A: Dear Joyce,
Thank you for the opportunity to provide some clarity about this issue. As you may be aware, soy has enjoyed considerable favourable press over the past decade. We have seen reports of soy reducing risk of heart disease, cancer, osteoporosis, and reducing symptoms of menopause. Just when soy seemed to be on top of the world, articles and websites began to appear claiming that this new-found health hero was really a villain in disguise. Soy bashers said all the hype about soy was really just propaganda, and that in truth, soy was not a health food, but rather a dangerous substance that should be carefully avoided by humans. In fact, anti-soy advocates claim that eating soy raises risk of cancer, osteoporosis, thyroid problems, birth defects, reproductive problems, nutritional deficiencies and Alzheimer's disease. This has left consumer wondering if soy is really a saint or a sinner.
First, it is important to understand that soy is not something new. The soybean has been used for food for centuries, particularly in the Orient. Traditional forms of soy foods included fresh or frozen beans from the soy pod (called edamame ), soy milk, tofu, and fermented foods, such as tempeh, miso and soy sauce. More recently soy has become a huge hit in North America, with all of the traditional forms of soy widely available in addition to numerous others, such as soy nuts, soy-based meat analogues, soy-based protein beverages, soy chips, soy ice cream, soy yogurt, and the list goes on. These products have become staples for many vegetarians and vegans. So, the question of the safety of soy is one that certainly deserves serious consideration.
Let's briefly address a few of the major claims against soy. For more detailed information about each of these issues, the following websites are most helpful:
Soy and Breast Cancer
Claim : Soy increases risk of breast cancer.
Among the very first health claims made for soy is that it may reduce incidence of breast cancer. It seemed so obvious when one compared the very low rates of breast cancer in Asian countries using large amounts of soy with rates in North American countries that used comparatively small amounts of soy. The risk reduction was thought to be due to the isoflavones (mainly genestein and daidzen) in soy. Isoflavones are a type of phytoestrogen (plant estrogen) that has been thought to interfere with the ability of the potent human form of estrogen to increase cell proliferation and, therefore, cancer risk. However, studies have been mixed. While some do indeed show soy acting as an anti-estrogen, others suggest soy may act as a weak estrogen itself, increasing cancer risk. Interestingly, some studies have shown that while small amounts of genestein increase cell growth, large amounts inhibit it. Finally, there is some evidence that women eating soy from an early age (especially during puberty) do reduce their breast cancer risk, while there seems to be less protection for those who begin to eat soy later in life.
Conclusion: We still do not know all the answers where soy and breast cancer are concerned. However, the evidence is sufficient to say that soy consumption does not increase risk of breast cancer and may reduce risk in some people, especially if soy is consumed from an early age. For those who have estrogen-positive breast cancer, it also appears safe to use soy in moderation.
Soy and Thyroid
Claim : Soy contains natural chemicals known as goitrogens that interfere with thyroid function. These can cause an enlargement of the thyroid gland (a “goiter”) and symptoms of hypothyroidism, such as lethargy, dullness, coldness, and depression.
It is true that soy contains goitrogens, as do many other foods such as cruciferous vegetables (cabbage, cauliflower, broccoli, and brussel sprouts), sweet potatoes, lima beans, and millet. However, these foods have been found only to cause problems when iodine intake is low , because goitrogens do their damage by interfering with the thyroid gland's ability to utilize iodine. Between 1951 and 1961, several cases of goiter were diagnosed in infants who had been fed infant formula made from soy flour. These cases are frequently cited by the anti-soy lobbyists to prove soy damages thyroid function (especially in infants). But not a single case of goiter in infants has been caused by soy formula since the 1960s. At that time the soy formula base was changed from soy flour to soy protein isolates, which are low in goitrogens, and manufacturers began fortifying soy formula with iodine.
Soy does not cause thyroid problems in healthy, well-nourished people who are not deficient in iodine. However, people who do not have a reliable source of iodine could increase their risk of thyroid problems if they eat a lot of soy and/or other foods rich in goitrogens. Iodized salt, dairy products, and fish are the main dietary sources of iodine, and most multivitamin/mineral supplements provide the recommended daily allowance. So the answer is not to avoid soy or cruciferous vegetables, but to get enough iodine.
Conclusion : There is no evidence that eating soy foods regularly causes thyroid problems in healthy people who include sufficient iodine in the diet.
Soy and Cognitive Function
Claim: Soyfoods, especially tofu, can cause mental deterioration and accelerate aging.
One study done in Hawaii (the Honolulu Heart Study) found that Japanese-American men who ate the most tofu in middle age had the greatest mental deterioration and dementia as seniors. This study is widely cited as evidence that tofu may cause a reduction in cognitive function. Interestingly, there have been at least three other studies that have suggested that soy provides significant beneficial effects on cognitive function. In addition populations with relatively high soy intake (about a serving a day), including people in Asia and Seventh-day Adventists, experience lower rates of dementia than those populations who eat little if any soy. While this does not prove that soy is beneficial, it does suggest that moderate soy consumption is likely not detrimental.
Conclusion. The weight of the evidence suggests that soy may offer some benefits to cognitive function, although more research is needed before firm conclusions can be made on this issue.
What about soy versus rice milk? It all depends. If you are sensitive to soy or use a lot of soy products, you may wish to use fortified rice milk. However, my preference is for soy – especially for children. Soy is a much richer source of high quality protein, vitamins and minerals. It also contains isoflavones, which are protective for heart health and against osteoporosis. I think it tastes better too.
Brenda Davis is a registered dietitian in Kelowna, B.C., a globetrotting lecturer and the author of several books, including Why Vegan and The New Becoming Vegetarian .