Soy and Cancer: What is the Connection?

The important question is what is the relationship between soy consumption and cancer in humans? Despite the allegations of those wishing to build a case against soy, the evidence strongly suggests not only that soy does not promote cancer, but that it reduces cancer risk. For example, the elders of Okinawa have repeatedly been shown to be healthiest and longest-lived people in the world. This was demonstrated conclusively in the renowned Okinawa Centenarian Study, a 25-year study sponsored by the Japanese Ministry of Health.

Soy and cancer – what study says

The researchers conducting the study analyzed the diet and health profiles of Okinawan elders, and compared them to other elder populations throughout the world. They concluded that high soy consumption is one of the main reasons that Okinawans are at extremely low risk for hormone-dependent cancers, including cancers of the breast, prostate, ovaries and colon. Compared to North Americans, they have a staggering 80 percent less breast cancer and prostate cancer, and less than half the ovarian cancer and colon cancer.

This enormously reduced cancer risk arises in part, the study’s authors say, from the Okinawans large consumption of isoflavones from soy. This is an important finding. The lowest cancer rates in the world are found in the Okinawans who consume the most soy.

Other studies have confirmed the link between soy consumption and reduced cancer risk. The Japan Public Health Center Study found the lowest breast cancer rates in those prefectures where women ate the most soy products. A recent study published in the British medical journal Lancet showed that women who ate the most flavonoids (mostly isoflavones from soy products) had a substantially lower risk of breast cancer than those who had lower flavonoid intake.

Perhaps most tellingly, a huge study published in the Journal of the National Cancer Institute in 2003 found that women with a high intake of soy reduced their risk of breast cancer by 54 percent compared to women with a low intake of soy.

The anti-soy campaigners repeatedly say that soyfoods raise the risk of cancer. But such charges are utterly incompatible with the findings of the prestigious Health Professionals Follow-up Study, which found a 70% reduction in prostate cancer for men who consume soymilk daily.

Kaayla Daniel is a protege of Fallon and Enig and the author of a prominent anti-soy book, titled The Whole Soy Story: The Dark Side of America’s Favorite Health Food. The book is edited by Fallon, who owns the small book company that publishes it. In her book, Daniel says that “soy can almost certainly be blamed for at least some of the increase in thyroid cancers in that soy isoflavones [the type of phytoestrogen found in soy] induce… thyroid tumors.” But that’s not what the Cancer Prevention Institute of California found when it undertook the Bay Area Thyroid Cancer Study. Quite the contrary. They found that those who consumed the most phytoestrogens from soy foods, whole grains, nuts and seeds, had a markedly lower risk of thyroid cancer. Women who consumed the most soy had about half the risk of thyroid cancer compared to those who consumed the least.

It is true that if you eat too much soy and your diet is deficient in iodine, your thyroid gland may become swollen and underactive, you may develop symptoms of hypothyroidism (such as lethargy and depression), and your risk of thyroid cancer could increase. But the answer isn’t to avoid soy. It’s to make sure you consume enough iodine. Soy does not cause thyroid problems in people who consume adequate amounts of iodine.

In the U.S., iodine deficiency is very rare, because common table salt is fortified with iodine, and a mere quarter teaspoon of iodized salt provides the needed daily dose. Those not consuming iodized salt, however, should make sure they are obtaining reliable sources of the mineral. The iodine content of plant foods depends greatly on the amounts found in the soil in which they are grown. Sea vegetables and seaweeds are excellent and reliable sources of the mineral, and most multi-vitamin supplements contain iodine.

Meanwhile, Kaayla Daniel’s book has misled many health conscious people into believing that soy increases the risk not only for thyroid problems and thyroid cancer, but for many other forms of cancer. As a result, increasing numbers of people have become frightened of eating soy.

Health researcher Syd Baumel was one of the first to challenge the promotion of soy as a miracle food, and to question the idea that the more soy you eat the better. But when he looked into Daniel’s claims, he was anything but impressed. He says Daniel’s book “consistently deceives and manipulates the reader in order to build a false case… Pretty well anywhere you dip into this book, the waters are muddied with half-truths, misrepresentations, errors, lies and other tricks of false persuasion.” Baumel gives this example:

“Daniel cites as five-year clinical trial in which six out of 179 post-menopausal women taking a very high dosage soy isoflavone supplement developed endometrial hyperplasia. None of the 197 women who took a placebo did. ‘Endometrial proliferation is a precursor of cancer,’ Daniel warns, implying the women can look forward to a date with the oncologist. She doesn’t mention that all of them developed the relatively benign, non-atypical form of endometrial hyperplasia. Research suggests this condition carries a 2 percent risk of progressing to endometrial cancer – little different from the 1 to 2 percent risk for women in general.”

In 1997, the American Institute for Cancer Research, in collaboration with its international affiliate, the World Cancer Research Fund, issued a major international report, Food, Nutrition and the Prevention of Cancer: A Global Perspective. This report analyzed more than 4,500 research studies, and its production involved the participation of more than 120 contributors and peer reviewers, including participants from the World Health Organization, the Food and Agriculture Organization of the United Nations, the International Agency on Research in Cancer, and the U.S. National Cancer Institute. In 2000, Riva Bitrum, the President of Research for the American Institute for Cancer Research, said that “Studies showing consistently that just one serving a day of soyfoods contributes to a reduction in cancer risk are encouraging.”

Of course, any foods with such potent biological properties — even healthful ones — are bound to have some unwanted side effects in some people under some circumstances. Although soy consumption on the whole reduces cancer incidence, there are questions about its effect on women who have estrogen-positive (ER+) breast tumors. These tumors are stimulated by estrogen. Might they therefore be stimulated by the weak estrogenic activity of the isoflavones found in soy? The jury is still out. There is some evidence this may be the case, though there is also evidence that soy consumption favorably alters the metabolism of estrogen so that it is less likely to stimulate tumor growth.   For healthy women, according to the American Institute for Cancer Research, “even two or three servings a day of soyfoods should be fine as one part of a mostly plant-based diet.”

Soy supplements are a different story. Soy pills and powders can contain amounts of isoflavones (usually daidzein and genistein) far in excess of the amounts possible to get through diet. Very little research has been done on the effects of such mega-doses. Although there is no firm evidence to demonstrate that ingestion of isoflavones has adverse effects on human beings, there is also no clear evidence that large doses are safe. Some manufacturers of soy protein isolates and supplements recommend that people consume 100 grams of soy protein a day (the equivalent of 7 or 8 soyburgers). I believe it’s probably safer, until more is learned, to avoid concentrated soy supplements entirely.

Source: http://foodrevolution.org/blog/the-truth-about-soy/