The soy bean (Glycine max) has been a staple food in Asia for 5000 years. Not only is it a good source of protein, but it has long enjoyed a reputation for providing a diverse range of health benefits. Modern science has determined that these benefits are largely due to the presence of a number of compounds called isoflavones, the principal ones being genistein and daidzein. The use of soy products and extracts has become very popular in Western countries in recent decades as evidence has accumulated regarding the beneficial effects of soy isoflavones in areas such as menopause, heart disease, and osteoporosis.
Isoflavones belong to a class of plant compounds known as phytoestrogens; the name reflecting the fact that their chemical structure is very similar to the estrogen produced naturally by the body’s endocrine system. This similarity enables isoflavones to interfere with the action of estrogen in two distinct ways: In situations where there is an excess of estrogen—causing unwanted side effects—isoflavones can reduce the hormone’s effects by competing with it for the same receptor sites. In the converse situation where there is a drop in estrogen level—during menopause, for example—isoflavones can mimic estrogen by binding to its receptor sites and thereby reducing the severity of menopausal symptoms.
The benefits of soy isoflavones on women’s hormonal health can clearly be seen by the relative prevalence of several conditions in Asia and the United States. For example, rates of breast cancer and endometriosis are lower in Asia, as are menopausal symptoms such as hot flashes. In fact, less than 10% of Asian women experience hot flashes, compared to over 80% of U.S. women. Osteoporosis is relatively rare in China and Japan, despite the fact that consumption of dairy products—generally considered to counteract bone deterioration—is much lower than it is in the U.S.
These demographic observational conclusions are increasingly supported by research trials and laboratory studies. Although some studies have been inconclusive, approximately twenty clinical trials have demonstrated the effectiveness of soy isoflavones in relieving menopausal symptoms. For example, a randomized placebo-controlled study of 80 postmenopausal women (Obstetrics and Gynecology, 2002; 99:389-394) found that 100mg of soy isoflavone per day gave a significant reduction in menopausal symptoms. Furthermore, the isoflavone users had reduced total and LDL (“bad”) cholesterol levels and none of the deleterious side effects—including endometrial thickening—commonly associated with estrogen treatment. The latter aspect is very important as many women are searching for a safe alternative to estrogen therapy due to fears of increased risk of cancer and heart disease. A recent review, which analyzed the data from all existing trials of soy isoflavones on menopause symptoms, concluded that hot flashes were reduced by 30 – 50% (Kurzer et al, 2003).
Another menopause-related condition that has been researched in relation to soy isoflavone is osteoporosis. A study of 478 postmenopausal Japanese women (Obstetrics & Gynecology 2001 Jan; 97(1):109-115) found a correlation between higher isoflavone intake and bone density. A review of fifteen different studies on bone health and soy isoflavones concluded that, in general, isoflavones reduce bone loss, especially in younger postmenopausal women.
A wealth of epidemiological and research data confirms the benefits of soy isoflavones for the maintenance of good health as a whole, and for menopausal women in particular. The average American consumes a much lower dietary amount than their Asian counterparts, so, for these people, a daily supplement of soy isoflavone makes good sense.
Because of the estrogen-like effect of soy isoflavone, pregnant or nursing mothers should not take these supplements.