WEDNESDAY, March 19, 2003 (HealthDayNews) -- If your kidneys are not healthy, a diet high in protein may speed up the damage to your organs, new research claims.
However, if your kidneys are healthy, eating lots of protein does not seem to hurt them, the study also concludes.
"We now have evidence to suggest that, in individuals with kidney disease there is some reason to suspect a high protein intake could be potentially detrimental," says study author Dr. Eric L. Knight, a researcher at Brigham and Women's Hospital and Massachusetts General Hospital in Boston. "Based on this study, I would say in individuals with normal kidney function, we have no reason to believe that a relatively high protein intake has a harmful effect on the kidneys."
The findings appear in the current issue of the Annals of Internal Medicine.
Patients with chronic kidney disease are often put on low-protein diets to decrease the workload of the kidneys and slow progression of the disease, Knight says. After the body uses the protein from food, a waste product called urea is made and excreted by the kidneys. If kidneys aren't working well, the excess urea may build up, so decreasing protein intake is often suggested.
However, the question of whether high protein intake affects those with milder disease or no disease has received less attention, and with high-protein weight-loss diets so popular now, the issue is timely.
"I think we are the first to study the impact of protein intake on kidney function in those with normal kidney function and mild kidney dysfunction," Knight says.
Knight's team evaluated 1,624 women who were part of the large, ongoing Nurses' Health Study and had given blood samples in 1989. One quarter of these women, or 489, had mild kidney problems, as detected by a blood test, and the other 1,135 had healthy kidneys.
Protein intake was measured in 1990 and in 1994 via food questionnaires, and blood samples were taken again in 2000 to test kidney function. Those in the highest category of protein intake "were 3.5 times more likely to have a significant decline in kidney function," Knight says. That was defined as at least a 15 percent decline in functioning. Animal protein in particular was associated with worsening kidney function.
The average weight of the women in the study was 154 pounds, and those in the highest intake category averaged 93 grams of protein a day, Knight says. That's about the amount of protein found in three chicken breasts, each about three ounces.
The Institute of Medicine's Food and Nutrition Board recommends a daily protein intake of .8 grams per kilogram of body weight. For a person who weighs 154 pounds, that translates to 56 grams of protein daily. No more than 35 percent of total daily calories should come from protein, the institute also recommends, with a range of 10 percent to 35 percent.
Susan Roberts, a professor of nutrition at Tufts University who has also researched the topic, says the conclusion of the new study is "probably valid and good" and points out the study is based on large numbers.
Anyone with kidney disease, she says, should keep an eye on protein intake, but not go lower than the amount recommended by the government.
"Those with pre-existing kidney disease should get advice from their doctors before starting a diet high in protein," Knight says. And since mild kidney problems usually are not accompanied by symptoms, people might not be aware their kidneys are ailing, he adds. A blood test can help determine the status of your kidneys.
More information
The National Kidney Foundation has more on the warning signs of kidney disease and special diets for those with kidney disease.
