physiological range include ajoene, which interacts with and inhibits the fibrinogen receptor (IC50 of 800nM)[286] which is a particular mechanism noted with mixed garlic extracts[287] and some other isolated compounds such as sodium 2-propenyl thiosulfate.[43] Some human evidence does note that following ingestion of garlic supplements (7,200mg of the aged garlic extract) there is a reduction of fibrinogen binding to platelets (by around 30%)[246] and inhibition has been noted at a lower dose of 2,400mg aged garlic extract as well;[134] other ligands, such as collagen or von Willebrand factor, are inhibited only at a higher dose of 7,200mg.[134]

S-Ethylcysteine and S-1-Proponyl-L-cysteine have been noted to have inhibitory effects on aggregation at 780nM.[32]

Most bioactives in garlic appear to exert inhibitory effects on platelet aggregation in vitro, but at higher concentrations than are likely to occur with oral supplements. When looking at compounds that are likely to be active, Ajoene seems to be a likely candidate and the main mechanism is preventing fibrinogen from binding to its receptor

When investigating platelet function in humans, 7,200mg of aged garlic extract for 10 months in hypercholesterolemic men appears to reduce platelet aggregation induced by ephedrine and collagen (but not ADP)[246] although reduced rates ADP aggregation has been noted elsewhere (5mL of juice in otherwise healthy persons over 13 weeks) independent of maximal aggregatory potential.[288] One study has noted that 2,400-7,200mg of aged garlic extract could reduce ephedrine and collagen aggregation at all doses yet only the highest dose tested inhibited ADP,[134] so it is possible that particular pro-aggregatory agent is more resistant to the effects of garlic.

Dietary levels of garlic (4.2g of raw garlic cloves) over the course of one week in otherwise healthy persons does not modify platelet function,[165] suggesting that despite the forementioned supplemental studies on garlic that moderate food ingestion has no effect.

In a comparative study, 80mg of ginkgo biloba (EGb-761) was more effective than 250mg garlic oil (21.5% diallyl disulfide) over 180 days both reduced blood viscosity to the same level, but ginkgo was deemed more effective due to a higher baseline level and larger statistical significance.[289]

Supplements of aged garlic extract appear to have an ability to inhibit platelet aggregation when consumed by both healthy persons and those with cardiovascular complications. Moderate consumption of garlic in the diet may not be associated with such as effect

6.7Triglycerides

In obese rats with metabolic syndrome given aged garlic extract (2.86g/kg bodyweight; 0.1% of the diet being S-allylcysteine) with or without physical exercise for four weeks, supplementation appeared to further the weight and fat loss seen with exercise and was additive in reducing triglycerides (but no interaction was seen with cholesterol nor inflammatory markers).[290]

Rodent research suggests that the reduction in triglycerides is additive with exercise

One study in otherwise healthy persons given 5mL of a Kyolic garlic juice product has been noted to trend to reduce triglycerides by 13%, although the effect was not statistically significant after thirteen weeks[288] and other studies in otherwise healthy persons without high triglycerides have failed to find a reducing effect of garlic (100mg/kg raw bulb in elderly).[291]

When selectively looking at studies of people without any problems in triglyceride levels (ie. normal levels), it seems that there are unpredictable but minor alterations in triglycerides

1,200mg of aged garlic supplementation for 12 weeks in persons with metabolic syndrome failed to reduce triglycerides more than placebo,[67] and a failure to significantly reduce triglycerides has also been noted in hypertensives[268] and hypercholesterolemics.[281] 

400mg of garlic (1mg allicin) twice daily for six weeks in persons with high cholesterol has been noted to reduce triglycerides by 6.3%.[292]

In otherwise healthy controls, there is no significant reduction in triglycerides with 250mg of garlic supplementation over two months,[268] 600mg of twice daily garlic supplementation,[282] 900mg garlic in three divided doses for 12 weeks,[280]

A 1994 meta-analysis on the effects of garlic supplementation on triglycerides, which noted that many of the preliminary trials had a bit of methodological problems, noted that garlic was associated with a statistically significant reduction of triglycerides by 0.31mmol/L (95% CI of 0.14-0.49)[293] although a later meta-analysis looking at a larger pool of evidence noted a trend to reduce triglycerides that failed to reach statistical significance with a 5.45mg/dL reduction yet a 95% CI between a 14.18mg/dL reduction and a 3.27mg/dL increase.[294]

