By Gary Foresman, M.D.
The potential health benefits of soybeans and soy foods have become increasingly recognized worldwide. This is due largely to the apparent health benefits imparted by the traditional Asian diet, which is very high in soy foods, as well as low in saturated fat, and high in dietary fiber.
Soybeans are legumes that are rich in phytoestrogens-plant compounds that are structurally similar to estrogen and possess weak estrogenic activity. Soy foods, in particular, are rich in a class of phytoestrogens known as soy isoflavones, which are believed to provide many of the health benefits of the traditional Asian diet.
The Health Benefits of Soy Isoflavones
Isoflavones appear to exert a variety of effects that may protect against symptoms associated with sex hormone decline or imbalance, as well as age-related diseases including cancer, cardiovascular disease, and osteoporosis.
Hormone Balancing Effects
Isoflavones are reported to have a balancing effect on reproductive hormones in both pre- and postmenopausal women. Isoflavones possess weak estrogenic activity and compete with the more potent estrogens, thereby balancing overall estrogenic activity.2 It is possible, therefore, that problems associated with estrogen imbalance, such as endometriosis, cervical dysplasia, breast cancer, menstrual irregularities, and symptoms commonly seen with both premenstrual syndrome (PMS) and menopause, may improve with soy isoflavone intake.
The hormone modulating effects of soy isoflavones have recently been demonstrated in several human studies. In one study, the consumption of 60 grams of soy protein (containing 45 mg of isoflavones) per day for one month was shown to positively affect reproductive hormone levels in premenopausal women, resulting in longer menstrual cycles (increased number of days between menstruation). Follicular phase length was increased by an average of 2.5 days, whereas no significant change in luteal phase length was observed.3 Such hormone modulating effects may have a significant, positive effect on breast cancer risk.
Postmenopausal Japanese women who consume soy foods experience significantly reduced symptoms of menopause and are less likely to use hormone replacement therapy (HRT) compared to American women who do not include soy foods in their diet.
In one study, women who received 45 grams of soy flour per day reportedly experienced a 40% reduction of hot flashes after a period of six weeks compared to women who received the same amount of wheat flour.5 All of the women included in the study had not experienced a menstrual cycle for at least 12 months and were experiencing at least 14 hot flashes per week.
While the exact mechanisms are not known, a significant body of research suggests that soy isoflavones may help to reduce cancer risk, specifically breast, prostate, and colon cancer.6 For example, women in China, where soy consumption is high, have a lower risk for breast cancer and 36% lower plasma estrogen levels when compared to women in Britain, where soy consumption is low.7 In addition, Asian men who smoke and drink alcohol, but consume soy foods, still have lower rates of some cancers, including prostate cancer, when compared to Western men.
In experimental models, soy consumption has been shown to reduce cardiovascular disease and prevent LDL cholesterol oxidation?one of the main causes of the progressive hardening and blocking of arteries. Sirtori et al. reported that the substitution of soy protein for animal protein in the diets of hypercholesterolemic humans led to a marked decrease in the concentration of serum total cholesterol, low-density lipoprotein (LDL) cholesterol, and triglycerides without significantly affecting high-density lipoprotein (HDL) cholesterol concentrations.10 While soy consumption is known to reduce cholesterol in hypercholesterolemic subjects, a recent study showed that a high isoflavone diet consumed for three menstrual cycles (approximately 129 mg per day) lowered LDL cholesterol up to 10% and lowered the ratio of LDL to HDL cholesterol by 13.8% in premenopausal women with normal cholesterol levels.
A number of studies have shown that isoflavones have antioxidant properties.12 Antioxidants work by attacking and neutralizing free radicals?compounds formed by various body functions or taken in from the environment that can cause oxidative damage and accelerate the aging process.
A recent study showed that soy isoflavone supplementation (50 to 100 mg per day) decreased oxidative damage to DNA in humans.12 Isoflavones can also exert their protective antioxidant effects on LDL cholesterol, thereby helping to promote cardiovascular health.
Osteoporosis is a bone disorder characterized by decreased bone mass, enhanced bone fragility, and increased susceptibility to bone fractures. In women, the menopausal decline of estrogen production leads to an acceleration of bone loss for about 5 to 10 years following menopause.13 The most widely used and most effective treatment for slowing this bone loss and preventing osteoporosis is HRT. However, this therapy has numerous side effects that have prompted researchers to study natural alternatives that are safe and effective.
A recent study of women aged 30 to 40 years who were followed for three years showed a strongly beneficial effect of dietary soy isoflavone intake in maintaining spinal bone mineral density.13 Additionally, Dalais et al. reported that 45 mg per day of soy isoflavones, consumed over two 12-week periods, increased bone density in 13 postmenopausal women.
Compelling research indicates that soy and its individual constituents have numerous health benefits. The primary isoflavones, genistein and daidzein, exert a wide array of effects that appear to offer protection against cancer, cardiovascular disease, and osteoporosis. Soy may also be helpful in relieving menopausal symptoms and promoting hormone balance. While the majority of studies demonstrating the benefits of soy have used whole soy foods, soy isoflavone supplements may provide many of the same benefits. The recommended intake is 60 mg to no more than 120 mg per day.
An area of concern that some may have regarding the consumption of soy foods or soy isoflavone supplements is the issue of genetically modified (GM) soybeans. While there is a great deal of controversy surrounding this subject, there are many sources of organic soy products available for those who are concerned. In addition, soy isoflavone supplements are available that use non-GM, or "identity preserved," soybeans. To be sure this is what you are getting, you should ask the manufacturer to provide documentation certifying that their product does not contain isoflavones derived from GM soybeans.
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