Dietary salt, bone strength, and mineral content in unloaded rat femurs.

Introduction: Reduced bone mineral and ultimate strength are regular consequences of unloading bone. The aim of this study was to determine if high dietary salt intake would reduce the bone density and strength to a greater extent in rats with unloaded bones compared to ambulatory control rats fed the same dietary calcium and phosphorus.

Methods: Mature male Sprague-Dawley rats were divided into four groups: two exposed to a spaceflight model that unloaded the hind limbs (HU) and two controls (C) with normal ambulation. Half the HU and C rats were fed normal dietary salt (0.26%, NNa) and half high dietary salt (8%, HNa). The calcium (Ca) and phosphorus (P) content of the diets was normal (Ca 0.5% and P 0.6%) in all four groups. After 4 wk of hind limb unloading, the bone mineral content (BMC) of excised femurs was measured by the ash weight and the ultimate torsional strength was determined by a torsional strength test device.

Results: Femoral BMC (mg) was lower in HUNNa than C rats fed normal salt diets. Femurs from HU rats fed normal salt diets showed lower (20-26%) torsional strength (Nmm), compared to all other groups.

Discussion: It appears that high salt diets with normal amounts of calcium and phosphorus may prevent the decrease in bone torsional strength and BMC induced by unloading the femurs in 6-mo-old rats.