How Much Protein Should We Consume?

Abstract & Commentary

Synopsis: A high intake of nondairy protein may accelerate renal function decline in women with mild renal insufficiency.

Source: Knight EL, et al. Ann Intern Med. 2003;138: 460-467.

In individuals with moderate-to-severe renal insufficiency, low protein intake may slow renal function decline. However, the long-term effect of protein intake on renal function in persons with normal renal function or mild insufficiency is unknown.

This study involved 1624 women enrolled in the Nurses’ Health Study who were aged 42-68 years in 1989 and gave blood samples in 1989 and 2000. Ninety-eight percent of the women were white, and 1% were African American. Their protein intake was measured in 1990 and 1994 by using a semi-quantitative food frequency questionnaire. Creatrinine concentration was used to estimate glomerular filtration rate (GFR) and creatinine clearance.

A high protein intake was not significantly associated with change in estimated GFR in women with normal renal function. In women with mild renal insufficiency (defined as an estimated GFR of > 55 mL/min per 1.73m2 but < 80 mL/min per 1.73 m2), increase in protein intake was significantly associated with a decline in renal function. A high intake of nondairy animal protein was associated with a significantly greater decrease in GFR.

High protein intake was not associated with renal function decline in women with normal renal function. However, a high intake of nondairy animal protein may accelerate renal function decline in women with mild renal insufficiency.

Comment by Ralph R. Hall, MD, FACP

There are several limitations to this study. Since participants were not randomly assigned to a specific protein intake and since only 2 estimations of protein intake were taken during the 11 years of the study, the dietary intake may have varied more than estimated. The population studied was predominately white; therefore, the results apply only to this population.

Another significant study that confirms and compliments this study was carried out by Wrome and associates.1 They studied the relationship between dietary protein and microalbuminuria (MA). The study included participants from the third National Health and Nutrition Examination Survey (NHANES III) and included large numbers of young men and women, elderly, black and Mexican Americans. There were 15,779 subjects aged 20-80 years with available measurements of dietary protein (DPI), urinary albumin, and creatinine. The dietary recall used assessed protein intake of animal and plant sources separately. DPI was not associated with an increase in MA in healthy persons or those with isolated hypertension or in diabetics without hypertension. However, in diabetics with hypertension there was an increase in MA in those on high-protein diets.

MA is associated with a significantly increased risk of cardiovascular disease. It is not a cause of cardiovascular disease but serves as a marker for those at increased risk. It is caused by glomerular capillary injury and is probably the result of endothelial disfunction.

As Knight and colleagues note, the potential consequences for the large number of undiagnosed diabetes and hypertensive patients and of patients with mild undiagnosed renal failure who are inclined to embark on a high-protein weight-loss diet are significant.

Dr. Hall is Emeritus Professor of Medicine, University of Missouri-Kansas City School of Medicine.

Reference

1. Wrone EM, et al. Am J Kidney Dis. 2003;41:580-587.