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.