Soy Isoflavone Supplements
Claims, Benefits: A “natural” way to replenish the aging body’s declining estrogen levels and thus relieve menopausal symptoms, such as hot flashes, as well as decrease the risk of heart disease and osteoporosis, without promoting breast cancer.
Bottom Line: You don’t know what you’re getting in these supplements—or even what would be a good formulation. And if you did know what’s in the pills, you couldn’t tell what they would do in your body. It’s risky business. Stick with soy foods.
Soy: Food, Not Pills
We have said that soy is good food, but that you shouldn’t take supplements made from isolated soy components, particularly isoflavones. Many readers have asked us why not. The simple answer is that you don’t know what you’re getting in these supplements—or even what would be a good formulation. And if you did know what’s in the pills, you couldn’t tell what they would do in your body. Here are more details.
Soy supplements are widely promoted as a “natural” way to reduce menopausal symptoms such as hot flashes, and for other proposed health benefits—especially for women who don’t want to eat soy foods regularly. Among the substances found in soy (and in certain herbs, grains, and seeds, notably flaxseed) are chemicals known as phytoestrogens (plant estrogens). These are supposed to replenish the aging body’s declining estrogen levels and thus relieve menopausal symptoms, as well as decrease the risk of heart disease and osteoporosis, without promoting breast cancer. At least that’s the theory.
Note: By “soy supplements” we mean capsules and pills—not soy powders or soy concentrates, which contain relatively low levels of isoflavones per serving.
Why it’s so unclear
Notable among phytoestrogens is a group of substances called isoflavones. Two of the primary soy isoflavones, genistein and daidzein, are found in many supplements. These may indeed affect the risk of cancer, especially breast cancer. But for better or worse?
• Conflicting results. Almost all of the research on isolated isoflavones has been done on animals or in the test tube. Some animal studies suggest that these substances may help maintain bone strength and inhibit certain cancers. Other studies suggest that it isn’t the genistein and daidzein—perhaps not any of the isoflavones—but something else in soy that provides these benefits. And then there have been other studies into various proposed health benefits of soy or soy compounds that have not found a positive effect. Some research has found that the isoflavones may inhibit thyroid function. But no one knows how these animal and test tube studies relate to humans.
• Estrogen-boosting or estrogen-blocking? Though theories about what they do are speculative, isoflavones have potentially contradictory effects. They can act both as estrogens and as anti-estrogens. One theory is that in premenopausal women, who have high hormone levels, phytoestrogens may act as anti-estrogens—that is, block some effects of estrogen—and thus protect against breast cancer (now thought to be promoted by high lifetime estrogen exposure). But after menopause, when estrogen levels are lower, phytoestrogens may act like estrogens, thus relieving hot flashes and other symptoms. It’s unknown how potent these phytoestrogens are—far less potent than regular estrogen, certainly.
Exactly what effect concentrated isoflavones have remains unclear. And that’s a concern. If you got an estrogen-boosting effect when you wanted an estrogen-blocking effect, for example, you could end up raising your risk of breast cancer. These complexities are seldom mentioned by advocates of soy supplements.
Another problem, hardly ever discussed, is the variable amounts of isoflavones in soy supplements, and how these differ from the levels in soy foods. The amounts of isoflavones listed for some pills are so small as to probably have no effects at all. But the pills may contain much more, or much less, than the label states, since there’s no regulation. In any case, no one knows how much you would need to get a benefit, if there is one, and how much would be too much.
Keep in mind: Soy foods are well worth adding to your diet. But supplements containing concentrated isoflavones are another matter. No one knows what the long-term effects are. Proponents and marketers of the supplements don’t mention all the unknowns and the possible adverse effects. If isolated isoflavones have unpredictable hormonal actions in the body, that’s risky business. Pregnant or nursing women, in particular, shouldn’t risk taking isoflavone supplements. In contrast, people have been eating soy foods for centuries, and there’s good evidence that these are healthful.
Issue: March 2008
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