We examined the response of net muscle protein synthesis to ingestion of amino acids after a bout of resistance exercise. A primed, constant infusion of L-[ring-²H₅]phenylalanine was used to measure net muscle protein balance 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 solution (PLA). Arterial amino acid concentrations increased ~150-640% above baseline during ingestion of MAA and EAA. Net muscle protein balance was significantly increased from negative during PLA ingestion (50 ± 23 nmol · min¹ · 100 ml leg volume¹) to positive during MAA ingestion (17 ± 13 nmol · min¹ · 100 ml leg volume¹) and EAA (29 ± 14 nmol · min¹ · 100 ml leg volume¹; 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 synthesis after heavy resistance exercise in humans similar to that seen when the amino acids were infused.

muscle protein synthesis; muscle protein breakdown; resistance exercise; stable isotopes

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