The deposition of fat and protein and the utilization of energy by growing rats offered diets ad lib. or in controlled amounts by gastric intubation has been investigated. Diets contained 50, 75, 100 or 200 g protein/kg, mainly as casein. 2. Gain of body-weight and protein increased with increasing dietary protein concentration when animals received the same energy intake, although the reverse was true for fat deposition. However, the differences in live-weight gain were almost entirely due to changes in body water. The dry-matter content of the gain in animals given low-protein diets was 770 g/kg compared to 360 g/kg in those given the control diet. 3. Energy retention was unaffected by dietary protein level in groups given the same energy intake by gastric intubation. In Expt 1 daily heat production increased significantly (P smaller than 0-05) with increasing protein level (50, 75 and 200 g protein/kg diet) when energy intake was constant, but in Expt 2 there was no significant effect of protein level (50, 100 and 200 g protein/kg diet). Problems arose in the selection of a suitable basis for comparison of heat production between groups because of the differences in body-weight and body composition. 4. The energy requirement for zero energy balance was approximately 10 percent lower for the low-protein groups than for those given the diet containing 200 g protein/kg when food intake was just above the maintenance level. When the requirement was expressed per unit metabolic body size (W0-75 KG) dietary protein level had no significant effect. The mean values for Expts 1 and 2 were 452 and 456 kJ respectively. 5. The energy cost of weight gain increased as dietary protein level decreased in pairs of groups gaining at the same rate. The extra energy ingested by the animals given the lower protein level was converted to body tissue with an efficiency of at least 0-70. 6. Striking differences were observed in body composition and energy retention of the two pairs of groups used for the comparison of tube-feeding and ad lib. feeding. With the diet containing 50 g protein/kg, tube-fed rats gained significantly more weight (P smaller than 0-01) and more fat, dry matter and energy (P smaller than 0-001) than their ad lib, counterparts given an isoenergetic intake. 7. The results demonstrate that dietary protein level has little or no effecon the utilization of energy by growing rats when the pattern of intake is controlled by gastric intubation.
