Postexercise net protein synthesis in human muscle from orally administered amino acids

We examined the response of net muscle protein synthesis to ingestion of am ino acids after a bout of resistance exercise. A primed, constant infusion of L-[ring-H-2(5)]phenylalanine was used to measure net muscle protein bala nce in three male and three female volunteers on three occasions. Subjects consumed in random order 1 liter of 1) a mixed amino acid (40 g) solution ( MAA), 2) an essential amino acid (40 g) solution (EAA), and 3) a placebo so lution (PLA). Arterial amino acid concentrations increased similar to 150-6 40% above baseline during ingestion of MAA and EAA. Net muscle protein bala nce was significantly increased from negative during PLA ingestion (-50 +/- 23 nmol.min(-1).100 ml leg volume(-1)) to positive during MAA ingestion (1 7 +/- 13 nmol.min(-1).100 ml leg volume(-1)) and EAA (29 +/- 14 nmol.min-(1 ).100 ml leg volume(-1); P < 0.05). Because net balance was similar for MAA and EAA, it does not appear necessary to include nonessential amino acids in a formulation designed to elicit an anabolic response from muscle after exercise. We concluded that ingestion of oral essential amino acids results in a change from net muscle protein degradation to net muscle protein synt hesis after heavy resistance exercise in humans similar to that seen when t he amino acids were infused.