1. An experiment was carried out on growing male rats to study the effects of a phosphorus deficiency on voluntary energy intake, estimated by the separate-feeding technique, as well as the consequences on growth and metabolic utilization of energy, protein and minerals. After a preliminary period of P deprivation, three groups of twelve animals were fed separately and simultaneously two dietary rations. A protein ration provided 1.2 g protein/d to which was added one of three levels of P in the form of monosodium phosphate: a normal level of 35.4 mg/d (treatment A), half the normal level (19.1 mg/d, treatment B) or one-quarter the normal level (9.6 mg/d, treatment C). Another protein-free ration was fed ad lib.
2. It was only when the level of P represented one quarter the normal level that a significant decrease in growth rate was noticed, accompanied by a definite decrease in daily energy consumption. The food conversion ratio (g dry matter intake/g body-weight gain) increased whereas the protein efficiency ratio (g body-weight gain/g protein intake) was lower. The nitrogen and energy retentions changed in the same way: both dectreased with treatment C compared to treatment A and B. Independently of the protein supply, the voluntary energy intake was closely related to the intensity of protein retention, which depended on the dietary level of P. The daily retention of P only slightly decreased at the lowest ingestion level (9.6 mg/d in treatment C), compared to that of the higher levels. On the other hand, the amount of calcium retained regularly decreased with the decreasing supply of P. The result of this was a progressive reduction in retained Ca:P as the level of P decreased.
3. From these results it appeared that the primary effect of a P deficiency in the growing rat was a decrease in bone mineralization. At a more advanced stage, the tissue P levels were affected and the resulting metabolic alterations reduced protein deposition and consequently the voluntary energy intake. The level of energy consumption, in separate-feeding, is more generally dependent on the level of protein deposition allowed by the limiting factor for growth, either protein, minerals or vitamins.