Effect of Low-Carbohydrate High-Protein Diets on Acid-Base Balance, Stone-Forming Propensity, and Calcium Metabolism

Abstract

Diets that restrict carbohydrate intake but allow unlimited fat and protein, such as the Atkins' diet, are much in vogue today. Whether a low-carbohydrate, high-protein (LCHP) diet increases the risk of bone loss or kidney stone formation was the question asked in this study of 10 healthy individuals ranging in age from 21 to 52 years. Initially they took their usual diet for 2 weeks and then an Atkins'-type induction diet (with severely restricted carbohydrate) for 2 weeks. An Atkins' maintenance diet then was consumed for 4 weeks in the maintenance phase. In the last week, the participants ate constant metabolic meals. Both the induction and maintenance LCHP diets contained nearly twice as much protein and fat as the subjects' usual diets but significantly less carbohydrate. Intakes of calcium, potassium, magnesium, sodium, and chloride did not differ significantly. Both weight loss and increased blood urea nitrogen levels attested to compliance with the diet. None of the 10 dieters developed clinically apparent metabolic acidosis. Serum sodium levels dropped slightly, but there were no changes in levels of potassium, chloride, calcium, or phosphorus. Urinary pH and citrate fell significantly with the induction and maintenance diets. Urinary ammonium, titratable acidity, and net acid excretion rose nearly 2-fold, as did urinary sulfate levels. Urinary calcium and phosphorus levels increased by about 2.25 mmol and 12.5 mmol/day, respectively, during the LCHP diet periods. Urinary magnesium decreased, but uric acid, oxalate, sodium, and potassium excretion did not change measurably levels measured during the baseline diet. The urinary content of undissociated uric acid doubled. Calcium balance decreased during both LCHP diets. Serum alkaline phosphatase decreased, but the bone-specific fraction did not change. Serum osteocalcin was significantly lower when subjects were on the LCHP diets. Urinary levels of deoxypyridinoline and N-telopeptide tended to increase, but not to a significant degree, and serum levels of calcitriol and parathyroid hormone did not change significantly. Endogenous creatinine clearance rose significantly during both LCH diets. This short-term study suggests that intake of a LCHP weight reduction diet, through delivering an exaggerated acid load, may heighten the risk of both kidney stone formation and bone loss. This possibility should be evaluated in a long-term prospective trial.