1. The relationships between the intakes of protein and of non-protein energy (NPE), nitrogcn retention and body protein synthesis have been studied in female pigs weighing 30 and 35 kg.

2. Four animals were assigned to three regimens and given a conventional (basal) diet supplemented with fat, carbohydrate or protein. After 1 week, measurements of N excretion in urine and faeces (7 d collection) and gaseous exchange (3–4 d) were made. At the end of the balance period a solution of [l-14C]leucine was infused at a constant rate. Body protein synthesis was then calculated as the difference between the apparent irreversible loss of blood leucine and the loss of 14C in expired air.

The animals were then offered the basal diet without supplement for 10 d and the measurements of N retention, energy retention and protein synthesis were repeated.

3. The intakes of metabolizable energy (ME; MJ/kg body-weight (W)0.75per d) were 1.75 for fat, 1.58 for carbohydrate, 1–25for protein and 1.18 for the basal diet; corresponding intakes of apparently digestible N (ADN; g N/kgW0.75 per d) were 2.30,2.31,4.35 and 2–17. Daily N retention, which during the period of basal feeding was 13.6 g was increased by between 3.4 and 7.2 g by the supplements. Daily fat deposition was also increased in the animals that received the diets supplemented with carbohydrate and fat.

4. The rate of leucine catabolism was significantly reduced in the animals receiving the diets that were supplemented with W Eand increased by the addition of protein to the diet.

5. When based on the specfic radioactivity of blood leucine both the synthesis and breakdown of body protein (per unit metabolic body-weight) were increased by 30% in the animals receiving the high-protein diet but the increases in protein synthesis associated with the addition of carbohydrate (+14%) and fat (+12%) were much less marked. Consideration of these results together with previous observations (Reeds et al. 1980) suggested that body protein synthesis(g N/d) increased by 0.88 for each g increase in daily ADN and by 0.93 for each MJ increase in daily ME intake.

6. Comparison of the results obtained with the animals given high-carbohydrate diets and those given high-protein diets suggested an increase in heat production of 14 KJ/g of additional fat deposition. A similar comparison of animals receiving the high-protein and basal diets suggested a heat increment of 23.5KJ/g additional protein deposition. The changes in heat production and protein synthesis in the animals given the protein supplement were compatible with a heat increment of 5.3 KJ/g additional protein synthesized. Because of the large proportion of heat production associated with the deposition of fat this could not be confirmed with either of the other supplements, but it is possible that the energy cost of protein accretion varies with the relative proportions of protein and NPE in the diet.