Muscle protein synthesis and balance responsiveness to essential amino acids ingestion in the presence of elevated plasma free fatty acid concentrations.

CONTEXT: Elevated plasma free fatty acid (FFA) concentrations are observed under various clinical circumstances and are associated with impaired glucose disposal in skeletal muscle.
OBJECTIVE: The aim of the study was to determine the effects of elevated plasma FFA concentrations on the response of protein synthesis and balance in muscle after essential amino acids (EAAs) ingestion.
DESIGN: Leg protein kinetics were determined in young healthy individuals before and after the ingestion of EAAs at 10 h after the initiation of either lipid (Liposyn/heparin+EAA) or saline (saline+EAA) infusions.
RESULTS: Plasma insulin responses where higher (P <0.05) in the Liposyn/heparin+EAA group than the saline+EAA group both before (14 +/- 4 vs. 6 +/- 1 microIU . ml(-1)) and after (1038 +/- 257 vs. 280 +/- 87 microIU . ml(-1) . 210 min(-1)) the EAA ingestion. After the EAA ingestion, the rates of both leg phenylalanine disappearance (Rd; nmol . min(-1) . kg lean leg mass(-1)) and muscle proteins fractional synthesis (FSR; % . h(-1)) increased (P <0.05) in both the Liposyn/heparin+EAA and saline+EAA groups, but these changes were not different between the two groups (Rd, 102 +/- 32 vs. 118 +/- 34; FSR, 0.014 +/- 0.005 vs. 0.018 +/- 0.007; P > 0.05). Although the leg phenylalanine rate of appearance (Ra; nmol . min(-1) . kg lean leg mass(-1)) was lower (381 +/- 47 vs. 518 +/- 40) and the balance was greater (-109 +/- 20 vs. -172 +/- 17) in the Liposyn/heparin+EAA group compared to the saline+EAA group before the EAA ingestion (P <0.05), the changes in both of these parameters were not different between groups after the EAA ingestion (P > 0.05).
CONCLUSIONS: Elevated plasma FFA concentrations do not interfere with the response of muscle protein synthesis and balance to a bolus ingestion of EAAs.