Concern has been raised recently as to whether soy, once thought to help
prevent breast cancer, could actually increase the risk. Michelle Holmes,
M.D., assistant professor of medicine at Harvard Medical School, believes
that women with a strong family history of breast cancer should limit soy
intake. She says several recent studies indicate a link between soy
consumption and increased breast cell proliferation.
"Breast cell proliferation is not breast cancer," says Holmes, "but the
worry is that if cells proliferate out of control, this can become cancer."
She also cites a 1998 study in the American Journal of Clinical Nutrition,
which observed 48 premenopausal women who were scheduled for breast biopsy.
Half were fed a high soy diet for two weeks before their biopsy, and half a
regular diet. After the biopsies, those who had eaten the high soy diet had
more microscopic markers of breast cell proliferation in their normal breast
tissue than did the women on the regular diet.
But don't toss out your tofu just yet. Margo Woods, D.Sc., associate
professor of nutrition at Tufts University School of Medicine, points to
studies that show postmenopausal Japanese women with breast cancer, who
consume an average of 3560 mg of phytoestrogens (genistein and daidzein
found in soy) per day, fare better than American women with breast cancer.
"So we assume that at least the soy is not a risk factor in women [who already have] breast cancer," she says.
Then there is the fact that three out of four epidemiological studies in
Asian populations, who consume more soy than any other group, showed an
association between decreased risk of breast cancer and soy intake. One
hypothesis for this could be the timing of the soy intake. In a 2001 study
of 1,500 Chinese women with breast cancer and 1,500 without, the women were
asked about their soy intake during adolescence. Those in the top 20 percent
of soy intake were half as likely to have breast cancer compared to women
who consumed the least amount of soy.
What does all this mean? The jury is still out on whether large amounts of
soy may have any untoward effects. Although there are no recommended daily
allowances for soy, Mark Messina, Ph.D., author of The Simple Soybean and
Your Health (Avery Publishers, 1994), suggests limiting your daily intake to
about 100 mg of soy isoflavones or 25 g of soy protein. That's about three
servings, which is equal to adding four ounces of tofu to a stir-fry, soy
milk on your cereal, and drinking a soy latte´.