New insights into the methodological issues of the indicator amino acid oxidation method in preterm neonates.

Background: We determined the effect of adaptation to the study diet on oxidation of the indicator amino acid and the required tracer washout time in preterms.
Methods: Subjects received a study diet for 6 d that entailed a 50% reduction in leucine. Tracer studies using enterally infused [(13)C]bicarbonate and [1-(13)C]phenylalanine were performed on days 1, 2, 4, and 6. Breath samples containing (13)CO2 were collected during steady state and measured by infrared spectrometric analysis, and the fraction of (13)CO2 recovery from (13)C oxidation (F(13)CO2) was calculated.
Results: Preterm infants (n = 11, birth weight 1.9 ± 0.1 kg, gestational age 32.6 ± 1.5 wk) received 166 mg/kg/d of leucine. Baseline enrichment changed significantly at day 1 of the study diet. F(13)CO2 did not change significantly between days 2 and 4 but was significantly lower at day 6. The tracer washout time was determined to be 7.5 h using a biphasic regression analysis.
Conclusion: One day of adaptation to a new diet is necessary to adapt to the (13)C enrichment of the study formula before starting infant requirement studies. Adaptation for a period of 5 d results in a protein-sparing response. The minimal time between two studies within the same subject is 7.5 h.