Low-carbohydrate, high-protein diets increase risk of kidney stones and may raise bone-loss risk


August 1, 2002

"Popular low-carbohydrate, high-protein diets may result in rapid weight loss, but they also appear to pose serious health problems, including increased risk of kidney stones and bone loss," report researchers from the University of Chicago and the University of Texas Southwestern in the August issue of the American Journal of Kidney Diseases.

"High protein, low carbohydrate diets clearly produced changes that substantially increase the risk of kidney stone formation if continued over time. Our study was too brief to show diet induced osteoporosis, but our data suggests this may be another potential risk," said Shalini Reddy, M.D., assistant professor of medicine at the University of Chicago and lead author of the study.

In this study, 10 healthy subjects ate a regular diet for two weeks. They followed that with two weeks on a highly restrictive diet that included some vegetables but no fruits and fewer than 20 grams of carbohydrates. Participants then ate a slightly less-restrictive diet for the final four weeks.

A diet heavy on animal proteins and light on carbohydrates does increase fat metabolism--which can increase the amount of acid in the blood. The researchers found that acid excretion--a marker for the acid load in the blood--increased as much as 90 percent while subjects were on diets that severely restricted carbohydrates. They also found that calcium absorption was unchanged but calcium excretion increased.

The diet produced changes in urine chemistry--higher levels of uric acid and calcium--that enhance the propensity to form stones. The increased acid load in the blood may also suppress the function of cells that make new bone and stimulate the cells that break down bone, suggesting that much of the calcium being excreted was leached from bone.

"This short-term metabolic study stresses that a low-carbohydrate high protein diet may enhance the risk for stone formation and bone loss," conclude the authors. Patients who pursue weight loss, they suggest, "should be made aware of a potential increase in risk for kidney stone formation and and unknown long-term risk to bone health." The study was funded in part by the United States Public Health Service.

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