Daily delivery of dietary nitrogen to the periphery is stable in rats adapted to increased protein intake

Dietary nitrogen was traced in rats adapted to a 50% protein diet and given a meal,containing 1.50 g N-15-labeled protein (HP-50. group). This group w as compared with rats usually consuming a 14% protein diet and fed a meal c ontaining either 0.42 g (AP-14 group) or 1.50 g (AP-50 group) of N-15-label ed protein. In the HP group, the muscle nonprotein nitrogen pool was double d when compared with the AP group. The main adaptation. was the enhancement of dietary nitrogen transferred to urea (2.2 +/- 0.5 vs. 1.3 +/- .1 mmol N /100 g body wt in the HP-50 and AP-50 groups, respectively). All amino acid s reaching the periphery except arginine and the branched-chain amino acids were depressed. Consequently, dietary nitrogen incorporation into muscle p rotein was paradoxically reduced in the HP-50 group, whereas more dietary n itrogen was accumulated in the free nitrogen pool. These results underline the important role played by splanchnic catabolism in adaptation to a high- protein diet, in contrast to muscle tissue. Digestive kinetics and splanchn ic anabolism participate to a lesser extent in the regulation processes.