COMBINED EFFECT OF CHRONIC PROTEIN RESTRICTION AND ACUTE PROTEIN AND LEUCINE SUPPLY ON LEUCINE OXIDATION MEASUREMENTS IN GROWING AND MATURERATS

We hypothesize that the physiological condition of an animal contribut es to the regulation of amino acid oxidation during feeding. In this e xperiment we have investigated different physiological conditions, nam ely chronic protein restriction in both growing and mature rats. Dieta ry supply during the leucine oxidation measurement was restricted to o ne level of protein intake, but two levels of leucine intake. Therefor e rats were conditioned on a high protein (HP) diet (210 g casein/kg f eed) or a low protein (LP) diet (75 g/kg). Overnight fasted rats were offered a 5 g HP meal for 30 min at the onset of a 7.5 h (CO2)-C-14 br eath test. The meal contained either L-[carboxyl-C-14]-leucine or L-[u niversal-C-14]-leucine. In the leucine group meals contained synthetic leucine (70 mg leucine/g protein) to investigate specific leucine ove rsupply. Protein restriction only caused a slightly lower cumulative ( CO2)-C-14 output in growing rats (P < 0.05), but only for the carboxyl data. During the first hour after the meal growing rats showed a slig htly lower (CO2)-C-14 expiration compared to mature rats (P< 0.01), ag ain only for the carboxyl data. The universal data indicated no effect of diet or age. Addition of leucine increased both the hourly and the cumulative (CO2)-C-14 expiration in all four groups (P< 0.001), with no marked difference between carboxyl and universal data. In conclusio n, both protein restriction and age showed only slight changes in post prandial leucine oxidation measurements, but the introduction of synth etic free leucine had a marked negative effect on dietary leucine util isation.