Urological Survey - Stone Disease

International Braz J Urol
Official Journal of the Brazilian Society of Urology

Vol. 28 (6): 571-572, November - December, 2002
 
Effect of low-carbohydrate high-protein diets on acid-base balance, stone-forming propensity, and calcium metabolism
     
MARGARET S. PEARLE
University of Texas Southwestern Med Ctr Dallas, Texas, USA
   
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. Editorial Comment The popularity of low carbohydrate-high protein diets prompted the authors to explore the potential effect of these diets on stone-forming risk and calcium balance. They enrolled 10 healthy volunteers to participate in a 3-phase trial mimicking the 2 phases of the Atkins’ diet, including induction (2 weeks) and maintenance (4 weeks), preceded by a baseline usual diet phase (2 weeks). During the last week of each phase of study, the subjects consumed constant metabolic diets corresponding to the appropriate phase of the Atkins’diet, at home for 3 days and as in inpatient in the General Clinical Research Unit for the last 4 days. On the last 2 days of each phase, two 24-hour urine samples were collected for stone risk factors, and blood was collected for serum electrolytes, markers of bone turnover, PTH and vitamin D. Additionally, fractional intestinal calcium absorption was measured. Urine pH decreased significantly during the diet, from 6.09 to 5.56 to 5.67, and net acid excretion increased by 56 mEq/d and 51 mEq/d from baseline during the 2 study phases, respectively. Accordingly, urinary citrate decreased from 763 mg/d to 449 mg/d and to 581 mg/d during the 3 phases. Urinary calcium increased significantly from 160 mg/d to 258 mg/d to 248 mg/d, respectively, despite no change in fractional intestinal calcium absorption. Consequently, estimated calcium balance decreased by 130 mg/d and 90 mg/d from baseline during the 2 study phases, respectively. Low carbohydrate-high protein diets have enjoyed increasing popularity in recent years, because of the effectiveness of the diet in weight reduction. However, the stone-forming propensity of a high protein diet has been well established in the literature based on the increased acid load, which results in an increase in urinary calcium and uric acid and a decrease in urinary citrate and pH. Likewise high protein consumption has been associated with a negative calcium balance and bone loss. This carefully executed study clearly demonstrates that a low carbohydrate high protein diet confers a marked acid load that increases stone risk and decreases calcium balance, potentially threatening bone health. Clearly, further study with a long-term trial is necessary; however, stone formers, and those with compromised bone mineral density, should be cautioned before embarking on low-carbohydrate, high-protein weight reduction plans. The authors additionally plan to investigate whether alkali therapy can counter the negative effects of the acid load, potentially reducing the risk of the diet.
 

Key Words: Urological Survey
Am J Kidney Dis. 2002; 40:265-74

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