Phytoestrogens are part of the plant’s natural defenses that act differently than hormones in humans.

Soyfoods contain complex mixtures of proteins, fats, carbohydrates, and bio-active compounds such as soy isoflavones. These isoflavones are phytoestrogens with approximately 1/1,000th the biological activity of circulating human estrogens.(1)

Mean isoflavone intake for the average U.S. person is only 2.35 mg/day, about the amount from one ounce of soymilk. (2) Soy oil and lecithin do not contain isoflavones.

As humans are not rats, studies of isoflavones in rodents give different results, which have fostered myths surrounding the intake of soyfoods. Rats injected with large doses of isoflavones might experience problems that are not seen in human studies.

In humans, studies show soy isoflavones do not lower testosterone levels, may improve the health of arteries, prevent certain cancers including breast and prostate cancers, and reduce menopausal symptoms. (3)

Brain Power

Recent data from clinical trails on humans find that soy may actually improve brain function. Studies are finding that soy isoflavones may enhance short term memory and executive function in women, but the data remain conflicting. In a study of 30 cognitively-healthy adults over the age of 60, those who reported eating soyfoods as part of their normal diet performed better on cognitive tests than those who did not. More research in this area is needed. Click here for more information on soy and memory (including references to the studies cited).


1. Setchell KD. Phytoestrogens: the biochemistry, physiology, and implications for human health of soy isoflavones. Am J Clin Nutr 1998; 68:1333S-1346S.

2. Bai W, Wang C, Ren C. Intakes of total and individual flavonoids by US adults. Int J Food Sci Nutr. 2013 Sep 11.

3. Kang J, Badger T, et al. Non-isoflavone Phytochemicals in Soy and Their Health Effect. J. Agric. Food Chem. 2010, 58, 8119?8133.