Soybeans: healthful or harmful?

A study from Tulane was widely reported in the news media to show that eating soybeans prevents heart attacks. That’s not what the study showed. The authors reviewed 41 recent articles on soybeans and blood cholesterol levels (American Journal of Cardiology, September 2006). They found that soybeans were unquestionably associated with lowering total cholesterol, the bad low-density cholesterol, and triglycerides, and increasing the good high-density cholesterol, and the more soybeans a person takes in, the greater the reduction in bad cholesterol. But no one has shown whether the benefits come from the soybeans themselves or from replacing other foods with soy products, which would reduce the amount of saturated fat, partially hydrogenated oils and cholesterol taken in. You might get the same results just by removing meat, chicken, full fat dairy products and so forth from your diet, even without eating any soybeans.

All plants contain chemicals that are healthful and chemicals that can harm us. Fortunately for us, our ancestors learned which plants are edible and healthful, and taught us to avoid those that are poisonous. However, if you eat very large amounts of one food, you can poison yourself, even though reasonable amounts are harmless or beneficial. For example, soybeans contain a plant estrogen called genistein, omega-3 fatty acids and fiber, all substances with known health benefits. But they also contain small amounts of trypsin inhibitors that could damage the pancreas; hemagglutinins that could cause clots to form; goitrogens that could block thyroid function; and phytates that can block the absorption of minerals. You would need to eat very large amounts of soy products to get any of these negative effects. Enjoy a moderate amount of soy foods, but do not let health claims lead you to eat huge amounts of soy to the exclusion of other foods. A healthful diet is a varied diet that includes plenty of vegetables, fruits, whole grains, beans and other seeds.

January 15, 2007

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