Original Investigations: Pathogenesis and Treatment of Kidney Disease and Hypertension
Effect of low-carbohydrate high-protein diets on acid-base balance, stone-forming propensity, and calcium metabolism,☆☆,

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Abstract

Background: Low-carbohydrate high-protein (LCHP) diets are used commonly for weight reduction. This study explores the relationship between such diets and acid-base balance, kidney-stone risk, and calcium and bone metabolism. Methods: Ten healthy subjects participated in a metabolic study. Subjects initially consumed their usual non-weight-reducing diet, then a severely carbohydrate-restricted induction diet for 2 weeks, followed by a moderately carbohydrate-restricted maintenance diet for 4 weeks. Results: Urine pH decreased from 6.09 (Usual) to 5.56 (Induction; P < 0.01) to 5.67 (Maintenance;P < 0.05). Net acid excretion increased by 56 mEq/d (Induction; P < 0.001) and 51 mEq/d (Maintenance; P < 0.001) from a baseline of 61 mEq/d. Urinary citrate levels decreased from 763 mg/d (3.98 mmol/d) to 449 mg/d (2.34 mmol/d; P < 0.01) to 581 mg/d (3.03 mmol/d; P < 0.05). Urinary saturation of undissociated uric acid increased more than twofold. Urinary calcium levels increased from 160 mg/d (3.99 mmol/d) to 258 mg/d (6.44 mmol/d; P < 0.001) to 248 mg/d (6.19 mmol/d; P < 0.01). This increase in urinary calcium levels was not compensated by a commensurate increase in fractional intestinal calcium absorption. Therefore, estimated calcium balance decreased by 130 mg/d (3.24 mmol/d; P < 0.001) and 90 mg/d (2.25 mmol/d; P < 0.05). Urinary deoxypyridinoline and N-telopeptide levels trended upward, whereas serum osteocalcin concentrations decreased significantly (P < 0.01). Conclusion: Consumption of an LCHP diet for 6 weeks delivers a marked acid load to the kidney, increases the risk for stone formation, decreases estimated calcium balance, and may increase the risk for bone loss. © 2002 by the National Kidney Foundation, Inc.

Section snippets

Experimental subjects

Subjects were recruited by advertisement. Included in the study were men and women with a body mass index (BMI) of 22 kg/m2 or greater and a desire to lose weight. Exclusion criteria were the presence or history of peptic ulcer disease, intestinal strictures, chronic diarrhea, renal calculi, metabolic acidosis or alkalosis, osteoporosis, gout, hyperuricemia, hyperkalemia, hypokalemia, arrhythmias, hypercalcemia, decreased endogenous creatinine clearance (≤0.6 mL/min/kg [0.0167 mL/s/kg]), and

Baseline demography

Eighty volunteers were screened for the study. Ten healthy subjects (seven women, three men; age, 21 to 52 years; mean, 38.4 years) participated in the study. Three subjects were Latin American, and seven subjects were white. At entry, subjects had a mean height of 166 cm (range, 152 to 189 cm), mean weight of 81.4 kg (range, 56.7 to 109.9 kg), and mean BMI of 29.4 kg/cm2 (range, 22.0 to 38.3 kg/cm2).

Composition of metabolic diets

The composition of each subject's three different metabolic diets was estimated by using US

Discussion

The objective of this study is to examine the effects of LCHP weight-reducing diets on acid-base balance, stone-forming propensity, and calcium metabolism in healthy individuals. Our data show that such a diet provides an exaggerated acid load, increasing risks for renal calculi formation and bone loss.

The increased acid load delivered by an LCHP diet was reflected in the increased urinary titratable acidity and urinary ammonium excretion. These diets were associated with a striking increase in

Acknowledgements

No grant support was obtained by any industry. None of the authors have a proprietary interest in any diet evaluated. The authors thank Beverley Adams-Huet, John Poindexter, Faye Britton, William Gitomer, Carolyn Griffith, Alan Stewart, Robert Butsch, and Rebecca Aricheta for assistance during this project.

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    Supported in part by grants no. P01-DK20543 and M01-RR00633USPHS from the US Public Health Service.

    ☆☆

    Address reprint requests to Shalini T. Reddy, MD, The University of Chicago, Department of Medicine, Section of General Internal Medicine, 5841 S Maryland Ave, MC 3051, Chicago, IL 60637. E-mail: [email protected]

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