SPLANCHNIC AND WHOLE-BODY LEUCINE KINETICS IN YOUNG AND ELDERLY MEN

Whole-body and splanchnic protein metabolism were determined in six yo ung (mean age: 22.7 y) and six old (68.2 y) men before and during a st andardized meal (41.8 kJ/kg) containing 15.6% protein, by using a comb ination of intravenous ([C-13]leucine) and oral ([H-2(3)]leucine) trac ers. In the postabsorptive state, leucine flux and oxidation were simi lar in both groups when corrected for lean body mass ((x) over bar +/- SEM: 1.80 +/- 0.09 compared with 1.79 +/- 0.07 mu mol . kg(-1). min(- 1) and 0.55 +/- 0.02 compared with 0.49 +/- 0.04 mu mol . kg(-1). min( -1) for young and old men, respectively, NS). The pattern of response to the meal was also similar in young and old men: increased flux and oxidation, decreased protein breakdown, and unchanged protein synthesi s. Splanchnic extraction of dietary leucine was twice as high in elder ly men (50 +/- 11% compared with 23 +/- 2%, P < 0.05), was inversely r elated to plasma leucine concentration (r = -0.771, P < 0.01), and was positively related to body mass index (r = 0.861, P < 0.001). In conc lusion, in elderly men there is higher leucine extraction by the gut, liver, or both during feeding, which could lead to a lower peripheral availability of dietary leucine.