Nutrition: How Good Is Soy? (cont.)

White, for one, worries that soy may speed the aging of brain cells. He recently found evidence that the brains of elderly people who ate tofu at least twice a week for 30 years were aging faster than normal. Tests designed to assess memory and analytical ability showed that their brains functioned as if they were four years older than their actual age, White says of his study published in the April 2000 issue of the Journal of the American College of Nutrition.

Another fear is that the estrogen-like substances in soy may dampen the function of the thyroid. Consuming 40 milligrams of isoflavones a day can slow the production of thyroid hormone, says Larrian Gillespie, MD, author of The Menopause Diet and The Goddess Diet. (One tablespoon of soy powder contains about 25 milligrams of isoflavones, while most isoflavone supplements come in 40-milligram pills.)

According to Gillespie, within a few weeks of regularly consuming 40 milligrams of isoflavones, some women feel fatigued, constipated, and achy all over. Some also gain weight and have heavier menstrual periods. Menopausal women are at particular risk, since they're already prone to hypothyroidism. "Women think it's because of hormones and don't realize they're symptoms of hypothyroidism," Gillespie says. "Once they stop the soy, they say, 'I'm feeling fine again.' "

Soy's Not All Bad

But if some studies point to dangers from soy, others suggest important benefits. For instance, isoflavones may prevent the growth of estrogen-dependent breast cancer cells, according to findings published in the March 2000 issue of the journal Cancer Research. That's because isoflavones appear to encourage the body to break down estrogen more quickly -- before it can stimulate cancer cells to grow. Instead of lingering in the blood, bits and pieces of estrogen molecules wind up in the urine.

Isoflavones can also slow prostate cancer cells from growing, according to a study published in the June 2000 issue of the International Journal of Oncology. Other studies hint that eating soy may help prevent heart disease, endometriosis, and even osteoporosis in women, Gillespie says. However, if you think you may have any of these conditions, see your doctor before making any substantive changes to your diet.

Soy's biggest impact is on cholesterol levels, according to a mound of studies. One published in the December 1998 issue of the American Journal of Clinical Nutrition found that men who ate a low-fat diet and relied on soy as their main protein source for five weeks saw their "bad" (LDL) cholesterol levels decrease by as much as 14% and their "good" (HDL) levels increase by as much as 8%. Men who ate a low-fat diet but instead relied on meat as protein also saw their cholesterol levels significantly improve, though not as much as the soy-eaters.