The term “soy” is used to refer to many products derived from the soybean. In terms of health and wellness, the two most important dietary supplements derived from soybeans are isolated/concentrated soy proteins and soy extracts which contain a high amount of compounds called isoflavones. The isoflavones have been associated with a wide variety of beneficial health effects including protection from cancer and osteoporosis to a reduction in hot flashes and other symptoms of menopause. Soy protein, which may or may not contain a high level of isoflavones (depending on how it is processed) has been associated with a reduction in serum cholesterol and triglyceride levels and may protect against the development of coronary heart disease. Soy products are generally marketed with claims for their benefits in reducing cholesterol and triglyceride levels, reducing the risk of heart disease, suppressing menopausal symptoms such as hot flashes, and slowing bone breakdown during menopause.
Depending on the method of processing, many soy foods contain a relatively high content of chemical compounds called isoflavones, which possess weak estrogen-like effects and relatively powerful antioxidant properties. Under conditions of high estrogen exposure, which may promote certain cancers, the isoflavone compounds tend to block the adverse effects of estrogen and may prevent growth of cancer cells. Under conditions of low estrogen exposure, such as during menopause, the isoflavones tend to act as weak estrogens, which may be just enough to help alleviate some of the symptoms associated with menopause, such as hot flashes, headaches and mood swings. In terms of heart disease risk, both isoflavones and concentrated soy proteins provide benefits via antioxidant effects (from the isoflavones) and cholesterol lowering (from the protein).
As a high quality protein source, soy-based protein powders provide an excellent amino acid profile along with the added health benefits for heart health, cancer protection, bone maintenance and, in postmenopausal women, relief from menopausal symptoms. For those women who cannot or choose not to select hormone replacement therapy following menopause, isoflavone supplements may provide an effective alternative to treating some of the symptoms associated with menopause, including hot flashes, night sweats, headaches, vaginal dryness and mood swings.
In general, soybeans, like most legumes, have a high protein content (35-40% for whole soybeans). After processing, protein powders composed of soy concentrate provide about 70% protein, while the even more purified protein source, soy isolate, may contain close to 90% protein. As a protein source, soy tends to be somewhat low in the sulfur-containing amino acids such as cysteine and methionine. This has led to the perception that soy protein is a less desirable source of dietary protein than those foods with higher sulfur content, such as meat, milk and eggs. When the digestibility of the protein is taken into account, however, isolated soy protein scores on par with egg white and milk proteins and there is no difference noted in either the increase in muscle strength or muscle cross-sectional area in men fed protein from beef or soy protein while performing resistance exercise for 12 weeks (Haub et al. 2002). Consumption of soy foods and soy proteins has been associated with beneficial health effects for heart disease, osteoporosis, cancer and menopausal symptoms. It is still unclear, however, whether these effects are due to the displacement of animal protein with vegetable protein or the combination of soy protein with the phytonutrients known as isoflavones.
Soy is the richest dietary source of isoflavones. Typical soy foods like tofu contain approximately 40-100mg of isoflavones per ounce. Soymilk provides about 100-150mg of isoflavones per 8-ounce glass. The isoflavones function as phytoestrogens in the body, where they possess weak estrogen-like effects. The two primary isoflavones found in soy are daidzein and genistein, both of which have been associated with the health benefits mentioned above. The chemical structure of isoflavones is similar enough to that of estrogen so that they can bind to the estrogen receptor on cells, yet different enough so that they only perform very weak estrogen effects. For the different soy-based protein powders on the market, the isoflavone content can vary significantly, from almost zero for those products extracted using alcohol, to certified levels of 2-5mg per gram of protein. In many Asian countries, where the incidence of heart disease, cancer and menopausal symptoms is low, the daily isoflavone intake is estimated at an average of 50-100mg per day- in contrast, the average Western intake is less than 5mg per day.
Results from a number of studies show the cholesterol-lowering benefits of including soy protein in the diet and the American Heart Association supports the use of soy protein as a reliable method of reducing serum cholesterol and Ldl levels (Jenkins et al. 2003). Reductions of 20-30% in total cholesterol, LDL, triglycerides, and C-reactive protein, with no lowering of HDL (good) cholesterol levels, has been shown with soy protein intakes of 25-50 grams per day, typically taken in 2-4 divided doses throughout the day (Jenkins et al. 2003). Cross sectional studies of typical soy isoflavone intakes have shown positive correlations between total isoflavone intake and HDL cholesterol levels and inverse associations of isoflavones with insulin levels – both of which may be associated with a lower overall risk of coronary vascular disease in postmenopausal women (Goodman-Gruen and Kritz-Silverstein 2001). These findings from a number of studies have promoted the FDA to approve a health claim for the prevention of coronary heart disease (CHD) by a higher consumption of soy protein (permitted on products with at least 6.25 grams of soy protein per serving). Such intakes have also been shown to improve arterial elasticity, vasodilatory function, and reduce the susceptibility of the LDL particles to become oxidized (Nestel et al. 1997, Steinberg et al. 2003, Wiseman et al. 2000) to a similar degree as hormone replacement therapy in postmenopausal women (Chiechi et al. 2002). In some studies, soy protein added to the diet has reduced cholesterol as significantly as HMG CoA reductase inhibitors such as lovastatin (Jenkins et al. 2003). Soy protein feeding (30-50 grams/day of isolated soy protein) for 4-16 weeks has also been shown to protect blood vessels via a reduction in plasma total homocysteine levels (Tonstad et al. 2002) and overall exposure to inflammatory cytokines (Jenkins et al. 2002).
