The effect of a high-protein, high-sodium diet on calcium and bone metabolism in postmenopausal women and its interaction with vitamin D receptor genotype

The influence of a high-Na, high-protein (calciuric) diet on Ca and bone metabolism was investigated in postmenopausal women (aged 50-67 years) who were stratified by vitamin D receptor (VDR) genotype. In a crossover trial, twenty-four women were randomly assigned to a diet high in protein (90g/d) and Na (180mmol/d) or a diet adequate in protein (70g/d) and low in Na (65 mmol/d) for 4 weeks, followed by crossover to the alternative dietary regimen for a further 4 weeks. Dietary Ca intake was maintained at usual intakes (about 20 mmol (800mg)/d). Urinary Na, K, Ca, N and type I collagen cross-linked N-telopeptide (NTx; a marker of bone resorption), plasma parathyroid hormone (PTH), serum 25-hydroxycholecalciferol (25(OH)D₃), 1,25-dihydroxycholecalciferol (1,25(OH)₂D₃), osteocalcin and bone-specific alkaline phosphatase (B-Alkphase) were measured in 24 h urine samples and fasting blood samples collected at the end of each dietary period. The calciuric diet significantly (P<0.05) increased mean urinary Na, N, K, Ca and NTx (by 19%) compared with the basal diet, but had no effect on circulating 25(OH)D₃, 1,25(OH)₂D₃, PTH, osteocalcin or B-Alkphase in the total group (n 24). There were no differences in serum markers or urinary minerals between the basal and calciuric diet in either VDR genotype groups. While the calciuric diet significantly increased urinary NTx (by 25.6%, P<0.01) in the f+ VDR group (n 10; carrying one or more (f) Fok I alleles), it had no effect in the f - VDR group (n 14; not carrying any Fok I alleles). It is concluded that the Na- and protein-induced urinary Ca loss is compensated for by increased bone resorption and that this response may be influenced by VDR genotype.