The soy plant (Glycine max, soybean, shoyu, and soya) belongs to the pea or legume family, and its beans are used medicinally. Soybeans were first cultivated in China as far back as the 11th century BC and are now grown in the U.S. and other parts of the world, including Japan and Europe.

Soybeans contain up to 25% fixed oil, 24% carbohydrates, and 50% protein. They are rich in minerals and vitamins and are considered a good source of fiber. The specific constituents include isoflavones (daidzein, genistein, biochanin A, and formononetin), coumestans (coumestrol and other coumestans), lignans (matairesinol and secoisolariciresinol), phytosterols (beta sitosterol), saponins (saponin B1 and B2), phytates (phytic acid), and protease inhibitors (Bowman-Birk inhibitor and Kunitz-trypsin inhibitor). Among these constituents, the isoflavones, coumestans, and lignans have chemical structures that are similar to estrogen. These are referred to as phytoestrogens. The most commonly studied isoflavones are daidzein and genistein. Since the phytoestrogens are similar in structure to the naturally occurring hormone estrogen, they have similar activities in the body. The metabolism of these agents in the body is variable. Soy has been used to treat hyperlipidemia and menopausal symptoms and to prevent osteoporosis and breast cancer. Soy has also been used for cyclic breast pain, hypertension, constipation, diarrhea, slowing the progression of kidney disease, decreasing urinary protein excretion, preventing hot flashes in breast cancer survivors, and preventing prostate and endometrial cancer.

The isoflavones bind to both alpha- and beta-estrogen receptors but have a higher affinity for the beta-receptors, which are predominantly found in the heart, vasculature, bones, and bladder. Soy estrogens may act as selective estrogen receptor modulators (SERMs). In premenopausal women, soy phytoestrogens have an antiestrogen effect, whereas in postmenopausal women with low estrogen levels, soy has a weak estrogen effect.

Soy is found in a variety of foods and herbal medicines. A few examples of food products are soy flour, soymilk, and candy-covered and roasted soybeans. Some examples of herbal products that contain soy phytoestrogens are Menopause Formula, Menopause Multiple, Soy Essentials, and Soy Isoflavone.

The clinical evidence varies regarding the efficacy of soy isoflavones for treating hot flashes associated with menopause. As with all herbal medicines, the clinical trials perf ormed to date have been small, and many are flawed. The FDA has approved the labeling of soy products for cholesterol reduction when used in combination with a low-fat diet. There is evidence of soy’s ability to reduce total and LDL cholesterol levels, with no effect on HDL cholesterol. Epidemiological studies have indicated that Asian women who consume a high-soy diet have fewer hot flashes. Although some clinical studies have shown a modest improvement, others show no improvement over that seen with placebo. Soy has been shown to be ineffective in preventing hot flashes in breast cancer surviviors. It may be effective in reducing the risk of osteoporosis by increasing bone mineral density. Because soy isoflavones do not appear to have a stimulatory effect upon the endometrium, they may not carry the risk of endometrial cancer associated with estrogen therapy.

The adverse effects of soy include gastrointestinal symptoms such as constipation, bloating, and nausea. It may cause an allergic reaction that manifests as a skin rash and itching. Other statements about soy’s risks need further research. These include claims regarding increased risk of breast cancer, stomach cancer from fermented soy products, and cognitive impairment in later life.

Theoretically, soy may competitively inhibit effects of estrogen replacement therapy. There is a preliminary indication that soy may antagonize the antitumor effects of tamoxifen; therefore, concurrent use should be avoided. No herbal or dietary supplement interactions have been reported to date. Individuals with asthma are at increased risk of soy hull allergy, as are people with allergic rhinitis. Patients who have breast cancer or a personal or family history of the disease should use soy cautiously, as its effects upon breast-cell proliferation are not yet well known. Soy should be used cautiously in hypothyroid patients, as it can inhibit thyroid hormone synthesis. Because soy may increase the risk of kidney stones due to its high oxalate content, patients with a history of kidney stones should avoid excessive consumption of soy. People with milk allergies may also be sensitive to soy and should be counseled to use soy products with caution or to avoid them altogether.

The recommended dose of soy protein varies depending upon the desired result: for lowering cholesterol, 20 to 50 g/day; for osteoporosis prevention, 40 g/day; and for hot flash prevention, 20 to 60 g/day (providing 30 to 90 mg of isoflavones per day).

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