Published online by Cambridge University Press: 09 March 2007
Effects of acute (meal) and chronic (diet) level of protein supply on metabolic leucine utilization were investigated in growing (10 weeks) and mature (> 1 year) rats. Rats were conditioned on a high-protein (HP) diet (210 g casein/kg feed) or a low-protein(LP) diet (75 g casein/kg feed) from 7 weeks of age. Overnight-fasted rats were offered a HP or LP meal during a 8 h 14CO2 breath test with a constant infusion of either L-[l-14C]leucine (carboxyl, CL) or L-[U-14C]leucine (universal, UL). Before the meal 14CO2 output was lower for overnight-fasted rats fed on LP than on HP (P < 0·001), and also lower for growing than for mature rats (P < 0·001). Meal ingestion resulted in a rapid increase in 14CO2 output. From 2 h after the start of the meal the effect of acute protein supply on 14CO2 output was significant (P < 0·001), while the effect of chronic protein supply disappeared for CL. After the meal 14CO2 output was transiently lower for growing than for mature rats (P < 0·05), especially after the LP meal. The difference in 14CO 2 output between CL and UL increased transiently after the meal, indicating an increase in decarboxylation relative to total oxidation of leucine. In conclusion: (1) metabolic leucine utilization after overnight fasting depends on the level of chronic protein supply and stage of development of the animal, (2) metabolic leucine utilization after feeding depends primarily on the level of acute protein supply, (3) the transient increase in non-protein label retention suggests a temporal oversupply ofleucine relative to the actual metabolic state.