Low-carbohydrate, high-protein diets that restrict potassium-rich fruits and vegetables promote calciuria

Abstract

In a recent issue of Osteoporosis International, Carter et al. reported that subjects adhering to a ketogenic, lowcarbohydrate/high-protein (LCHP) diet for weight loss did not display increased bone turnover markers after 3 months compared to control subjects consuming a typical, unrestricted diet [1]. In this trial, weight loss averaged 6.4 kg versus 1.1 kg in the dieters and controls respectively (p=0.0008), whereas the mean urinary N-telopeptide increased by 1.6 versus 1.9 nmol BCE/mmol creatinine in the dieters and controls, respectively (p=0.86). Although this trial supports the view that high-protein diets do not adversely affect bone [2, 3], the dietary level of alkalizing minerals was not considered. High-protein diets generate excessive amounts of acid due to the obligatory catabolism of sulfur amino acids, and bone may respond by releasing basic salts into the circulation [4]. Hence, if high-protein intakes are not accompanied by an adequate intake of alkalizing minerals, which have a neutralizing effect in vivo, bone health may be adversely affected [4–6]. Here we discuss unpublished data regarding acid/base metabolism from our recent trial comparing ketogenic and nonketogenic LCHP diets [7]. In this trial, 19 adults (body mass index: 34.4T1.0 kg/m) were randomly assigned to a ketogenic LCHP diet (k-LCHP; 60% of energy as fat; 5% energy as carbohydrate) or to a nonketogenic LCHP diet (n-LCHP; 30% of energy as fat; 40% of energy as carbohydrate). The n-LCHP diet was composed mainly of low-fat dairy products, fruits and vegetables, and low-fat meats, whereas the k-LCHP diet consisted primarily of high-fat meats and some vegetables. The diets were similar in protein content (~20 g/mJ), but daily intakes of potassium (1,935 versus 3,535 mg) and calcium (715 versus 1,110 mg) were markedly different for the k-LCHP versus n-LCHP diets. Blood and urine samples were collected at baseline and at week 2. Weight loss, blood ketone concentrations and anion gap values, and urinary calcium concentrations at baseline and week 2 are shown in Table 1. The blood anion gap, the interval between the sum of blood cations and blood anions, is a direct indication of the acidity of blood. For comparison, Table 1 also displays values for urine calcium calculated from predictive equations based on the dietary protein:potassium ratio [8, 9]. These data indicate that n-LCHP diets are associated with lower acid loads and lower rates of calcium excretion as compared to k-LCHP diets. Moreover, anion gap values and urinary calcium were highly correlated (r=0.542, p=0.016). Since the predicted calciuria was similar to actual values, it would appear that potassium-rich foods favorably influence acid/base metabolism and reduce calciuria. Alexy et al. [10] have presented longitudinal evidence that inadequate intakes of alkalizing minerals from fruits and vegetables may adversely impact bone health. Also, based on data from Lewis et al. [11], the calcium-rich Osteoporos Int (2006) 17:1820–1821 DOI 10.1007/s00198-006-0214-y