Soy and flax are combined in everything from waes to chips. But how much of these health foods should you really eat?
Meanwhile, the jury is still out on the effect of soy protein and phytoestrogens on bone density. Some studies show that for postmenopausal women, eating soy slightly reduces bone loss; others show it brings about no improvement.
Fears about soy intake increasing the risk of breast cancer have arisen largely in the past five years. William Helferich, a professor of nutrition at the University of Illinois, demonstrated that soy protein (isoflavones) appears to promote breast cancer. However, Harvard researchers found that isoflavones prevented tumor growth. Finally, a 2004 study showed that when women between the ages of 49 and 65 ate soy for one year, breast density (which is associated with cancer risk) did not increase. "The study suggests that the soy wasn't beneficial, but it wasn't harmful either," says Messina.
Perhaps the most certain data, which comes largely from population studies, suggests that those of us who ate tofu scrambles and soybean stews as children are the best protected. Soy eaten during childhood likely alters the structure of the developing breast, lessening the risk of cancer—a possible explanation for low breast cancer rates in Asia.
"But if that's the case," says Mindy Kurzer, soy researcher and professor of nutrition at the University of Minnesota in St. Paul, "then we know much less about whether an adult woman should consume soy. It may have no [additional] benefit or there may be a small possibility that it's stimulating cancer cells to grow."
Still, there is no research that suggests a greater risk of breast cancer in people who eat soy than those who do not. And studies of large populations associate soy with lowered risk. "It's theoretically possible that soy phytoestrogens stimulate tumor cells that have estrogen receptors, but that's not been proven," says Kurzer.
She tells friends who've had breast cancer, or who are at risk for it, to continue eating soy if they always have—but not to start if they haven't. She does not recommend supplements, since little is known about them. Messina takes this recommendation a bit further: "You need to be aware of the controversy, and you can live happily without eating soy. But the evidence suggests that soy is safe for everyone, including breast cancer survivors."
The flaxseed story is simpler: Some research shows that flaxseed reduces the risk of breast cancer and slows tumor growth, says Thompson. And evidence points to a possible long-term boon for flax lovers: It may have some protective effect in children as well, a fabulous inheritance to leave to your kids.
The central gift of flaxseed is the heart-healthy omega-3 fatty acids that help lower cholesterol and reduce clogging in the arteries. Those acids may also help fight inflammation in the prostate gland, keep sperm healthy, and improve penile blood flow, which in turn checks impotence. And the body simply needs those fatty acids—often difficult for vegetarians to come by—to help cell membranes best accept nutrients while barring harm.
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