The kidney function of people with type 2 diabetes seems to be improved by dietary soy protein, with the added benefit that their levels of “good” cholesterol also go up a bit, preliminary research suggests.

Kidney function often becomes impaired with long-standing diabetes. The study of 14 older men with diabetes-related kidney disease found that adding a soy product to their diets reduced the amount of protein in their urine - an indicator of improved kidney function.

The study is too small to draw conclusions, but the results provide “initial evidence” that isolated soy protein may help reduce diabetics’ risk of kidney and heart disease, the researchers say.

Dr. John W. Erdman Jr., one of the study’s authors, told Reuters Health he hopes the work will spur larger studies.

It’s unclear why soy protein might aid in diabetic kidney disease, but estrogen-like plant compounds called isoflavones could be involved, said Erdman, a professor of food science at the University of Illinois at Urbana-Champaign.

He and his colleagues there and with the Veterans Affairs Illiana Health Care System in Danville, Illinois, report the findings in the Journal of Nutrition.

For eight weeks, men in the study used an isolated soy protein powder that could be added to a drink or food. For another eight weeks, they used a milk-based protein powder.

The goal, Erdman explained, was to have the men replace part of their usual protein intake with the soy or milk protein; however, the patients failed to follow the diet instructions and instead added the protein powders to their normal routine.

Yet even with the extra protein intake, the men’s excretion of protein in urine fell an average of nearly 10 percent when they consumed the soy product, the researchers found. In contrast, protein levels in the urine increased with the milk-based powder. [

In addition, eight weeks on the soy powder boosted the men’s levels of heart-healthy HDL cholesterol by about four percent, while it tended to dip while the men were on the milk protein.

It’s possible, Erdman and his colleagues note, that the estrogen-like activity of soy isoflavones explain the kidney effects they found, because kidney disease seems to progress more slowly in women than men, and estrogen may be a factor. In this study, blood tests showed that as the men’s isoflavone levels increased, their protein excretion declined.

Another possibility, Erdman said, is related to the fact that soy protein and animal protein have different compositions. While using the soy powder, the men’s blood levels of amino acid called arginine increased; arginine is a precursor to nitric oxide, a compound that helps dilate blood vessels.

The study received partial funding from Protein Technologies International, maker of the soy- and milk-based protein powders used in the research.

SOURCE: Journal of Nutrition, August 2004.