SOY & WOMEN

Western women have a number of specific health concerns including breast cancer, osteoporosis and symptoms of menopause. All these conditions are related in varying extents to diet. In some cases, that relationship is fairly well understood. In others, researchers are only beginning to uncover the diet-causation link.

Because soybeans contain phytoestrogens (also called isoflavones), which possess weak estrogen-like activity, there is increasing interest in the role that soyfoods can play in women's health. The nutrient make-up of soybeans and soyfoods and the type of protein in these foods may also impact disease risk in women.

Breast cancer is the second most common cause of death in American women and is the first cause of death in those between the ages of 35 and 44. However, there are striking differences in breast cancer rates among countries. For example, American women are four times as likely to die from breast cancer as Japanese women.

Although it is likely that a variety of factors contribute to this difference, soyfoods consumption may play a role. Soyfoods are rich in the isoflavone genistein, containing as much as one milligram per gram of soyfood (dry weight). Genistein has been shown to inhibit estrogen-dependent breast cancer cells in vitro. Like other isoflavones, genistein exhibits very weak estrogen activity. Because it is thought that weak estrogens compete with endogenously produced estrogen for binding to estrogen receptors, they may act as antiestrogens and interfere with the growth of estrogen-dependent tumors.

But, in vitro, genistein also inhibits breast cancer cells that are estrogen-independent. This may be because genistein inhibits enzymes that control cell growth. The results of case control studies are, however, equivocal. But, among those epidemiological studies that show soy to be protective, the consumption of just one serving of soyfoods (1/2 cup tofu, tempeh, soybeans or textured vegetable protein, or 1 cup soy milk) is associated with reduced cancer risk.

The decrease in estrogen production that signals menopause can produce a variety of symptoms. These include difficulty in regulating body temperature that manifests as night sweats and hot flashes. However, the severity of menopause symptoms varies throughout the world. For example, Asian women are typically one-third as likely as American women to report symptoms. Although, to some extent, this may reflect cultural differences regarding willingness to discuss these symptoms, there appears to be an actual difference in the severity and incidence of symptoms.

Soy consumption has been proposed as one explanation for these differences. The estrogenic activity of soybean isoflavones may help to offset the effects of reduced estrogen production by the ovaries. Even though soy isoflavones exhibit very weak activity (1/100 to 1/100,000 the potency of endogenous estrogen) in women consuming soyfoods, blood levels of isoflavones can be 1,000 times higher than endogenously produced estrogen levels.

Most important, in a recent Australian study, researchers found that women who consumed 45 grams of soy flour per day experienced a 40 percent decrease in menopause symptoms. Several studies examining the effects of soy on menopause symptoms are under way.

Osteoporosis is a worldwide health problem. It affects women more than men since women have smaller bone mass and experience dramatic increases in bone loss following menopause. Estimates are that women may lose as much as 15 to 50 percent of their bone mass in the first 10 years following menopause.

Many factors, including exercise, diet and genetics, affect bone health. Some dietary factors can compromise bone health by increasing the rate at which calcium is released from the skeleton and excreted from the body. Although adequate calcium throughout life is essential for good bone health, the amount of calcium retained by the bones is more important than the actual amount consumed. Balance studies indicate that urinary calcium loss, rather than calcium intake, is the predominant factor explaining variations in calcium balance among women.

One dietary factor that increases bone calcium loss is protein. Diets high in animal protein, in particular, increase urinary calcium losses. Although soy protein is equivalent to animal protein in quality, soy protein, relative to animal protein, causes less calcium to be excreted in the urine.

Soyfoods may play other roles in protecting bone health. The isoflavones daidzein and genistein, found in significant amounts only in soybeans and soyfoods, may directly inhibit bone resorption. Finally, many soyfoods are rich in calcium. Whole soybeans, textured vegetable protein, fortified soy milk and calcium-set tofu are all high-calcium foods. Furthermore, recent studies show the fractional absorption rate of calcium from some soyfoods to be quite high, comparable to that of milk.

Although women of childbearing age have lower heart disease rates than men, those rates increase after menopause. Soyfoods may play a role in helping to reduce risk of heart disease. Besides being low in saturated fat and cholesterol-free, the protein in soyfoods has been shown to reduce cholesterol levels. A dose response relationship exists; higher amounts of soy protein increase the effects. Soy protein is also primarily effective in patients with high cholesterol levels (>240 mg.dl). Although 25 grams of soy protein has been found to produce decreases in cholesterol levels, as much as 40 grams of soy protein - or the amount in three to four servings of soyfoods - is needed to produce larger effects.

Soy may also offer protection because of its antioxidant activity. A recent human study found that soy inhibited LDL-cholesterol oxidation. Decreased cholesterol oxidation may be extremely important for decreasing heart disease risk. Finally, the isoflavone genistein inhibits platelet aggregation and smooth muscle cell proliferation in vitro. Thus, soy intake may reduce clot formation and plaque development.

Some concern has been raised about the effects of soy consumption on fertility. A recent study found that soy consumption increased the length of the menstrual cycle by an average of 2.5 days. Soy consumption was also associated with a marked decrease in both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels. The decrease in LH levels has lead to speculation that soy might interfere with ovulation. However, there was no evidence of anovulation in the study and there have been no reports of adverse effects on fertility among women who consume soy in either Asian or western populations.

Soyfoods appear to offer benefits to women in a variety of ways. The strength of the evidence for these different roles for soy varies considerably; much of the information is speculative. However, soy offers an opportunity for women to make a relatively easy addition to their diet that offers no disadvantages and that has the potential to offer considerable benefits.