What does soy do?
The isoflavones in soy, primarily genistein and daidzein, have been well researched by scientists for their antioxidant and phytoestrogenic properties.1 Saponins enhance immune function and bind to cholesterol to limit its absorption in the intestine. Phytosterols and other components of soy have been reported to lower cholesterol levels.
The soy isoflavone genistein has been reported to inhibit angiogenesis,2 the growth of new blood vessels that, when abnormal, can contribute to the development of cancer. Soy isoflavones have also been shown to inhibit 5 alpha-reductase,3 the enzyme that activates testosterone in the prostate gland and other tissues. 5 Alpha-reductase inhibition represents a potentially effective therapeutic approach to benign prostate enlargement and male pattern baldness.
Isoflavones may reduce the risk of hormone-dependent cancers, such as breast and prostate cancer, as well as other cancers. One study of soy research found that 65% of 26 animal-based cancer studies showed a protective effect of soy or soy isoflavones.4 Human research also suggests a protective role of soy against cancer,5 6 but the data are currently insufficient to form any solid conclusions.7
A review of 38 different studies revealed that soy consumption reduced cholesterol levels in 89% of the studies. A meta-analysis of these studies indicated that eating soy resulted, on average, in a cholesterol reduction of 23 mg per deciliter.8 Exactly how soy lowers cholesterol remains in debate,9 though isoflavones appear to be one key component.10
The mild estrogenic activity of soy isoflavones may ease menopause symptoms for some women, without creating estrogen-related problems. In one double-blind trial, supplementation with 60 grams of soy protein per day for 12 weeks led to a 45% decrease in the number of hot flashes, compared with a 30% reduction in the placebo group, a statistically significant difference.11 In addition, soy may help regulate hormone levels in premenopausal women.12
Soy may also be beneficial in preventing osteoporosis. Isoflavones from soy have protected against bone loss in animal studies.13 In a double-blind study of postmenopausal women, supplementation with 40 grams of soy protein powder per day (containing 90 mg of isoflavones per day) protected against bone mineral loss in the spine.14 Although the use of soy in the prevention of osteoporosis looks hopeful, no long-term human studies have examined the effects of soy or soy-derived isoflavones on bone density or fracture risk.
Where is soy found?
Soy has been used in connection with the following conditions
Who is likely to be deficient of soy?
Although deficiencies do not occur, people who do not consume soy foods will not gain the benefits of soy.
How much soy is usually taken?
The ideal intake of soy is not known. Researchers suggest the equivalent of one serving of soy foods per day supports good health, and the benefits increase as soy intake increases.15 Societies in which large amounts of soy are consumed ingest between 50 and 100 mg per day of soy isoflavones. The cholesterol-lowering effects of soy have been observed at amounts as low as 20 grams of soy protein per day, if it replaces animal protein in the diet.16
Are there any side effects or interactions with soy?
Soy products and cooked soybeans are safe at a wide range of intakes. However, a small percentage of people have allergies to soybeans and thus should avoid soy products.
Soy isoflavones have been reported to reduce thyroid function in some people.17 A preliminary trial of soy supplementation among healthy Japanese, found that 30 grams (about one ounce) per day of soybeans for three months, led to a slight reduction in the hormone that stimulates the thyroid gland.18 Some participants complained of malaise, constipation, sleepiness, and even goiter. These symptoms resolved within a month of discontinuing soy supplements. However, a variety of soy products have been shown to either cause an increase in thyroid function19 or produce no change in thyroid function.20 The clinical importance of interactions between soy and thyroid function remains unclear. However, in infants with congenital hypothyroidism, soy formula must not be added, nor removed from the diet, without consultation with a physician, because ingestion of soy may interfere with the absorption of thyroid medication.21
Most research, including animal studies, report anticancer effects of soy extracts,22 though occasional animal studies have reported cancer-enhancing effects.23 The findings of several recent studies suggest that consuming soy might, under some circumstances, increase the risk of breast cancer. When ovaries have been removed from animals—a situation related to the condition of women who have had a total hysterectomy—dietary genistein has been reported to increase the proliferation of breast cancer cells.24 When pregnant rats were given genistein injections, their female offspring were reported to be at greater risk of breast cancer.25 Although premenopausal women have shown decreases in estrogen levels in response to soy,26 27 pro-estrogenic effects have also been reported.28 When pre-menopausal women were given soy isoflavones, an increase in breast secretions resulted—an effect thought to elevate the risk of breast cancer.29 In yet another trial, healthy breast cells from women previously given soy supplements containing isoflavones showed an increase in proliferation rates—an effect that might also increase the risk of breast cancer.30
Soy contains a compound called phytic acid, which can interfere with mineral absorption.
Are there any drug interactions?