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-17β 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% re duction 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.
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