Low-Carb Diets May Increase Risk of Kidney Stones, Bone Loss

CHICAGO--Low-carbohydrate, high-protein diets may increase the risk of kidney stones and bone loss by causing a reduction in calcium metabolism, according to researchers from the University of Chicago and colleagues at Dallas' University of Texas (UT) Southwestern. The study--which was published in the August issue of the American Journal of Kidney Diseases (40, 2:265-74, 2002) (www.ajkd.org)--involved 10 healthy subjects who were observed for eight weeks.

Subjects consumed their usual diet for two weeks, followed by two weeks on a severely carbohydrate-restricted diet and four weeks on a moderately carbohydrate-restricted diet. During both carbohydrate-restricted diets, subjects exhibited a decrease in urine pH. Urinary citrate levels (high levels of which restrict kidney stone formation) also decreased, dropping from 763 mg/d at baseline to 449 mg/d during the severely restricted diet. Researchers also noted an increase in net acid excretion, which can suppress the function of bone-forming cells and induce bone resorption (loss). Urinary calcium levels also increased significantly, from 160 mg/d at baseline to 258 mg/d during the severely restricted diet and 248 mg/d during the moderately restricted diet.

"This type of diet increases the propensity to develop kidney stones," said Chia-Ying Wang, M.D., a study author, in a story released by UT Southwestern (www.swmed.edu). "On the basis of this study alone, there was an increased risk of developing kidney stones and a possible increase in the risk of bone loss. We already know that osteoporosis is going to be a major issue as the population ages, and if people are going to eat this kind of diet on a long-term basis, it's unknown what the implications would be for your bones."