Osteoporosis in Aging Rats Induced by High Phosphorus Diets

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Abstract
Bone resorption was monitored in 45Ca-labeled rats fed different concentrations of phosphorus in the diet from 8 to 14 months of age. The diets contained 0.6% Ca and 0.3%, 0.6%, 1.2% or 1.8% P. The cumulative excretion of 45Ca was 16% greater over the 6-month period in the 0.6% P group than in the 0.3% P group, and was 37% greater in the 1.2% P group. Urinary calcium was depressed by about 50% in animals which consumed more than 0.3% P in the diet. The dry fat-free weight, ash, calcium and phosphorus content of the femurs, lumbar vertebrae and mandibles were significantly lower (P < 0.05 or < 0.01) in the animals fed the 1.2% P or 1.8% P diets than in those fed the diets lower in phosphorus, and these parameters tended to be lower in the 0.6% P group than in the 0.3% P group. Urinary hydroxyproline excretion was elevated when the diet contained 1.8% P. The intestinal absorption of calcium was unaffected by the phosphorus content of the diet. These findings confirm the results of previous experiments in this series which have indicated that the level of dietary phosphorus intake is a significant factor affecting the development of osteoporosis in aging rats and mice. The accelerating effect of excess dietary P on bone resorption is attributed to a secondary hyperparathyroidism induced by a slight depression of serum calcium concentration associated with increased circulating phosphate.