How Soy Fights
Cholesterol Is Discovered
By John Martin
Researchers say they've discovered why diets high in soy can lower your cholesterol in some instances. It's well known that diets high in soy can have dramatic effects on your cholesterol level. Now, scientists in North Carolina have found, apparently for the first time, what it is in soy that inhibits cholesterol.
In their study published in the current issue of the Archives of Internal Medicine, researchers claim that the cholesterol-lowering effect of soy depends largely on the amount of isoflavones that are present, which are plant estrogens. The higher the amount of isoflavones in the soy, the larger the effect, they say.
In the same way, the scientists say isoflavones that have been extracted from soy do not lower cholesterol, nor does a relatively low dose of this estrogen.
In the study, 156 people with moderately high cholesterol levels were picked at random to eat one of several diets over a period of nine weeks: one that contained levels of casein, a protein isolated from milk; and others that contained varying levels of isoflavones.
"What we saw was that there was no effect of the casein, and that the soy reduced levels of total and low-density (so called "bad") cholesterol," said John Crouse, MD, a professor of Internal Medicine and Public Health Sciences at Wake Forest University who led the research.
In the group of people who ate the highest level of soy protein, the results were dramatic. In just over two months, levels of "bad" cholesterol dropped by 10 percent among those who had the highest cholesterol levels in that group. That "bad" cholesterol, also known as low-density lipoprotein (LDL), is the substance that doctors believe increases the risk of heart disease.
Even when the researchers combined all those with varying LDL cholesterol levels, high amounts of soy still reduced that cholesterol by as much as 6 percent.
By contrast, those people who ate diets that contained the lowest levels of soy, or diets in which isoflavones were extracted, showed little decline in cholesterol levels.
There was also evidence of what health experts call a "dose response relationship"; that is, an increase in the amount of isoflavones caused a direct decrease in cholesterol levels. Higher levels of the estrogen also dropped blood pressure levels among women who participated.
However, other research has found that the isoflavones alone do not provide the same effect. It only works, Dr. Crouse says, if they are consumed in soy protein.
Initially, all participants were placed on casein milk shakes, then certain individuals were selected at random to change their diets to soy protein, explained Dr. Crouse. The researchers determined their conclusions by measuring blood lipid levels before the soy diets were introduced, as well as afterwards.
Lipids are organic substances made up of fats or fatty acids. High lipid levels in the bloodstream can potentially be a precursor to atherosclerosis.
Dr. Crouse and his colleagues also noted that people living in Asia consume 30 to 50 times more soy than their counterparts in the Western Hemisphere. People in those countries, as a result, have a much lower prevalence disease.
In fact, nearly a thousand times as many isoflavones are found in the urine of those living in Asia, compared to American populations, based on statistics.
"This is a subject of active investigation at the present time, but it seems that soy may have a number of effects that are beneficial in chronic diseases, including ... breast cancer, possibly has an effect on menopausal symptoms in women ... and also on cardiovascular disease and stroke," Crouse tells
One reason is that the chemical structure of isoflavones is similar to estrogen, he said, adding that a range of animal studies at Wake Forest have confirmed that isoflavones provided this disease protection.
For people with cholesterol problems, there is one common food that contains this soy ingredient.
"Probably tofu is the foodstuff that has the highest concentration of isoflavones," Crouse said.
The highest levels of soy protein used in this study were equal to consuming about a half to two-thirds of a tofu cake each day.
"If you stop the diet, the cholesterol would drift back up again," said Crouse. "Elevated cholesterol is a chronic problem that needs chronic attention."