Leucine kinetics in fed low-birth-weight infants: importance of splanchnic tissues

Whole body Leu kinetics were determined in fed low-birth-weight (LBW) infants. To assess the importance of first-pass splanchnic extraction of ingested amino acids, two tracers were simultaneously infused by intravenous (L-[1-13C]Leu) and intragastric (L-[5,5,5-2H3]Leu) routes in 13 LBW infants [1,742 +/- 169 (SE) g] fed with protein-enriched human milk (protein intake 3.23 +/- 0.97 g.kg-1.day-1). Splanchnic extraction estimated from plasma [2H3]Leu appearance was 48.2 +/- 15.6% of Leu intake. Total Leu flux, endogenous Leu flux (index of protein catabolism), and nonoxidative Leu disposal (NOLD, index of protein synthesis) were 3.45 +/- 0.57, 1.90 +/- 0.74, and 2.54 +/- 0.62 mumol.kg-1.min-1, respectively. Higher estimates were obtained when using alpha-ketoisocaproate as a precursor pool. There was a wide individual variation of protein intake due to the use of human milk, and, over this range, Leu intake was correlated negatively with endogenous Leu flux (r = 0.88, P less than 0.01) whereas NOLD remained fairly constant. Thus, in LBW infants, 1) splanchnic extraction is two times as high as in adults and might reflect an elevated splanchnic protein turnover and 2) increasing protein intake probably promotes protein gain mainly by inhibiting protein catabolism.