Soy foods and supplements have been studied for many clinical conditions that affect women. The most well known are those that involve the treatment of menopausal hot flashes, effects on bone loss and bone density, heart disease, the menstrual cycle, breast cancer and endometrial cancer.
Research on the use of soy for relief of hot flashes in perimenopausal and postmenopausal women has been mixed. A systematic review published in the European Menopause Journal in 2004 concluded that of the most rigorous studies that have been done, "there is some evidence for the efficacy of soy preparations for perimenopausal symptoms."1 A systematic review published in one of the most conventional of U.S. gynecology medical journals concluded that the soy foods and soy extracts do not improve hot flushes or other menopausal symptoms.2
Soy appears to have some beneficial effect on bone density, although considerably weaker than its effect on estrogen. What this means for the regular soy consumer is a little bit denser bones and a little bit slower bone loss. This is a desirable effect, especially when we consider the rates of osteoporosis and bone fractures in aging women. Animal studies have shown that the addition of soy to the diet inhibits bone loss and maintains bone in surgically menopausal rats3 and maintains bone in another.4
Research has shown mixed results in women. One study shows an increase in bone mass in postmenopausal women5 while another showed no effect in Chinese women.6
Soy is certainly beneficial for bones as a source of calcium. Some soy foods, such as soy milk (up to 300 mg calcium per serving) and firm tofu (up to 500 mg), can offer as much or more calcium than a serving of dairy products. In any case, including high-calcium soy foods in the diet is a good way to help maintain strong bones.
Although we do not yet understand all the mechanisms that make it so, there is evidence that consuming more soy protein can be an effective method of improving one's cholesterol. In fact, in October 1999, the U.S. Food and Drug Administration announced that eating 25 grams of soy protein a day lowers the risk of coronary heart disease—possibly one of the most significant declarations in the history of the natural foods industry. The evidence that soy lowers cholesterol is substantial. Many studies have demonstrated this effect. Perhaps the best evidence comes from a review of 38 clinical trials concluding that consumption of soy protein rather than animal protein significantly decreased serum concentrations of total cholesterol, LDL ("bad") cholesterol, and triglycerides.7
One of the greatest fears for women contemplating menopause treatment options is the concern about estrogen replacement therapy increasing the risk of breast cancer the longer it is used. Most of the research we have on soy is in relationship to lowering the risk of breast cancer in women who consume more soy in their diets from a young age (Japanese and Chinese women). In addition, we do have some laboratory research to glean theories from, regarding the safety of using soy products in women who have had or have breast cancer. The best review article on the topic of soy for breast cancer survivors was published in 2001 in the Journal of Nutrition.8 The scientists who authored this paper concluded a good guideline, (likely to be safe for most people) is to consume 1-2 servings per day of soy foods, (about what is consumed in the typical Asian diet). A very recent review of population studies concluded a protective effect of soy for breast cancer, prostate cancer and colon cancer.9
Like breast cancer, scientific data supports the conclusion the soybeans do not have a pro estrogen effect on the lining of the uterus. It is in fact probably an estrogen blocker and is associated with low rates of uterine cancer in countries where soy intake is high.10
In another study, 114 mg daily isoflavones for 3 months had no effect on the lining of the uterus. In one study of 5 years duration, postmenopausal women who had an intake of 150 mg of soy isoflavones per day did exhibit a thickening of the lining of the uterus (an estrogenic effect), although there were no precancerous cells.11 However, 150 mg of soy isoflavones per day is significantly higher than an average daily intake of soy foods in the diet–about 3 times the Asian guideline.