1. Measurements were made of the nitrogen and energy balances of pigs of 30, 60 and 90 kg given a conventional diet at various daily rates. 2. Body protein synthesis was estimated from the irreversible loss of leucine from the blood following the infusion of [1-14C]leucine, and from the oxidation of the labelled amino acid. 3. Protein synthesis (g/d) increased by 2.17 for each 1 g increase in daily protein accretion and by 1.55 for each 1 g increase in daily protein intake. 4. At 30 kg, pigs close to energy equilibrium synthesized 270 g protein daily compared with 406 g and 512 g when their ration supplied twice and three times their maintenance requirement. 5. There was a close correlation between the daily urinary excretion of urea + ammonia and total amino acid catabolism estimated from the catabolism of leucine, but the latter underestimated the observed excretion by 2.5 g N/d. 6. The results imply that protein turnover accounts for only a proportion of the heat production associated with protein deposition.

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 weighmg 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 "C 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)0'76 per d) were 1.75 for fat, 1.58 for carbohydrate, 1-25 for protein and 1.18 for the basal diet; corresponding intakes of apparently digestible N (ADN; g N/kgWo'6 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 E and increased by the addition of protein to the diet.5. When based on the spec& 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 uf. 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 233KJ/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. In growing pigs (Reeds et al. 1980) and children (Golden et al. 1977) increases in N retention associated with increases in food intake a...

1. The interrelations between protein accretion and whole-body protein turnover were studied by varying the quantity and quality of protein given to growing pigs.2. Diets with 150 or 290g lysine-deficient protein/kg were given in hourly meals, with or without lysine supplementation, to female pigs (mean weight 47 kg).3. After the animals were adapted to the diets, a constant infusion of [14C]urea was given intra-arterially for 30 h, during the last 6 h of which an infusion of [4,S-SH]leucine was also infused at a constant rate. At the same time, yeast-protein labelled with 15N was given in the diet for 50 h.4. The rate of urea synthesis was estimated from the specific radioactivity (SR) of plasma urea. The rate of leucine flux was estimated from the SR of plasma leucine. The irrevocable breakdown of leucine was estimated from the *H-labelling of body water. Total N flux was estimated from the 16N-labelling of urinary urea.5. Addition of lysine to the low-protein diet significantly increased N retention, with a substantial reduction in leucine breakdown, but there was no significant change in the flux of leucine or of total N.6. Increasing the quantity of the unsupplemented protein also increased N retention significantly, with concomitant increases in leucine breakdown and in the fluxes of leucine and of total N. 7. It is concluded that a doubling of protein accretion brought about by the improvement of dietary protein quality is not necessarily associated with an increased rate of whole-body protein turnover.The energy cost of growth, that is, the increase in food energy required to promote a unit increase in growth, greatly exceeds the heat of combustion of the tissue formed. The energy cost of protein accretion by growing 'pigs has been estimated to be 44 kJ/g (Agriculturab Research Council, 1981). The heat of combustion of mixed body proteins is 23.7 kJ/g so that each 1 g protein accretion apparently involves the expenditure of approximately 20 k ! (44 -23.7). The energy expended in protein synthesis, on the other hand, is generally estimated to be 3.5 kJ/g (Millward et al. 1976). In previous work with growing pigs (Reeds et al. 1980) the rate of protein synthesis was found to increase by 2.2 g for each 1 g increase in the rate of protein accretion. From these experiments and others in which single nutrients were varied (Reeds et ai. 1981) we suggested that the energy expended in protein synthesis could account for only part of the energy cost of protein accretion. In those experiments the changes in protein accretion were achieved by large changes in energy intake or protein intake or both. To discover whether increases in protein accretion are invariably linked to changes in whole-body protein synthesis and heat production, we have now examined the effects of doubling the rate of protein accretion with virtually no change in energy or protein intake by increasing the supply of the limiting amino acid in an imbalanced protein.The effects on heat production were described in the previous paper (Fuller et al....

