Self-regulation of phosphate intake by growing rats

Groups of growing rats (100-150 g) were offered a choice of two diets, one containing a deficient concentration of phosphorus (0.1%) and the other containing 0.3, 0.6, 1.2 or 1.8% phosphorus. All diets contained 0.6% Ca and were isocaloric. Except for the groups that were offered the 0.1 and 0.3% phosphorus diets, all the animals selected mixtures of diets containing nearly identical phosphorus contents (0.33-0.36%). The group offered the two diets lowest in phosphorus selected 83% of their food from the higher (0.3%) phosphorus diet to obtain a mixture containing 0.25% phosphorus. Irrespective of the phosphorus content of the diets available, all groups ate similar amounts of food, made similar weight gains and maintained normal plasma levels of calcium and phosphorus. The experiment demonstrates the existence of a feedback mechanism by which growing rats regulate their phosphorus intake within narrow limits when allowed to self-select among diets of markedly different phosphorus contents. It is postulated that feedback regulation of phosphorus intake is mediated by changes in plasma calcium homeostasis.