Leucine kinetics after a brief fast and in response to feeding in premature infants

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Abstract
To examine how feeding affects changes in leucine and protein metabolism, leucine kinetics were determined in nine preterm infants (32 ± 2 wk gestation; x̄ ± SD) after a brief fast and again during hourly feedings. Rates of leucine oxidation were similar during the fasting and feeding periods (31 ± 4 vs 37 ± 6 µmol · kg−1 · h−1; x̄ ± SE). The nonoxidative disposal rates of leucine (a reflection of protein synthesis) were also similar during both periods (228 ± 20 vs 205 ± 10 µmol · kg−1 · h−1; x̄ ± SE). In contrast, the rates of leucine release from endogenous protein (an indication of protein breakdown) were significantly reduced by feeding (259 ± 23 vs 185 ± 11 µmol · kg−1 · h−1; x̄ ± SE, P = 0.02). A significant positive correlation was demonstrated between the fasting rate of leucine release from endogenous protein and the degree of suppression produced by feeding (r2 = 0.796, P = 0.001). Conversely, a significant inverse correlation was shown between the nonoxidative disposal rate of leucine during fasting and the increase in response to feeding (r2 = 0.848, P < 0.001). These data suggest that premature infants respond to feeding after a brief fast by suppressing protein breakdown, rather than by increasing protein synthesis, and changes in protein metabolism produced by feeding in premature newborns may be influenced by the prevailing rates of protein breakdown and synthesis during fasting.