[Comparisons of bioavailability of various calcium salts. Utilization incisor dentin in parathyroidectomized rats]

In the present study, we compared the bioavailability of various calcium salts in the diet such as calcium lactate, calcium carbonate and calcium phosphate by examining the effects of these calcium salts on serum calcium concentration and on the mineralization of incisor dentin in parathyroidectomized (PTXed) rats. Each rat was given daily 12 g of a low calcium (0.1% Ca) diet beginning from 3 days before the operation and continuing for 11 days after. Then the diet was changed to those containing various calcium salts (1.5% Ca). The serum calcium and phosphorous concentrations were examined every three days, just before feeding time and 6 hours after. The degree and site of the mineralization of incisor dentin were examined histologically by a time marking method using lead acetate. In the PTXed rats maintained on a low calcium diet, the serum calcium concentration decreased to about 4.5 mg/dl and the mineralization of incisor dentin was inhibited. On the 11th day, just before feeding on diets containing various calcium salts, the serum calcium concentrations were very similar for each group, but at 6 hours after feeding, concentration was 7.6 mg/dl in the calcium lactate group, 6.2 mg/dl in the calcium carbonate group and 4.8 mg/dl in the calcium phosphate group. On the 17th day following administration of the high calcium diets, the serum calcium concentrations were 9.8 mg/dl in the calcium lactate group, 8.4 mg/dl in the calcium carbonate group and 4.4 mg/dl in the calcium phosphate group. Mineralization of the incisor dentin was best in the calcium lactate group, moderate in the calcium carbonate group and poor in the calcium phosphate group. We also examined the effects of dietary phosphorus contents on serum calcium concentrations and on the mineralization of incisor dentin using PTXed rats. On the 17th day following the administration of diets (1.5% Ca, with calcium lactate) containing various amounts of phosphorus, the serum calcium concentrations in the calcium lactate group were 8.8 mg/dl (0.4% P), 6.9 mg/dl (0.8% P) and 4.2 mg/dl (1.6% P) respectively. Mineralization of incisor dentin was also inhibited in rats fed a high phosphorus diet. These results suggest that in PTXed rats, absorption of calcium from the intestines and mineralization of the incisor dentin is best by the administration of calcium lactate, moderate by calcium carbonate and poor by calcium phosphate, and that phosphorus in the diet inhibits calcium absorption from the intestines.