RESUMO
The effects of intravenously administered amino acids and of varying amounts of energy on metabolic rate were studied and potential mechanisms examined in 19 healthy 4- to 6-day-old preterm (30 to 32 weeks gestation) infants. The infants were randomized to three groups. Group 1 (n = 6) received nonprotein energy 38 kcal/kg/d; group 2 (n = 5), 64 kcal/kg/d; and group 3 (n = 8), 64 kcal/kg/d plus 1 to 2 g/kg/d crystalline amino acids. Thirty-six hours after beginning the infusion, oxygen consumption (VO2) was measured by indirect calorimetry for 5 to 6 hours. Simultaneously, urine was collected for urinary norepinephrine excretion, which was determined using liquid chromatography with electrochemical detection. Serum thyroxine (T4) and triiodothyronine (T3) concentrations were determined by radioimmunoassay. Group 1 had lower VO2 and urinary norepinephrine excretion than did groups 2 and 3, which did not differ. T4 and T3 were not different among the three groups. The demonstrated simultaneous changes in VO2 and norepinephrine excretion with varying energy intakes independent of age supports energy intake as a modulator of the sympathetic nervous system, which in turn controls metabolic rate. Moderate amounts of intravenously administered amino acids do not appear to play an active role in this process; nor do they alter T3 and T4 valves. When VO2 increased with increasing energy intake, T3 and T4 were unaffected, supporting a passive role for thyroid hormones in diet-induced thermogenesis.