Kinetic aspects of threonine (Thr) metabolism were examined in eight pigs fed hourly with a diet containing either 0.68% (LT group) or 0.81% (HT group) of Thr (wt/wt), corresponding to 10 and 30% Thr excess, respectively, compared with an "ideal" diet. Primary production (PR) and disposal (DR) rates were obtained for Thr, glycine (Gly), and 2-keto-butyrate (KB) after a 12-h continuous infusion of L-[U-14C]-Thr together with [1-13C]Gly and a 6-h continuous infusion of [1-14C]KB. Transfer of Thr into secondary pools was also monitored, and from these the rates of Thr oxidation through the catabolic pathways of L-Thr 3-dehydrogenase (DR(Thr-Gly)) and threonine dehydratase (DR(Thr-KB)) were estimated. For the LT group the results were (mumol.kg-1.h-1) PR(Thr) 314 +/- 3, PR(Gly) 551 +/- 24, PR(KB) 41 +/- 3, DR(Thr-Gly) 22 +/- 2, and DR(Thr-KB) 7 +/- 1. For the HT group they were PR(Thr) 301 +/- 23, PR(Gly) 598 +/- 55, PR(KB) 39 +/- 4, DR(Thr-Gly) 32 +/- 2, and DR(Thr-KB) 8 +/- 1. The increase in Thr intake (14 mumol.kg-1.h-1, P less than 0.01) induced a commensurate increase in the sum of DR(Thr-Gly) and DR(Thr-KB) (14 mumol.kg-1.h-1, P less than 0.001) when liver was used as the precursor pool. This was mainly due to the increased DR(Thr-Gly) (13 mumol.kg-1.h-1, P less than 0.01); the change in DR(Thr-KB) was not statistically significant. By comparison of intracellular-to-plasma ratios of specific activities (or enrichments) for different tissues with each type of infusion, liver was shown to be the major site of production of Gly and KB from Thr. These data suggest that in fed growing pigs a 30% excess of Thr in the diet does not alter the partition of Thr oxidation, since 80% of Thr oxidation occurs through the L-Thr 3-dehydrogenase pathway for both LT and HT groups.

1. Eight pigs with a mean weight of 48 kg were given, at a constant daily rate, diets of low (0.15) or high (0.30) protein content, very deficient in lysine, with or without a supplement of L-lysine (3-7 g/kg).2. Measurements of nitrogen and energy metabolism were made in four successive 14 d periods in a Latinsquare design.3. The rate of protein accretion was substantially increased by increases in both protein and lysine supply, but the rate of heat production was not significantly changed.4. The rate of fat deposition varied inversely with the rate of protein accretion, being reduced by both protein and lysine supplements.5. The relation between heat production and protein accretion (allowing for a constant energy cost of fat deposition) suggested that heat production increased with additional protein accretion less when protein quality was improved than when more protein was given.It was Kielanowski (1965) who first proposed, and who later elaborated (Kielanowski, 1966) the idea that the energy requirement of a growing animal can be considered to be the sum of three components : the energy requirements for maintenance, protein accretion (A) and fat deposition. Using multiple regression, he computed the association of metabolizable energy (ME) with A and fat deposition. The residual quantity was assumed to be the energy required for maintenance. In recent years, experiments in several species have been made to estimate the magnitude of these specific costs. As far as growing pigs are concerned, a survey of experimental evidence (Fowler et al. 1980; Agricultural Research Council, 198 1) suggested that the ME required for A is, on average, 44 kJ/g and for fat deposition 54 kJ/g. By subtracting from these values the heats of combustion of body protein and fat (23.7 and 39.6 kJ/g; Franke & Weniger, 1958) the increases in heat production associated with protein and fat deposition are 20.3 and 14.4 kJ/g respectively. These estimates are based on statistical associations, rather than physiological causation. It is simply to say that when the rate of A is increased, energy expenditure increases in constant proportion : it is not to say that there is necessarily any direct causal link between the two.By contrast, it has been estimated (Millward et al. 1976) that 4 mol ATP are required for each peptide bond formed and probably another 1 mol for additional associated energy expenditures in amino acid transport, RNA synthesis, etc. The synthesis of 5 mol ATP typically requires 400 kJ ME; assuming that the average molecular weight of amino acid residues is 116, this suggests that 3.45 kJ ME are expended for each 1 g protein synthesized.It might be thought that these estimates, 3.45 kJ/g for protein synthesis (S) and 20.3 kJ/g for A , could be reconciled if protein turnover exceeded A by a factor of 6, which indeed it commonly does (Waterlow et al. 1978). However, what is important is not the overall ratio, S: A but the marginal ratio, that is, the change in S associated with a change in A. In a previous paper (Reeds et...