Soy may not be the antioxidant powerhouse that some research has suggested, according to a new study.

Researchers found that 6 weeks on a soy-rich diet did not seem to diminish “oxidative stress” among 42 middle-aged adults with borderline-High cholesterol and no effect was seen with a diet supplemented with soy-derived antioxidants called isoflavones.

Oxidative stress refers to the potentially cell-damaging effects of oxygen free radicals, molecules that are generated by normal metabolism. The oxidation of “bad” LDL cholesterol, for example, may contribute to the “hardening” and narrowing of arteries that can lead to heart attack  and stroke.

LDL particles carry cholesterol to the arteries, and it’s thought that the greater the number of particles that are oxidized, that is, interact with oxygen, the more likely they are to be deposited into artery-clogging plaques.

Antioxidants, which include various nutrients such as vitamins C and E, help absorb the oxygen free radicals roaming the body. Diets rich in soy products, such as tofu, have been linked to lower cholesterol levels and lower rates of heart disease and stroke, and some lab research has suggested that the reason may rest in the ability of soy isoflavones to fight oxidative stress.

The new study, however, found that neither soy nor isolated isoflavones had much of an impact on oxidative stress.

“There was some protection against oxidation, but nothing too striking,” said Dr. Sonia Vega-Lopez, a researcher at the U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University in Boston.

That doesn’t mean it’s time to swap your tofu for T-bone steak, though. Soy is a good source of low-fat, plant-based protein, Vega-Lopez noted in an interview.

“Soy might not be the magic bullet some have thought,” she said, “but it’s still a good food.”

Vega-Lopez and her colleagues report their findings in the American Journal of Clinical Nutrition.

The study involved 42 men and women, age 50 or older, who had elevated LDL cholesterol levels. All participants followed each of four 6-week diets at some point in the study: one diet rich in soy, one containing soy with the isoflavones depleted, one with animal protein supplemented with isoflavones, and one based on animal protein and no added isoflavones.

During the last week of each diet, the researchers collected blood samples to look at several markers of oxidative stress, including the oxidation of LDL cholesterol.

They found that antioxidant activity in participants’ blood was slightly higher on the soy-protein diet, regardless of the isoflavone content, and soy protein did seem to reduce one measure of oxidative stress. There were no effects on LDL oxidation or other markers of oxidative stress, however.

All of the diets, Vega-Lopez noted, contained healthy servings of fruits and vegetables, which are prime antioxidant sources. So it’s possible that those effects masked some antioxidant benefits of the soy or isoflavones.

An editorial published with the report agrees that soy eaters should not give up on the food. More research is needed to determine the potential health benefits of soy, according to Penny M. Kris-Etherton and Sheila G. West of Pennsylvania State University in University Park.

“Until then,” they write, “it remains prudent to recommend soy products in a heart healthy diet because of their nutritional value and as a healthy substitute for protein sources that are higher in saturated fat and cholesterol.”

SOURCE: American Journal of Clinical Nutrition, January 2005.