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Is It True What They Say About Soy?

Soybeans and their products, such as tofu (soybean curd) and soy milk, enjoy a reputation as very healthy foods, and they have risen in popularity in this country. Certain soy foods have earned the right to be labeled "heart-healthy." Yet not everybody is on board, judging by a recent spate of queries from our readers. "I won't allow soy in the house," one writes. Others ask if soy can fuel breast cancer, cause thyroid disease, or promote Alzheimer's. One unnerving report suggested that soy milk damages infant immune systems. A lot of people these days are afraid of soy.

Soybeans contain a complex mix of phytochemicals, including isoflavones. Some of these may act as estrogens or as anti-estrogens (the latter may block the effects of estrogens in the human body). Soy isoflavones may also act as antioxidants and have other beneficial effects on blood vessels and the heart. There's still a lot to learn about soy.

Soy and isoflavone supplements are in another category entirely. Since they concentrate the hormone-like substances in soy, they may well have a downside. Moreover, you have no guarantee as to what's in the supplements, and too little is known about them. We hope you will cross them off your list.

But what about soy foods? First, here are the potential problems and the conclusions so far:

Breast cancer: The high intake of soy foods in Asian countries has long been credited, at least by some researchers, for the lower rate of breast cancer among Asian women, compared with women in countries where little soy is consumed. But some confusion arises when you look at genistein, the main soy isoflavone and a plant estrogen. Does it protect against breast cancer or, on the other hand, promote the growth of existing cancer cells? Some studies have suggested the latter. Researchers at the Mayo Clinic recently reviewed all the evidence and concluded that soy has not been shown to fuel breast cancer cells. "If breast cancer patients enjoy soy products," they concluded, "it seems reason-able for them to continue to use them."
Whether soy actually protects against breast cancer is still unknown.

Soy milk and infants: A study published last year in the Proceedings of the National Academy of Sciences found that the highly concentrated phytoestrogens in soy formula might weaken the immune systems of babies. The formulas have more of these compounds than soy foods do. But researchers emphasized that this risk is largely theoretical. There's no evidence that soy formula is unsafe, or that infants drinking it have been harmed. Breast milk is still the first choice, however, followed by milk-based formulas. Only infants allergic to milk should drink soy formula.

Thyroid disease: One study suggested that soy protein supplements can interfere with the absorption of thyroid medications. Other research tentatively showed that soy foods may actually interfere with normal thyroid function, perhaps leading to goiter (swelling of the thyroid gland, located in the neck). But there's no risk of goiter in healthy people consuming soy who are not deficient in iodine. Strict vegetarians, who eat no iodine-rich fish or dairy products, might be at risk—and eating lots of soy might boost the risk. The answer is not to give up soy, but to increase iodine intake. One way is to use a small amount of iodized salt. And vary your diet as much as possible.

Alzheimer's disease: As we reported two years ago, a study of middle-aged Japanese-American men showed that those who ate at least two servings of tofu a week had a faster decline in mental ability as they aged and were more prone to Alzheimer's than men who ate no tofu. But this study raised more questions than it answered. No such effects have been seen in Japan, where life expectancy is high and tofu is a staple of the diet. Indeed, soy foods may actually protect the brain. The findings of this study are questionable and should not lead you to avoid tofu or other soy foods.

Kidney stones: If you've ever had calcium-oxalate kidney stones, the most common type, you should limit your intake of soy. Many soy foods are rich in oxalates and thus may promote the formation of such stones in those at risk, according to a study last year.

Now, a look at the positive side of soy foods. Not much is certain, but the outlook is promising:

Heart disease: Many researchers believe that the high intake of soy in Asian countries helps explain the lower incidence of heart disease there, and the FDA has okayed a "heart-healthy" claim for soy foods. Those that contain at least 6.25 grams (about one-quarter of an ounce) of soy protein per serving can claim on the label to reduce the risk of heart disease, when consumed as part of a healthy diet. Soy helps lower high blood cholesterol and may work in other ways to benefit blood vessels and the heart. So far, of all the potential health benefits of soy, this one has the most solid evidence.

Prostate cancer: In countries where soy is a dietary staple, men are less likely to develop prostate cancer. In animal studies, soy has slowed the growth of this cancer. This anti-cancer effect is still only hypothetical; more research is needed.

Menopausal symptoms: Soy supplements (capsules or pills) containing high levels of isoflavones, as well as soy foods with lower levels, have been promoted as effective remedies for menopausal symptoms such as hot flashes, irregular sleep patterns, and vaginal dryness. But no one knows how effective these plant hormones are—the evidence is contradictory—or whether they are safe. If isoflavone supplements act like hormones, they could pose some of the same dangers. Soy foods, on the other hand, may not have enough plant hormones to combat menopausal symptoms, but at least they aren't harmful. It can't hurt to try them, as part of a healthy diet.

Osteoporosis: According to research in Japan, women who consume a lot of soy tend to have greater bone mass. Japanese women also have a lower rate of hip fractures than American women, but that might be because of genetics or other factors. So far, there's reason to think that consuming soy is beneficial to bones, but long-term studies are still needed.

Keep in mind: Soy foods are well worth adding to your diet, since they may reduce the risk of heart disease. Other possible health benefits—not so well established—include protection against breast and prostate cancer and osteoporosis. Soy is not magical; it cannot fix up a poor diet. On the other hand, there is no convincing evidence that soy foods are harmful. People have been eating them for millennia, particularly in Asia. Remember that not all soy products are created equal—soy sauce, for instance, contains no soy protein. Even soy foods with a heart-healthy label may be high in salt, sugar, and calories. Be sure to read the labels.

UC Berkeley Wellness Letter, November 2002