A new study in healthy, young adults reports significant improvements in short-term visual performance by eating dark compared to milk chocolate. Such results are fodder for sensational headlines, so we’ve combed through the study to give you a quick hype-free analysis.
This was a randomized, single-masked crossover study with 30 healthy, young adults (age ≈26) who had 20/20 vision in each eye and no history of eyesight diseases. All participants were tested with milk and dark chocolate but sessions were separated by a minimum of 72 hours. On testing days, subjects consumed either 47 g of Trader Joe’s 72% Cacao Dark Chocolate (316 mg flavanols) or 40 g of a Trader Joe’s Crispy Rice Milk Chocolate (12.4 mg flavanols) followed by a round of visual acuity testing within 2 hours of eating the chocolate.
Visual performance was assessed using three tests:
High-contrast visual acuity
Small-letter contrast sensitivity
Large-letter contrast sensitivity
Envision reading letters off a chart at your eye doctor appointment; these tests were similar.
All 30 participants completed all rounds of testing. Significant improvements were only seen in small-letter contrast sensitivity. Neither the high-contrast visual acuity or large-letter contrast sensitivity significantly improved.
The authors speculated that what improvements had been seen might be due to the cocoa flavanols increasing “retinal, visual pathway, and/or cerebral blood flow ... enhancing bioavailability of oxygen and nutrients to metabolically active sites.”
While it is well documented that cocoa flavanols can improve blood flow, only one other trial has tested their effects on visual performance in healthy subjects. In that trial, subjects who consumed an acute cocoa flavanol dose of 720 mg showed improvement in some visual testing measures 2 hours after ingestion. However, flavonoids have shown promise in improving vision function in those with glaucoma or ocular hypertension.
There are some limitations to note in this study. Chiefly, the flavanol content of the chocolates used were not directly measured. The 72% dark chocolate flavanol content was assumed based off of a 2014 analysis done by ConsumerLab. It is unclear how the flavanol content of the milk chocolate was obtained.
Quantifying the flavanol content is very important because not all chocolate is processed in a manner that preserves flavanols. Cocoa treated with alkali (aka dutching), roasted at high temperatures, or fermented can have greatly reduced flavanol concentrations. Shorter roasting and fermenting durations can help offset this loss, though. Simply buying dark chocolate does not guarantee it will have appreciable amounts of flavanols. The cacao percentage is also not a reliable indicator of flavanol content. Additionally, cocoa flavanol concentrations can vary from batch to batch depending on growing conditions. Thus, the actual flavanol intake of the study participants is uncertain.
It is not known if the vision improvements seen in this trial would lasts beyond the 2 hour mark, if long-term cocoa flavanol ingestion would have any clinically relevant impact on eyesight, or if the same improvements would be seen in those with poor vision or eyesight diseases. The results of this study should be viewed as very preliminary. Much more research is needed before any confident conclusions can be made about cocoa flavanols and visual performance.
It is far too soon for the “eating dark chocolate improves your eyesight” headlines. There was some improvement in small-letter contrast sensitivity, but that was only measured within two hours after eating chocolate. A lot more research needs to be conducted in this area.
- Effects of chocolate, cocoa, and flavan-3-ols on cardiovascular health: a systematic review and meta-analysis of randomized trials. Am J Clin Nutr. (2012) Hooper L, et al.
- Consumption of cocoa flavanols results in an acute improvement in visual and cognitive functions. Physiol Behav. (2011) Field DT, Williams CM, Butler LT.
- The effect of flavonoids on visual function in patients with glaucoma or ocular hypertension: a systematic review and meta-analysis. Graefes Arch Clin Exp Ophthalmol. (2015) Patel S, et al.
- Impact of alkalization on the antioxidant and flavanol content of commercial cocoa powders. J Agric Food Chem. (2008) Miller KB, et al.
- Effect of fermentation and drying on cocoa polyphenols. J Agric Food Chem. (2015) Albertini B, et al.
- Correlation between Antimicrobial, Antioxidant Activity, and Polyphenols of Alkalized/Nonalkalized Cocoa Powders. J Food Sci. (2017) Todorovic V, et al.
- Flavanol and flavonol contents of cocoa powder products: influence of the manufacturing process. J Agric Food Chem. (2008) Andres-Lacueva C, et al.
- Is it the dark in dark chocolate?. Circulation. (2007) Hollenberg NK, Fisher ND.