Leucine metabolism in aging humans: Effect of insulin and substrate availability

To elucidate the relative roles of insulin (I) and amino acid (AA) availability on body protein economy and AA kinetics, we compared whole body leucine kinetic responses, using a 360-min constant infusion of L-[1-¹³C]leucine, during administration of an L-AA solution to six healthy young (21-25 yr) and six healthy old (72-87 yr) men (study 1) to those when the AA solution was given in conjunction with a euglycemic I clamp (study 2). In study 1, serum I increased significantly (P < 0.02) by 4 ± 1 and 4 ± 2 μU/ml in young (Y) and old (O) men, respectively. In study 2, I was raised to 91 ± 7 (Y) and 88 ± 7 (O) μU/ml; the glucose infusion to maintain euglycemia in the Y was significantly greater than in the O (8.0 ± 0.1 vs. 6.8 ± 1.9 mg · kg^-1 · min^-1). Leucine flux and oxidation increased significantly in both age groups during the administration of AA. Estimates of leucine released from protein breakdown declined (P < 0.01) by 18 and 20% in study 1 and 2, respectively, in the young and by 12 and 44%, respectively, in the elderly. Rates of leucine incorporation into protein increased (P < 0.01) similarly in both age groups and in both studies. These findings emphasize the importance of AA availability in the stimulation of protein synthesis and suggest that insulin's major role in vivo is to repress whole body proteolysis. Furthermore, despite evidence of an age-related decline in glucose disposal, the elderly had similar leucine kinetic responses to hyperaminoacidemia.