How Foods Can Affect Cancer

By Andrew Weil, M.D.

What do steak, tofu and sushi have to do with cancer? Plenty, it seems, if several new studies served up at the American Association for Cancer Research in Boston are to be believed. And not all bear good news: the latest report from the sprawling Nurse's Health Study, for example, detected an unsettling association between red meat and breast cancer.

The report that interested me most looked at the association between breast cancer and soy-based foods. This is a controversial topic because soy contains isoflavones, some of which in isolated form can stimulate the growth of estrogen-receptor-positive breast-cancer cells. That's why many Western doctors warn women against eating soy. Yet the epidemiological evidence has been promising: Asian women on diets rich in soy have significantly lower rates of breast cancer than Western women have.

So I was particularly gratified by a new study of Asian-American women done by the National Cancer Institute (NCI). It looked at women who ate a lot of soy-based foods as children, adolescents and adults. The strongest and most consistent association was among women who ate the most soy-based foods from ages 5 to 11. They reduced their risk of developing hormone-fueled breast cancer 58%, compared with women who ate the least. The reduction for women who ate a lot of soy as adolescents and adults was 25%. Regular, moderate consumption of whole-soy foods (such as soy nuts, edamame, soy milk, tofu and tempeh) probably affects the development of breast tissue in young females, possibly making it more resistant to carcinogens, including estrogenic agents in the environment.

The lead researcher of the NCI study says it would be premature to recommend changes in children's diets on the basis of these results, but I don't agree. Women who have a family history of breast cancer ought to be introducing their kids to soy foods as early as possible. Substituting soy milk for cow's milk is one way to start. I believe the same thing will be shown to hold true for boys; a similar diet may lower their future risk of prostate cancer.

The other piece of good news came out of a large population study of more than 22,000 U.S. physicians. It found that men who ate fish five or more times a week had a 40% lower risk of developing colorectal cancer than men who ate it less than once a week. I've long believed that the omega-3 fatty acids in oily fish inhibit the COX-2 enzyme that increases both inflammation and cell proliferation.

The bad news came in another large population study, this one of more than 90,000 nurses. A report published in the Archives of Internal Medicine found that the risk of estrogen- and progesterone-receptor-positive breast cancer increased most in those nurses who ate the most red meat. Women who ate more than 1 1/2 servings of red meat a day had nearly double the risk, compared with those who ate three or fewer servings a week. The authors offered several theories for what's behind the correlation. One possibility is that red meat delivers too much iron in a form that promotes cancer. Another is that carcinogens form in meat as it is cooked. Yet another (and one that I would bet on) is that conventionally raised beef carries residues of the hormones ranchers give cattle to make them grow faster.

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