Epidemiological studies have suggested that Asian diets may provide protection from several cancers, including those of the breast, prostate gland and colon. As mentioned above, the action of isoflavones as weak estrogens allows them to bind to estrogen receptors and block some of the detrimental effects of estrogen – such as promotion of cancer cell growth (Kumar et al. 2002). Tamoxifen, a prescription drug for treating breast cancer, is though to act as an anti-estrogen by binding to the estrogen receptor and “blocking” the growth-promoting effects of estrogen in cancer cells. Women using tamoxifen have a lower incidence of breast cancer and a 30-40% reduction in breast cancer cell growth rate. The isoflavones in soy are chemically similar to tamoxifen and, therefore, may also reduce the risk of hormone-dependent cancers via the same “estrogen-blocking” mechanism. Dietary supplementation with 40mg of isoflavones per day over a 12-week period resulted in a reduction of serum levels of estradiol and estrone and a lengthening of the menstrual cycle by 3.52 days – effects that may suggest a potential to reduce the risk of breast cancer (Kumar et al. 2002). Studies of the effects of isoflavone supplementation on breast tissue and endometrial epithelial cell proliferation have shown no effects in postmenopausal women at doses of 45mg/dayfor 14 days (Hargreaves et al. 1999) or 120mg/day for 93 days (Duncan et al. 2000), whereas other studies have suggested a stimulation of breast tissue proliferation following 14 days of 45mg isoflavone supplementation in women with malignant or benign breast disease (MsMichael-Phillips et al. 1998).
Soy protein consumption has been shown to reduce bone breakdown and slow calcium loss in animal models of osteoporosis, suggesting a possible beneficial role in preventing osteoporosis in humans. A diet high in soy protein has been shown to improve bone density in peri- and postmenopausal women in both cross-sectional (Kritz-Silverstein and Goodman-Gruen 2002) and after 3-6 months of intervention (Alekel et al. 2000, Anderson et al. 2002, Dalais et al. 1998), but no significant effect on bone density is noted following consumption of soy protein or isolated soy isoflavones (90mg/day) in normally menstruating young (21-25 years of age) women even after a period of 12 months (Anderson et al. 2002). Healthy men (59 years of age) supplemented with 40 grams of soy protein for 3 months have shown a higher level of serum insulin-like growth factor-1 (IGF-1), which is associated with higher bone mass, although short-term changes in markers of bone formation and resorption do not change (Khalil et al. 2002). It is also interesting to note that soy protein seems to cause a reduced loss of calcium from the body compared to other dietary sources of protein, which may promote calcium loss and bone breakdown when consumed at high levels. Ipriflavone, a synthetic isoflavone drug prescribed in Europe, and available in the United States as a dietary supplement, may be metabolized in the body into daidzein, and has potent effects on reducing bone resorption in postmenopausal women.
Safety / Dosage
Dietary consumption of soy-based protein concentrate or soy isolate is not associated with any significant side effects aside from the mild gastrointestinal issues (bloating, flatulence) associated with any high protein diet. High doses of concentrated isoflavone extracts are probably safe at levels up to about 200mg per day (the maximum estimated amount contained in the average Japanese diet). Since the long term effects of isolated isoflavone supplements is unknown and the potential for pro-estrogenic effects may exist for mega-dose isoflavone consumption, it is prudent to keep total isoflavone intake close to those levels found in dietary amounts.
•As a protein supplement – as needed. (Typical protein recommendations are 1-2 grams of protein per kg of body weight per day).
•For heart health, 25-50 grams of soy protein isolate per day is effective in reducing cholesterol and triglyceride levels and in preventing the development of CHD (FDA-approved health claim).
•For menopausal symptoms, 25-50mg of isoflavones per day is effective in alleviating some of the symptoms associated with menopause (e.g. hot flashes). Do not exceed more than 200mg per day.
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EDITOR'S NOTE: This monograph can be found in The Health Professional's Guide to Dietary Supplements (Lippincott, Williams & Wilkins) by Shawn M. Talbott, PhD and Kerry Hughes, MS.