Soy, a staple food in many Asian countries, contains valuable constituents, including protein, isoflavones, saponins, and phytosterols. Soy protein provides essential amino acids. When eaten with rice, soy provides protein comparable with that found in animal products. Soy is low in fat and cholesterol-free.

The isoflavones in soy, primarily genistein and daidzein, have been well researched by scientists for their antioxidant and phytoestrogenic properties. 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, 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, 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. Human research also suggests a protective role of soy against cancer, but the data are currently insufficient to form any solid conclusions.

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. Exactly how soy lowers cholesterol remains in debate, though isoflavones appear to be one key component.

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. In addition, soy may help regulate hormone levels in premenopausal women.

Soy may also be beneficial in preventing osteoporosis. Isoflavones from soy have protected against bone loss in animal studies. 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. 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.