6.8Cholesterol

Mechanistically, allylmercaptan (5-125µg/mL) appears to reduce cholesterol synthesis with an IC50 value of around 25µg/mL (450µM)[295][296] and causing 20% inhibition at 5µg/mL.[295] Despite this potency, it tends to be the weakest of the tested sulfur compounds.[297] Diallyl disulfide (DADS, a more potent compound) has an IC50 of 64+/-7µM in inhibiting cholesterol synthesis[296] and S-allylcysteine (SAC) seems active as well.[298]

Inhibition of HMG-CoA has been reported with the water soluble bioactives Allicin (IC50 17+/-2μM), Ajoene, S-allylcysteine, S-ethylcysteine, and S-prpoylcysteine although Aliin is inactive[298][299][300] and components in the oil fragment such as diallyl disulfide (inhibitory at 5μM).[297]

The components of garlic appear to be inhibitors of HMG-CoA (molecular target of statin drugs) and secondary to that can inhibit cholesterol synthesis. This mechanism appears to occur potently enough and at a low enough concentration with some of the stable garlic products that it likely explains the observed human effects

The first meta-analysis on garlic and cholesterol (1993) noted that low doses of supplemental garlic, correlating to approximately one half to a full clove of garlic day, are associated with reductions in cholesterol in persons with high cholesterol by 0.59mmol/L (around 9%);[301] subsequently, a meta-analysis on the influence of garlic on cholesterol noted that there was a statistically significant reduction in total cholesterol by 0.77mmol/L (95% CI of 0.65-0.89) which was a 12% reduction relative to control.[293] Further meta-analyses have been conducted on the topic as well, concluding a 0.41 mmol/L reduction [302] and an 8% reduction[294] which were similar in magnitude to the first two meta-analyses although they had differeing conclusions; one meta-analysis stating that the study quality was of questionable clinical relevance[302] and the other directly stating clinical relevance and an alternative to statin therapy (in some populations).[294]

These meta-analyses tended to assess studies that used a lower dose garlic supplementation of around 700-1,200mg, and tended to last lengths of over 8 weeks (with some studies lasting up to 24 weeks). The subjects were those with hypercholesterolemia and a total cholesterol level exceeding 200g/dL.

When looking at all meta-analyses conducted, they all report a reduction in circulating cholesterol associated with garlic supplementation and this reduction seems to be of a similar magnitude in all analyses at around 10% for persons with high cholesterol (over 200mg/dL)

While the above meta-analyses tended to only assess total cholesterol, LDL and HDL have both been directly investigated.

In regards to LDL, there are mostly reductions associated with 7,200mg of the extract (4.6%),[281] 900mg over twelve weeks (14.2%),[280] 600mg of garlic extract known as Allicor over 12 weeks (13.8%),[282] 400mg over 12 weeks (17.3%),[292] whereas 250mg of the pearls[268] and 1,200mg of the extract in persons with metabolic syndrome over 12 weeks[67] have failed.

Improvements (increases) in HDL-C have been noted with 600mg of garlic (Allicor) over 12 weeks (11.5%),[282] 400mg over 12 weeks (15.7%),[292] while nonsignificant reductions have been seen with 7,200mg of garlic[281] and 900mg over 12 weeks,[280] and no significant influence with 250mg of garlic pearls[268] or 1,200mg over 12 weeks in persons with metabolic syndrome.[67]

Garlic seems to reduce LDL-C in a fairly reliable manner (although some negative evidence does exist) whereas it is a bit less reliable on HDL-C yet still seems to have a beneficial effect there. The magnitude of change for LDL and HDL appears to be in the range of 10-15% when looking at persons with high cholesterol

7Interactions with Glucose Metabolism

7.1Absorption

An in vitro assessment of garlic extract has noted that, against the α-glucoside enzyme, that garlic has an inhibitory effect with an IC50 of 136.3µg/mL[303] although elsewhere garlic extracts have failed to have any inhibitory potential on both α-glucoside and α-amylase when 100µL (1mg dry plant weight) was tested.[137] It was also noted that inhibitory actions of spices on the enzymes of carbohydrate metabolism correlated well with the flavonoid content[137] which garlic is known to be pretty low in.

