The effect of high salt and high protein intake on calcium metabolism, bone composition and bone resorption in the rat.

The effects of salt (NaCl) supplementation of rat diets (50 g/kg diet), with normal (200 g/kg) or high (500 g/kg) dietary casein content, were studied in 3-week-old male rats over a 3-week period. Weight gain was reduced by dietary salt but was unaffected by dietary casein. Salt-supplemented rats exhibited a two- and three-fold increase in urinary Mg and Ca excretion respectively, irrespective of dietary casein content. Dietary casein had no effect on urinary Ca or Mg. Salt reduced femoral mass but not femoral mass expressed relative to body weight, but neither variable was affected by dietary casein. Femoral Mg and P contents and concentrations were unaffected by dietary salt or casein. While femoral Ca concentration was unaffected by dietary salt, the Ca content was reduced by salt supplementation, irrespective of dietary casein content. Neither the content nor concentration of Ca in femora was affected by dietary casein. Urinary pyridinoline and deoxypyridinoline levels were increased by salt supplementation, irrespective of dietary casein content, but were unaffected by casein. Net Ca absorption was unaffected by dietary salt or casein. In conclusion, these results show that salt supplementation over the short-term increased the rate of bone resorption in rats. This was as a consequence of Na-induced calciuria. On the other hand, a high dietary protein intake had no effect on Ca metabolism, bone composition or bone resorption, nor did it augment the Na-induced calciuria or increased rate of bone resorption.