In October 1999, the US Food and Drug Administration authorized the use on food labels of health claims associated with
soy protein and the reduced risk of
coronary heart disease. Several studies have indicated that a total daily intake of 25 g of
soy protein paired with a
low-fat diet resulted in clinically important reductions of total
cholesterol and
low-density lipoprotein (
LDL) cholesterol levels. Soybeans are a rich source of
isoflavones, a class of
phytoestrogens found predominantly in legumes and beans. Soy
isoflavones are heterocyclic
phenols with structural similarity to
estradiol-17beta and
selective estrogen receptor modulators. Actions at the cellular level depend on the target tissue, receptor status of the tissue, and the level of endogenous
estrogen. Studies of soy-based diets evaluating the relation between soy consumption and serum
lipid concentrations revealed that soy consumption significantly decreased total
cholesterol, LDL cholesterol, and
triglyceride levels. However, the soy
isoflavones do not increase
high-density lipoprotein cholesterol or
triglyceride levels. The effects of
soy protein on other target tissues reflect estrogenlike agonist and antagonist effects. Epidemiological studies suggest a protective effect of
soy protein on breast tissue as evidenced by the lower rates of
breast cancer in East Asian countries where soy is a predominant part of the diet. Data available from human studies on the effect of
isoflavones on
osteoporosis are limited, and additional studies are needed to support a role in
osteoporosis prevention. Thus far, there is no evidence for a stimulatory effect of
isoflavones on the endometrium. A few studies reveal a minimal effect of soy on
hot flashes, with soy reducing
hot flashes 45% and placebo causing a 30% reduction compared with an approximate 70% reduction in
hot flashes with
estrogen replacement therapy. Evidence from laboratory studies reveals neither a positive nor a negative effect of soy
isoflavones on cognition. To date, no adverse effects of short- or long-term use of
soy proteins are known in humans. The only adverse effects known are those reported in animals (
infertility in sheep and quails grazing on
phytoestrogen-rich pastures). In conclusion, soy
isoflavones are biologically active compounds. Current data are insufficient to draw definitive conclusions regarding the use of
isoflavones as an alternative to
estrogen for
hormone replacement in postmenopausal women. Although epidemiological and basic laboratory studies allude to the possible protective effects of soy
isoflavones at specific target tissues, randomized, placebo-controlled clinical trials are necessary to address these important issues.