The purpose of this study was to determine muscle protein turnover and fractional synthetic rates during endurance exercise and recovery with carbohydrate (CHO) or CHO plus protein (CHO+P) ingestion. On two occasions, eight healthy males were studied at rest and during 3 h cycling and 3 h recovery. Leg muscle protein turnover was determined from stable isotope infusion (L-[ring-13C6]phenylalanine), arterial femoral-venous blood sampling and plasma flow measurements. Fractional synthetic rates (FSR) of the vastus lateralis were determined from the incorporation of L-[ring-13C6]phenylalanine into bound protein. Leg phenylalanine release, which is indicative of muscle protein breakdown, was ∼twofold higher during exercise than at rest (p<0.05), and was not different between CHO and CHO+P (p>0.05). CHO+P ingestion increased leg phenylalanine uptake during exercise (p<0.05) and increased muscle intracellular phenylalanine concentration (p<0.05). However, FSR were not different between CHO and CHO+P (0.029±0.004 and 0.030±0.003%/h respectively, p=0.88). During the recovery period, FSR were 0.070±0.003 and 0.105±0.013%/h for CHO and CHO+P, respectively (p<0.01). Furthermore, ingesting CHO+P during recovery resulted in a net phenylalanine uptake, compared with a net phenylalanine release in the CHO trial. The authors conclude: (1) that endurance exercise increases muscle protein breakdown, (2) protein ingestion does not reduce muscle protein breakdown or increase muscle protein synthesis during exercise, (3) protein ingestion augments post-exercise muscle protein synthesis and (4) protein ingestion is essential to achieve positive muscle protein balance during recovery from endurance exercise.
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