Maca (Lepidium meyenii) is a starchy tuber that looks like a radish. It grows high in the Andes and is used as food by Andean Indians. Supplements made from it are reputed to enhance fertility and sexual performance in both men and women. Tieraona Low Dog, M.D., an internationally recognized expert in the fields of integrative medicine, dietary supplements and women's health, notes that maca is widely used to relieve menopausal symptoms. I can find no evidence to back claims that it can treat cancer and improve stamina in cancer patients.
Some animal studies have found that maca increases sexual function, but I've been able to find only two human studies, both questionable. One showed improved libido among men between the ages of 21 and 56 who took either 1,500 or 3,000 mg of maca for 12 weeks. The Peruvian researchers who conducted the study found no difference in hormone levels among the men, suggesting that the change in libido couldn't be attributed to a direct effect of maca on male hormones. Statistical flaws in the study also weaken its conclusions. A second study conducted by same research group showed an improvement in sperm production and sperm motility (movement) in nine healthy men aged 24 to 44 who took either 1500 or 3000 mg of maca daily for four months. Again, no changes in hormone levels were seen, and, again, poor methodology casts doubt on the validity of the findings.
Dr. Low Dog notes that there is a growing body of basic science studies of maca as well as some animal data. While the safety profile of maca appears to be good, we have virtually no evidence from clinical trials demonstrating effectiveness in humans for anything. I'm sure maca is a nutritious component of the Andean diet, but given the lack of scientific information on any of the claimed health benefits, I see no reason to take maca supplements.
Andrew Weil, M.D.