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 CO 2 were collected during steady state and measured by infrared spectrometric analysis, and the fraction of 13 CO 2 recovery from 13 C oxidation (F 13 CO 2) 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 CO 2 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. © 2013 International Pediatric Research Foundation, Inc.