Protein Redistribution From Skeletal Muscle to Splanchnic Tissue on Fasting and Refeeding in Young and Older Healthy Individuals

BackgroundDuring aging, a shift of protein metabolism from muscle to splanchnic tissue contributes to increased muscle protein loss after a period of metabolic stress (eg, fasting).ObjectiveTo study the adaptation of protein metabolism in the whole body and tissue (ie, skeletal muscle and splanchnic area) to metabolic stress, such as short-term fasting and refeeding, in aged people.Design and participantsWe studied splanchnic and muscle protein metabolism after 38 hours of fasting and refeeding in 7 young (5 men/2 women, 24.4 ± 2.0 years) and 8 elderly individuals (6 men/2 women, 70.6 ± 3.1 years).MeasurementsWe used intravenous (IV) L-¹³C₆]phenylalanine, IV L-²H₃]leucine, and oral L-¹³C₁]leucine to obtain (1) whole-body protein kinetics, (2) muscle and albumin fractional synthesis rate (FSR, %/d; ¹³C₆-Phe, and ¹³C₁-Leu), and (3) splanchnic extraction during fasting and refeeding (%, ²H₃- and ¹³C₁-Leu).ResultsWhole-body protein breakdown was activated during fasting in young and older individuals (P < .01 vs fasted state). Muscle FSR remained unchanged in both groups and not stimulated by refeeding in either group with either IV ¹³C Phe or oral ¹³C Leu, probably because of high plasma levels of essential amino acids (EAAs) and branched-chain amino acids (BCAAs). Splanchnic extraction of leucine was 42% higher in the elderly individuals (P = .03 vs young) and was associated with an increased albumin synthesis rate in elderly individuals in the fed state (P < .05 vs young).ConclusionSplanchnic protein metabolism is modified by age, but this metabolic change is not associated with a lower synthesis rate of muscle protein, provided high plasma levels of essential EAAs are maintained. Our data also suggest that splanchnic protein synthesis is a metabolic priority during recovery after metabolic stress in healthy elderly persons and that it might be even more affected in polymedicated older individuals having chronic diseases.