Two servings a day of soy protein -- such as that found in tofu, soy milk or soy powder -- can lower cholesterol levels by as much as 9 percent as long as the raw soy is uncooked, a study said on Monday.
Soy-fortified muffins, cereals or nutritional bars in which the soy protein was baked at high temperatures do not provide the benefit, study author James Anderson said.
An 8 percent to 9 percent drop in low-density lipoprotein, the so-called bad cholesterol that can lead to heart disease, can be gained from eating uncooked soy protein in the form of two 12-ounce (340 ml) servings of soy milk daily or two 2-ounce (57 grams) servings of tofu, said Anderson, a scientist at the University of Kentucky in Lexington. Cooking the tofu does not destroy the key proteins because they have been stabilized, he said.
The health benefit also is found in such products as soy nuts, soy powder sprinkled on food or in milkshakes, or edamame, a raw or parboiled edible form of soybeans popular in Japan, he said in a telephone interview.
Anderson presented his analysis of 57 previous studies on soy protein's impact on blood cholesterol to a scientific conference on soy being held in Chicago this week.
"Soy protein increases the activity of low-density lipoprotein receptors primarily on the liver that clears it from the body," he said. "Eating soy protein increases the activity of these enzymes that break down the cholesterol."
In the studies reviewed, the cholesterol drop showed up after about a month.
If the recommended two servings of soy were doubled in size, that would lower blood cholesterol only by another 1 percent to 2 percent, so the optimal effect comes from two modest servings, he said.
Eating the soy foods all at once would overwhelm the body's ability to process so they must be consumed in two separate servings, he said, comparing soy to a fast-acting drug that must be taken in doses hours apart to gain the effects.
Cooking scrambles the amino acids contained in soy proteins and all but eliminates the health benefit, he said.
A decade ago, Anderson was instrumental in publicizing the link between eating soy protein and lowering cholesterol, though he said his 1995 analysis was slightly flawed.
His claim that regular consumption of soy-based foods lowered "bad" cholesterol by 13 percent was too high, he said, because some Italian studies that he used in his analysis exaggerated the effect by examining particularly unhealthy patients.
Since Anderson's initial study, statins prescribed to lower cholesterol have become the best-selling drugs in the world.