Too much protein may cause reduced kidney function

Researchers at Brigham and Women’s Hospital (BWH) have found that high-protein diets may be associated with kidney function decline in women who already have mildly reduced kidney function. On further analysis, the risk was only significant for animal proteins, indicating that the source of protein may be an important factor. Researchers observed no association between high protein intake and decline in kidney function in women with normally functioning kidneys. These findings appear in the March 18 issue of Annals of Internal Medicine.

“The potential impact of protein consumption on renal function has important public health implications given the prevalence of high-protein diets and use of protein supplements,” said Eric C. Knight, BWH researcher. “We found that among women with mildly reduced kidney function – about 25 percent of individuals in our study – a higher-protein diet may lead to accelerated decline in kidney function compared with a lower-protein diet.

“Importantly, however, we also demonstrated that for women with normal renal function, high-protein diets appeared to have no adverse impact on their kidney function.”

Approximately 20 million people suffer from chronic kidney disease and more than 20 million others are at increased risk. If the disease progresses to what is known as end-stage renal disease (ESRD), dialysis or transplantation is required for survival.

The research team analyzed kidney function by measuring the estimated glomerular filtration rate (GFR) – an indication of how effectively the kidney is filtering the blood in order to remove waste from the body. Women with mildly reduced kidney function who consumed diets high in protein experienced the most significant decline in GFR. Compared with women with the lowest protein intake, women who consumed the greatest amounts of protein were more than three times as likely to have a significant decline in kidney function.

The data suggested that the type of protein consumption was also an important variable. A change in GFR was only significant among women consuming large amounts of nondairy, animal proteins.

“Recently, we have witnessed a growing popularity in low-carbohydrate, high-protein diets,” said Knight, also of Harvard Medical School and Massachusetts General Hospital. “As millions of Americans experiment with these diets and protein supplements, caregivers and researchers continue to question what the long-term health outcomes will be. While many questions still remain unanswered, we certainly have some evidence that among women with reduced kidney function, these diets may have adverse effects.”

While the researchers observed that protein consumption had an adverse impact on women with mild kidney problems, the same did not hold true for women with normal kidney function. “There are theoretical reasons to believe high-protein diets may be harmful to the kidney,” Knight said. “However, we observed no adverse effect of higher protein intake in women with normal kidney function.”

It has previously been hypothesized that high-protein diets adversely affect the kidney because of the stress required to process proteins. Increased protein consumption leads to hyperfiltration – a state in which the kidney faces increased pressure in order to filter and remove waste from the body. Over the long term, hyperfiltration may lead to kidney damage.

“The preliminary good news is that for women with normal renal function who want to try a high-protein diet and are worried about the impact of this regimen on their kidney health, it is probably safe,” Knight said. “However, older women, who as a group are more likely to have reduced kidney function, may want to consult a physician to analyze their kidney function before starting a high-protein diet.”

These findings were based on questionnaires and blood samples collected from 1,634 women enrolled in the BWH-based Nurses’ Health Study. The participants, who were 42 to 68 years of age at the onset of the study in 1989, were followed for 11 years.