Leucine metabolism in human newborns.

The present study was designed to 1) determine whether a relationship exists between newborn birth weight and leucine metabolism and 2) compare leucine and energy metabolism in a period of rapid growth and development (i.e., newborn) with a constant nongrowth period (i.e., adult). Leucine kinetics and energy expenditure were measured in the postabsorptive state in 12 normal full-term newborns in early neonatal life and in 11 normal adults using a primed constant L-[1-13C]leucine infusion combined with respiratory calorimetry. A significant positive correlation between newborn birth weight and leucine flux was observed. Leucine flux per kilogram body weight was significantly greater in newborns compared with adults, as was leucine oxidation and energy expenditure. When normalized to metabolic body weight (W0.75), newborns and adults had similar rates of energy expenditure, leucine flux, and oxidation. The allometric exponent relating leucine flux to body weight (0.80) was nearly equivalent to that describing the energy expenditure-to-body weight relationship (0.79) and both were similar to the 0.75 metabolic weight exponent. These data suggest the following. 1) A relationship exists between newborn birth weight and protein metabolism, as reflected by the correlation between leucine flux when expressed as micromoles per kilogram per hour and birth weight. It is speculated that this relationship may be the result of differences in previous protein and energy supplies. 2) The high rate of leucine flux measured in newborns probably reflects the rapid remodeling of protein that occurs in this period of development, even during fasting.(ABSTRACT TRUNCATED AT 250 WORDS)