1. The effect of dietary protein and energy restriction on heat production and growth costs has been examined in rats fed on a marginal (MP) or high (HP) protein diet, containing 9.2 % or 22 % respectively of the gross energy content as casein. Diets were given either ad fib. or at approximately 25, 50 or 75 % of the ad lib. intake.
2. Heat production (kJ/kg body-weight (W)0.75 per d) was increased by 23% in rats fed on the MP diet ad Lib., as compared with their HP controls (P < 0.01).
3. Factorial analysis of the data showed that the overall cost of energy deposition (kJ/kJ; Ee) was elevated on the MP diet (MP 1.7, HP 1.28; P < 0.001). Maintenance requirements (kJ/kg W0.75 per d) for zero energy balance were unchanged (MP 562, HP 573).
The partial energy cost of protein deposition (Ep) varied with dietary manipulation. If the partial energy cost of fat deposition (Ef) was assumed constant at 1.25 kJ/kJ, and maintenance requirements were assumed to vary with metabolic body size (W0.75), Ep was elevated on the MP diet. On both diets, Ep was reduced at low energy intakes.
5. The significance of these results is discussed in the context of current approaches to the analysis and interpretation of findings describing dietary induced changes in the rate of heat production.