High protein diets and kidney function.

High protein diets are popular once again, with supporters claiming that they promote weight loss. Two recent studies suggest that, at least for some people, high dietary protein can have harmful effects on kidney function. In the first study, researchers from Harvard University looked at protein intakes of more than 1,600 older women. They also evaluated the women's kidney function using a measurement called the glomerular filtration rate, or GFR. Dietary protein had no significant effect on kidney function in women who had normal kidney function at the start of this study. However, in women with mild impairment of kidney function, a higher total protein intake (median of 92 grams per day, compared to 61 grams per day) was associated with a faster decline in kidney function. This was especially true for protein from meat, fish, poultry, and eggs. These results certainly suggest that women with some impairment of kidney function should avoid high protein diets and that there may be some advantage to use of plant proteins as compared to animal proteins.

The second study looked at both men and women, aged 20 to 80 years. This time microalbuminuria was measured. Microalbuminuria refers to a small increase in albumin, a protein, in the urine and indicates a problem with kidney function. Microalbuminuria is also associated with increased risk of stroke and heart attacks. In people without high blood pressure or diabetes, dietary protein was not associated with microalbuminuria. However, in people with both high blood pressure and diabetes, high levels of dietary protein (average intake of 111 grams per day, or about 24 percent of calories) increased the risk of having microalbuminuria. Unfortunately, results were not separated for animal protein and plant protein, so we don't know if the source of the protein would have any effect on the findings of this study. These results do suggest that people with both high blood pressure and diabetes should consider means other than high protein diets for weight loss.

Knight EL, Stampfer MJ, Hankinson SE, et al. 2003. The impact of protein intake on renal function decline in women with normal renal function or mild insufficiency. Ann Intern Med 138:460-7.

Wrone EM, Camethon MR, Palaniappan L, Fortmann SP. 2003. Association of dietary protein intake and microalbuminuria in healthy adults: Third National Health and Nutrition Examination Survey. Am J Kidney Dis 41:580-7.