WINSTON-SALEM, NC -- The cholesterol-lowering effect of soy is the result of natural compounds in the soy known as isoflavones, Wake Forest University Baptist Medical Center researchers reported March 20 at an American Heart Association meeting in Santa Fe, NM.
John R. Crouse III, M.D., professor of internal medicine (endocrinology/metabolism) and public health sciences (epidemiology), said the project was the first to dissect the effects of isoflavones in soy from the overall soybean.
Crouse and his colleagues studied 156 patients with moderately elevated cholesterol levels who were randomly assigned to receive a soy drink containing soy protein either with or without isoflavones. Another group of patients got a beverage containing casein, the principal protein of cow's milk, but no soy protein or isoflavones.
Soy drinks containing isoflavones reduced both total cholesterol and low-density lipoproteins, LDL, the so-called bad cholesterol, said Crouse. In patients who started with a high LDL cholesterol, the effect was even more dramatic -- a 10 percent reduction in just nine weeks. Soy protein from which isoflavones were removed by alcohol extraction had no such effect.
"The conclusion is that the active ingredient in soy is the isoflavones," Crouse told the AHA's 38th Annual Conference of Cardiovascular Disease, Epidemiology and Prevention.
Scientists have long known that cardiovascular deaths and illnesses were lower in people who lived in the Far East than in Western countries. They suspected that the soybean might be part of the difference, since soy consumption is high in the Pacific Rim countries and uncommon in the West. Naturally occurring soy contains high concentrations of isoflavones, also known as phytoestrogens.
Studies in monkeys, mostly conducted at Wake Forest, showed that soy containing these phytoestrogens had many of the same benefits as the standard estrogen replacement therapy, including lowering of cholesterol, reducing the risk of hardening of the arteries, and overall, improving the function of arteries.
Crouse said prior studies of the effects of soy on blood cholesterol in people had showed variable results, though overall these earlier studies did show a reduction in total cholesterol, LDL and triglycerides.
But none of these earlier studies attempted to determine whether the cholesterol-lowering effect came from the soy protein or from the isoflavones in soy.
Crouse's study gave patients 25 grams of proteins at each serving from identical containers with 25, 42, 58 or 4 milligrams of isoflavone or the inert casein. Neither patients nor staff knew which patients were on which treatment. The study produced stepwise results: the higher the concentration of isoflavones, the greater the reduction in both total and LDL cholesterol. The alcohol-extracted soy drink with just the 4 milligrams of isoflavone and the casein drink had no effect.
The study's 156 participants included 38 post-menopausal women, 24 pre-menopausal women, and 94 men, whose average age was 52. The average total cholesterol was 241, which is considered moderately elevated. The average LDL cholesterol was 164, also moderately elevated.
"Overall the soy product was extremely well tolerated and no participant dropped out of the study because of adverse effects of the soy product," Crouse said.
Though tofu is a soy product containing isoflavones, the Wake Forest investigators used soy drinks supplied by Protein Technologies International of St. Louis, Mo. Protein Technologies also paid for the study.
Materials provided by Bowman Gray/Baptist Hospital Medical Center. Note: Content may be edited for style and length.
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