FRIDAY, Aug. 8, 2008 (HealthDay News) -- Eating foods with soy protein has been promoted as a way to lower cholesterol, but a new study finds it has no significant effect on cholesterol levels.
The findings "do not support the current health claims for soy protein in a general population," said study author Peter R.C. Howe, director of the Nutritional Physiology Research Centre at the Sansom Institute for Health Research at the University of South Australia in Adelaide.
He's referring to the health claims approved for soy foods in both the United States and the United Kingdom that link daily consumption of 25 grams of soy protein to a reduction in heart disease risk through a lowering of LDL, or "bad," cholesterol.
Howe's team studied 35 men and 58 women, average age 52, who had mildly high cholesterol levels. He assigned each participant to rotate through one of three diets for six weeks each. Each diet had varying amounts of soy protein and isoflavones, substances in soy that some experts say may have cholesterol-lowering powers.
One diet contributed 24 grams of soy protein and 71 milligrams of isoflavone equivalents, one had 12 grams of dairy protein and 12 of soy protein, with 76 milligrams of isoflavones. The dairy diet, which served as the control, had 24 grams of dairy protein without isoflavones.
Howe's team measured each person's blood cholesterol -- LDL, HDL and trigylcerides -- at the start of the study and after each six-week diet.
They found no significant effect of the diets with either 24 grams or 12 grams of soy protein on LDL levels.
In his research, Howe also looked closely at whether a person's ability to maximize the body's response to soy protein had a better cholesterol-lowering effect. These people are termed "equol producers" because of their above-average ability to make equol, a substance produced in the intestines as a metabolite of a potent soy isoflavone called daidzen. Equol is thought to inhibit LDL.
When Howe compared the cholesterol-lowering effects of those who were equol producers with those who were not, he found no differences.
Howe's study was confined to those with mildly high cholesterol; he said it may have an effect on those with higher cholesterol levels. And the soy diets did lower triglycerides, a blood fat, by 4 percent.
The findings were published in the August issue of The American Journal of Clinical Nutrition.
Even though the study found no effect of the soy protein on LDL cholesterol, Dr. Frank Sacks, a professor of nutrition at Harvard School of Public Health, called the research interesting. One facet he finds especially intriguing, he said, is the finding that equol producers have no benefit either.
After a series of studies on soy and its effect on cholesterol, the American Heart Association's Nutrition Committee, of which Sacks is vice-chairman, reviewed the evidence and issued an advisory, saying there is "nothing special" about soy or isoflavones for improving cholesterol and that the heart association doesn't recommend isoflavone supplements.
However, "there are other benefits to soy foods," Sacks said. They are healthy due to high levels of polyunsaturated fats, fiber, vitamins and minerals. But, he added, "forget soy protein for lowering LDL."
More information
To learn more about LDL cholesterol, visit the American Heart Association.
