Effects Of Dietary Soy Protein On Renal Function - Brief Article

Nutrition Research Newsletter, Jan, 1999

A preponderance of evidence links low-protein diets with a slowing in the progression of chronic renal insufficiency. Excessive protein intake may result in a deterioration of kidney function by making the kidney respond to the increased workload.

A new soy-protein hypothesis, developed by James W. Anderson et al., states that "substitution of soy protein for animal protein in diabetic individuals results in less hyperfiltration and hypertension, with resultant protection from diabetic nephropathy." Although this hypothesis has not been comprehensively tested, previous evidence found from animal studies appears to support the theory.

In comparison to high-protein diets, low-protein diets have been associated with decreased renal plasma flow, lower glomerular filtration rates (GFR), and reduced albumin excretion. It also appears that the type of dietary protein is related to post-prandial changes in renal function. Previous studies indicate that the largest changes in post-prandial renal function are from beef, followed by chicken and fish. Soy protein does not appear to significantly stimulate a post-prandial response. A study of nephrectomized rats suggested that soy protein intake may significantly preserve renal function in a damaged kidney better than animal protein.

Eight type II diabetic males with proteinuria participated in this study to examine the potential benefits of soy protein on kidney function. All of the men were being treated with insulin, and had fair glycemic control with glycated hemoglobin values [is less than] 8%. All of the men were hypertensive and obese (mean body mass index = 35.1). They all were free of other significant illnesses and impaired kidney function. Participants were initially asked to follow an 8-week run-in period diet similar to the study diet to provide them with education, adjust insulin therapy to achieve desirable glycemic control, and attain homeostasis. A registered dietitian provided extensive education with sample recipes and meal plans that adhered to the guidelines to supply ~55% of energy from carbohydrate, ~30%, from fat, and 15% (1 g/kg body weight) from protein. At least 50% of the protein in the soy-protein diet was from foods made with soy. Three-day diet records were completed and analyzed to monitor participant compliance.

After the run-in period, there was a four-week washout period prior to repeating the same 8-week cycle. Renal function was assessed by analysis of serum urea nitrogen, creatinine clearance, urine protein, and GFR (using nuclear medicine scanning). Throughout the study, glycemic control remained stable. For the subjects consuming soy protein, glycated hemoglobin values decreased form 8.1% to 7.3%. Although the difference between groups was not significant, those eating animal proteins decreased only from 7.7% to 7.6%. Diets with soy protein significantly decreased both serum and LDL cholesterol more than diets with animal protein (~9% decrease). Serum triacylglycerol levels were significantly reduced from the soy protein diet (-23%). Serum HDL levels were not significantly affected by either diet.

Unfortunately, the results of soy protein intake on renal function were not as positive. Although serum urea nitrogen decreased significantly more following soy protein intake than animal protein intake, the change in GFR and creatinine clearance was similar for both diets. Surprisingly, urine protein excretion remained stable with the animal protein diet, but actually increased significantly on the soy protein diet. However, given previous results, further research is warranted to investigate this soy-protein hypothesis.

James W. Anderson, Jill E. Blake, Jan Turner, and Belinda M. Smith, Effects of Soy Protein on Renal Function and Proteinuria in Patients with Type 2 Diabetes, American Journal of Clinical Nutrition 68(suppl.): 1347S-1353S (December 1998) [Correspondence: JW Anderson, Medical Service, 111C, 2250 Leestown Road, Lexington KY 40511, E-mail: jwandersmd@aol.com]