Normal energy expenditure in the infant with presymptomatic cystic fibrosis

To investigate whether a fundamental lesion in energy metabolism is a feature of cystic fibrosis, we measured total energy expenditure using the doubly labeled water technique in 19 infants with presymptomatic cystic fibrosis (0.8 to 6.6 months of age) identified through newborn screening. Total energy expenditure data collected in a cohort of healthy infants by the same method were used for comparison. Energy balance studies were additionally performed in 10 of the infants with cystic fibrosis. Total energy expenditure levels in infants with cystic fibrosis, expressed as either kilocalories per day or kilocalories per kilogram of fat free mass per day, did not differ from control levels. When expressed on the basis of body weight (in kilocalories per kilogram per day), expenditure levels in infants with cystic fibrosis were greater (p < 0.05) than control levels. No differences in expenditure were observed between those infants who were homozygous (n = 10) for the delta F508 mutation and the heterozygous infants (n = 7), regardless of how expenditure was expressed. Assessment of energy balance indicated that infants with cystic fibrosis grow at a normal rate for metabolizable energy intakes similar to those reported for healthy infants. We conclude that there are no differences in energy expenditure between infants with presymptomatic cystic fibrosis and healthy infants, once differences in body composition are taken into account. This finding indicates that the primary cystic fibrosis defect is not an energy-requiring one.