Oral amino acid intake stimulates muscle protein anabolism in the elderly as in the young

The reduction of muscle mass associated with aging might be the result of an altered response of muscle protein anabolism to oral amino acid intake, possibly due to age-related differences in the utilization of alimentary amino acids. Thus, we studied 4 elderly (E, age 68±1 yrs, BMI 25±1 kg/m²) and 4 young (Y, age 31±3 yrs, BMI 24±1 kg/m²) healthy volunteers in the post-absorptive state (basal) and during the oral administration of an amino acid mixture (OAA, E 3.2±0.1, Y 3.1±0.1 mg·kg^-1-min^-1) given as small drinks every 10 minutes for 3 hours. We measured leg muscle protein kinetics by the means of a three compartment model which requires L-[ring-²H₅]phenylalanine infusion, blood samples from femoral artery and vein blood samples, and leg muscle biopsies. In addition, first pass splanchnic phenylalanine extraction was measured by adding L-[ring-¹³C₆]phenylalanine to the drink. The oral amino acid mixture increased (p<0.02 vs. basal) phenylalanine transport into the muscle (E: basal 78±14, OAA 133±35; Y: basal 79±21, OAA 207±47; nmol·min^-1·100ml of leg^-1), and protein synthesis (E: basal 33±12, OAA 61±14; Y: basal 43±16, OAA 62±15; nmol·min^-1·100ml of leg^-1) with no significant differences between the two groups. Phenylalanine net balance (nmol·min^-1·100ml of leg^-1) became positive to the same extent in E (basal -24±8, OAA 10±2) and in Y (basal -19±7, OAA 15±4). However, phenylalanine first pass splanchnic extraction was higher in the elderly (48±5% vs. 30±5%, E vs. Y, p<0.05). In conclusion, oral amino acids stimulate muscle protein synthesis and net muscle protein anabolism in the elderly as in the young despite an age-related increase in first pass splanchnic uptake.