Soy foods have been part of the traditional Asian diet for thousands of years. Today, soy foods and protein powders are commonplace in the diets of people from around the world.
Soy is rich in isoflavones, which are phytoestrogens (they have estrogen-like effects in your body). The isoflavone content of soy foods and soy protein powders varies widely, making it difficult to know how much you are consuming unless the manufacturer specifically tells you.
Due to its popularity and possible health effects (many of which are attributed to its isoflavone content), soy has been the subject of numerous studies, often financed by the soy industry. Financing by private interests does not automatically disqualify a study, but it should be kept in mind when reading the findings.
Soy does not appear to affect thyroid activity in humans.
Soy-protein supplementation benefits LDL-C levels, blood pressure, and endothelial function, but only slightly, so the benefit to your health is uncertain.
In men, regular intake of soy protein may reduce the risk of developing prostate cancer. Soy protein also has the potential to reduce testosterone levels and interfere with fertility, but only when consumed in excess — no such effects have been observed from the daily consumption of 10–70 grams of soy protein or 60–240 mg of isoflavones.
In women, soy-protein intake is associated with a reduced risk of breast-cancer incidence and mortality. In premenopausal women, soy protein appears to increase menstrual cycle length and has unknown effects on fertility. In postmenopausal women, soy protein appears to modestly increase estradiol concentrations and bone mineral density. Soy protein also appears to reduce menopausal symptoms.
Finally, soy infant formulas should be used with caution. Animal studies suggest that soy formulas interfere with sexual development. Actual human studies are scarce, but associations between soy formulas and altered sexual development have been observed in infant girls. Additionally, while soy formulas do not impair the growth of healthy, full-term infants, they can cause growth problems and rickets in premature infants.