Saw Palmetto is a supplement which is derived from the fruit of the plant Serenoa repens. The supplement (saw palmetto) has a caloric value, as it is a concoction of fatty acids.
The fatty acids in question have the ability to block an enzyme that converts testosterone into dihydrotestosterone (DHT), the latter of which is a more androgenic form and can cause hairloss in the genetically susceptible.
Its effects on DHT production have also led to saw palmetto being used for benign prostatic hyperplasia (BPH) and lower urinary tract symptoms (LUTS) in men. Some studies have shown positive results, but larger and more well-designed studies have brought its efficacy into question.