There is not a lot of evidence on this particular topic overall, but the one study that suggested garlic may have a potential inhibitory effect on glucose absorption is directly contradicted by another; practical significance of garlic and carbohydrate absorption not yet clear but doesn't look promising

7.2Insulin

In vitro, S-allylcysteine sulfoxide appears to stimulate insulin secretion from pancreatic β-cells isolated from normal rats.[304]

Supplementation of 1,200mg aged garlic in persons with metabolic syndrome for three months has failed to reduce fasting insulin or to improve insulin sensitivity.[67]

7.3Blood Glucose

In healthy as well as diabetic rabbits, 250-350mg/kg of garlic extract appeared to have a dose-dependent reduction in blood glucose after four hours with a potency comparable to 250-500mg/kg Metformin.[305] Acute effects like this have been noted with isolated components such as S-allylcysteine sulfoxide[306] and with the garlic extract except with an oral glucose tolerance test.[307]

Supplementation of 1,200mg of aged garlic for 12 weeks in persons with metabolic syndrome has failed to significantly reduce blood glucose.[67]

7.4Glycation

Advanced glycation end products (AGEs) are small molecules developed from the process of glycation, and their production in states of hyperglycemia are thought to mediate damage to tissues and the genome in diabetes.[308][309] AGEs include compounds like carboxymethyllysine, which is the major AGE formed from oxidative breakdown[310] and is at two-fold higher concentrations in diabetics relative to normoglycemic persons[311] and can induce inflammatory damage via NF-kB.[312]

Advanced glycation end products (AGEs) are formed in the blood when a combination of high blood glucose and high oxidation are seen for long periods of time, and the production of AGEs is mostly relevant to diabetes and mediates a fair bit of organ damage seen in diabetics (the -pathy comorbidities)

Aged garlic extract at 28-84mg/mL range has been shown to inhibit over 50% glycation in vitro, which was seen with 30-80mM of isolated S-allylcysteine[313] and S-allylcysteine was noted to have Amadorin like activity and inhibited carboxymethyllysine formation at the lowest tested concentration of 10mM;[313] sulfur containing compounds (DAS, DADS) are less effective than cysteine containing compounds in attenuating glycation (LDL)[241] although it may be beneficial when oxidation is also a stressor[242] and while S-ethylcysteine has once been reported to be most effective[242] but seems comparable to S-allylcysteine and S-propylcysteine in the 5-15µM range.[241]

Amadorin-like compounds are those that ultimately inhibit AGE production, but inhibits the previous step of an amadori compound being oxidized into an AGE (of which the reference drug for this mechanism is Pyridorin).[314] Garlic extract also has also been noted to normalize the activity of both enzymes of polyol synthesis (aldose reductase and sorbitol dehydrogenase) after oral ingestion in diabetic rats,[315] so it is possible multiple pathways are involved.

Multiple sulfur bearing components in garlic are known to reduce the rate of glycation of proteins (hemoglobin and LDL mostly) and it isn't clear which ones are most potent in a living system. Beyond that aforementioned glycation (and the AGE pathway) there also seems to be some interaction with the polyol pathway which ultimately results in similar organ damage

In streptozotocin induced diabetic rats, there is a reduction in glycation of the kidneys and red blood cells seen with 500mg/kg of fresh garlic extract over eight weeks, which occurred alongside a reduction in blood glucose (57%) and slight restoration of insulin; glycated hemoglobin (HbA1c) was reduced 43%[316] and elsewhere in rats a normalization of blood glucose (250-500mg/kg garlic extract) after 30 days was associated with a normalization of the enzymes in polyol synthesis.[315]

Some studies that note a failure to reduce blood glucose also note no changes in HbA1c, although it seems that organ protection from diabetes may still occur when neither of the aforementioned two do.[317]

When blood glucose is reduced there seems to also be a reduction in HbA1c in rodents, but blood glucose reductions with garlic are not necessarily commonplace. Even if blood glucose is not reduced though, there seem to be some protective organ effects

7.5Type II Diabetes

Garlic appears to be quite a popular alternative medicine for diabetes, being one of the most popular reported alternative medicine (2001 data)[318] and being used in the Malaysian region as well (13.3% of those who reported using complementary medicine, less than both Momordica Charantia and