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Urea synthesis and leucine turnover in growing pigs: changes during 2 d following the addition of carbohydrate or fat to the diet

1. Studies have been made of the time-sequence of protein metabolic and hormonal changes following an abrupt increase in carbohydrate or fat intake. [3H]leucine and [14C]urea were infused for 72 h, via the aorta, into fourteen female pigs (30–38 kg body-weight). At 24 h after the start of the infusion their feed was either changed to one of two isonitrogenous diets containing additional starch (group BS, five animals) or fat (group BF, five animals), or remained unaltered (group BB, four animals). The distribution space of urea was measured by the dilution of a single dose of [14C]urea given both 48 h before and 48 h after the change in diet. The changes in the concentration and specific radioactivity of blood leucine were used to infer changes in protein turnover and those of plasma urea to measure total amino acid catabolism. The concentrations of blood glucose and plasma insulin and cortisol were also measured at approximately two-hourly intervals for the 48 h period following the change in diet. 2. Within 4 h of either change in diet blood leucine concentration was lowered and the leucine specific radioactivity was raised above that in group BB, but after 24 h both the concentration and specific radioactivity of leucine returned to values similar to those in group BB. Eventually the blood leucine specific radioactivity was slightly but not significantly reduced below that of group BB. 3. The addition of starch to the diet significantly reduced the synthesis and concentration of urea within 4 h but, although the addition of fat to the diet eventually lowered the urea concentration and synthesis, both changes were delayed for 18–24 h. 4. In group BS plasma glucose and insulin rose after the addition of starch, but after 24–36 h both returned to values that were the same as those in the animals that received the same diet throughout (group BB). The addition of fat to the diet altered neither blood glucose nor plasma insulin concentrations. The addition of either carbohydrate or fat to the diet eventually reduced pIasma cortisol concentrations, but the change did not occur until 24 h after the change in diet. 5. The results suggest that alterations in non-protein energy supply exert their immediate effect on the degradation of whole-body protein. They do not exclude the possibility that these early changes may reflect opposing changes at different sites. The results also suggest that the rate of urea synthesis may be controlled by the balance between the concentrations of insulin and cortisol, but that under the conditions of these experiments there was little relation between these hormones and the turnover of body protein as measured by the turnover of blood leucine.