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A little does a lot.

June 26, 2000 -- Awakening to the sound of a whirring blender and the sharp scent of fresh soybeans on Saturday mornings meant only one thing: a breakfast of Grandma's warm, sweet soy milk. I loved to sit and watch as she squeezed the milk out of ground soybeans wrapped in a cheesecloth.

Countless glasses later, I discovered that soy milk has a lot more to offer than fond childhood memories. Packed in every yellow bean are estrogen-like molecules, called isoflavones, which may help fight heart disease, osteoporosis, cancer, and other diseases. Based on just some of the latest findings, the Food and Drug Administration last year gave food makers permission to extol soy's cholesterol-lowering prowess on package labels.

That's great, if you happen to believe soy is a healthy choice for everyone. But with soy showing up in everything from breakfast cereal and pasta to energy bars and smoothies, some researchers now worry that too much of a good thing could be harmful.

"People ought to know that there ain't no free lunch," says Lon White, MD, MPH, senior neuroepidemiologist at the University of Hawaii. "At some point -- if these molecules are as potent as [we think] they are -- there will be potent [adverse] effects."

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.' "

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