Co-dependence of calcium and phosphorus for growth and bone development under conditions of varying deficiency

The purpose of this study was to quantify the effect of variation in calcium intake, with and without supplemental phosphorus, on growth and bone development in growing animals under conditions of varying calcium and phosphorus deficiency. Nine groups of weanling male rats were fed a diet nutritionally complete, except for calcium and phosphorus, for 28 days. This diet provided nine levels of varying calcium and phosphorus repletion, using either calcium carbonate, dicalcium phosphate, or tricalcium phosphate. Body weights and diet consumption were measured throughout the test period. At term, the femurs from each animal were weighed, measured for tensile strength, bone mineral content (BMC), and bone density, and analyzed for ash, calcium, phosphorus, and histology. As expected, at equivalent levels of calcium supplementation, the two phosphorus-containing salts promoted significantly greater improvement in all the bone variables measured, as well as greater body weight gain and diet and calcium utilization, compared to animals supplemented with calcium only. Histomorphometric analysis confirmed the results of the mineral analysis and showed the structural impact of the inadequate mineral intake. The mean values for ash weight, BMC, and tensile strength in the nine diet groups were well fit (R2 values ranging from 0.93 to 0.99) by multivariate models incorporating only the diet content values for calcium and phosphorus. In these models, the value for the phosphorus coefficient was three to sixfold larger than that for the calcium term, indicating a substantially greater effect of varying phosphorus intake than of varying calcium. These results demonstrate both the co-dependence of calcium and phosphorus in bone development and the importance of providing both minerals to support soft tissue and bone growth.