Lipid, Antioxidant Effects May Not Explain Soy's Vascular Benefits

DAVIS, Calif.--Daily consumption of soy protein replete with isoflavones results in positive effects on the vascular system, although these effects appear to be independent of lipid-altering and antioxidant effects, according to researchers at the University of California (UC), Davis. Their randomized, double blind, crossover study was published in the July issue of The American Journal of Clinical Nutrition (78, 1:123-30, 2003) (www.ajcn.org).

In studying soy's effects on vascular function, the UC Davis researchers assigned 28 healthy postmenopausal women to consume three different protein products (25 g/d of protein) with varying levels of isoflavones for six weeks each separated by washout periods. The first product was an isolated soy protein with 107 mg/d of isoflavones, the second was ethanol-washed isolated soy protein with 2 mg/d of isoflavones, and the third was total milk protein with no isoflavones.

To measure antioxidant function, researchers analyzed conjugated diene formation, which is a measure of LDL oxidation. "If you're looking at measuring the change of oxidation of these LDL particles, the most common way is to look at conjugated diene formation," said Francene M. Steinberg, an assistant professor at UC Davis. Because she and her team did not detect changes in conjugated diene formation with any of the protein diets, they concluded soy was not exerting an antioxidant effect in their particular study population.

"Soy has been proposed to be an antioxidant in other populations," Steinberg said. "In this study, we had such an incredibly healthy population--they were eating a diet low in fat, high in fruits and vegetables, and were taking vitamin E and vitamin C supplements-- there was no change. I don't think soy was able to make a big enough change to see a difference."

While the researchers determined soy was not exerting antioxidant effects, plasma isoflavones and metabolites were significantly higher after women consumed the isoflavone-rich soy protein compared to the other two regimens. In addition, the women exhibited a significantly lower post-occlusion peak flow velocity of the brachial artery after treatment with the isoflavone-rich supplement compared to treatment with total milk protein.

"The brachial artery reactivity looks at how much the artery can expand after it's been occluded," Steinberg explained. "A blood pressure cuff is put on the arm to cause a little bit of pressure on that artery, and when it's released, healthy arteries respond by expanding to let blood flow go through more rapidly. In people with high lipids and atherosclerosis, the expansion doesn't happen very well. When pressure is applied to the brachial artery and then released, a healthy response is for the arteries to expand to allow blood flow to increase. With the soy, even in this very healthy population, it allowed a greater flow to occur, and therefore we interpreted that as being a greater dilation."

While vascular function improved, markers of cardiovascular health remained unchanged between the groups. Blood pressure remained unchanged among the groups, as did lipid levels, according to the study. Researchers also measured additional markers that would indicate a risk of atherosclerosis, and those markers remained unchanged as well. "Basically, our population was very healthy and we didn't see any changes in any of those markers," Steinberg said.