Isolated soy protein added to the diets of 14 men, all military veterans under treatment for advanced stages of type 2 diabetes, significantly lowered unwanted proteins in their urine and slightly raised desired HDL cholesterol levels in their blood, researchers say.
The two improved areas are linked to kidney disease and coronary heart disease, respectively, in patients with type 2 diabetes. The U.S. Centers for Disease Control estimates that 18 million Americans have diabetes, with more than 90% being type 2 cases.
The study's findings, published in the August 2004 issue of the Journal of Nutrition, surprisingly exceeded the expectations of participating scientists at the University of Illinois at Urbana-Champaign and suggest that more widespread testing is warranted.
"The number of type 2 diabetics is increasing to epidemic proportions, with the disease being found in younger and younger individuals everyday," said Sandra R. Teixeira, who had pursued the research as the focus of her doctoral work at Illinois. "As a result, the rate also increases tremendously for diabetic complications, which include diabetic kidney disease and cardiovascular disease."
The human findings confirmed those of a study published a year earlier using mice, said Teixeira, now a researcher focusing on type 2 diabetes for the Novartis Institutes for Biomedical Research in Cambridge, Mass.
"Our most remarkable result was that soy protein added to the diet, compared to animal-source protein, in this case casein, resulted in a significant reduction in the amount of protein in the urine," said John Erdman, a professor of nutrition in both the University of Illinois College of Medicine at Urbana-Champaign and the College of Agricultural, Consumer and Environmental Sciences. "Patients eating casein actually had an increase of urine protein levels."
Participants in the study were men ranging in age from 53-73 who were recruited at the Veterans Affairs Illiana Health Care System (formerly known as the Danville Veterans Administration Medical Center) in Danville, Ill. Veterans diagnosed with diabetic nephropathy (kidney disease) took part in the study, but over the course of three seven-month study periods, several had to be dropped because of changes in medications. Data from the remaining 14 men were used for statistical analysis.
The men consumed pre-measured amounts, based on each man's weight, of either vanilla-flavored isolated soy protein made from soybeans or casein, an animal-based protein. They were supposed to have substituted 50% of their daily protein intake with the pre-measured powders, but instead, they consumed the powders as part of additional dishes or drinks in their diet.
"Our hypothesis was that we could slow down or maintain the same protein levels in the urine by introducing soy protein," said Lea Ann Carson, a research dietitian in the department of food science and human nutrition at Illinois. "We actually had a reduction."
Those consuming the soy protein had a 9.5% reduction in the excretion of urinary albumin, an undesired buildup of globular protein indicative of worsening kidney function. Participants eating casein had an 11.1% increase of urinary albumin. A common treatment for diabetic kidney disease has been a reduction in protein in the diet.
In Teixeira's earlier study of type 2 diabetic mice, casein consumption increased protein levels in the urine, while soy protein only prevented the levels from worsening.
Levels of HDL, often referred to as good cholesterol, were improved by 4.3% in the human study, but overall cholesterol ratios improved only slightly. HDL dropped just slightly in the men who consumed casein.
"The results suggest that a dietary modification as easy to implement as consuming soy-protein-rich foods may help to prevent diabetic kidney disease in addition to improving blood-lipid profile," Teixeira said. "This is quite important in light of the growing diabetic population, and larger trials are warranted to confirm the findings."
Why the soy protein worked as it did is not known, but Erdman theorized that isoflavones, estrogen-like components in soy, may play a role because they are thought to have improved blood lipids in earlier soy studies. Another possibility, he said, is the elevated level of serum arginine, a chemical precursor to nitrogen oxide that dilates arteries, that was observed in the men. Arginine may improve blood flow in the kidney, he said.
The study, despite focusing on just 14 participants, "is probably the strongest one done so far from the numbers of subjects and length of time, because it involved an intervention program designed for type 2 diabetes," said Erdman, who last year was elected to the Institute of Medicine of the National Academies.