Data on bilirubin production in breast-fed infants are scarce. The principal objective of our study was to compare bilirubin production rates in breast-fed and formula-fed infants. We measured the end-tidal carbon monoxide level corrected for inhaled CO (ETCOc), an index of total bilirubin production, in breast-fed only (B) or formula-fed only (F), well term infants, using an electrochemical CO detector system. Inclusion criteria: cesarean delivery, birthweight >2500g, gestational age >37 weeks, and absence of any illness in the neonatal period. ETCOc measurements were obtained between 2 and 8 hours of age, serum bilirubin levels at 72 +12 hrs, and weights daily. The mean ETCOc for B infants (1.3 +0.7 ppm, n = 66) did not differ from that of F infants (1.3 + 0.8ppm, n = 210). The mean serum bilirubin level at 72±12 hrs for B infants (8.5 + 3.4 mg/dl) was higher than that of F infants (6.8 + 3.4 mg/dl; p<0.001). Nine (14%) B infants and 12 (6%) F infants had levels >12 mg/dl at 72 + 12 hrs. B infants lost a mean of 8.6% of their birth weight in the first five days of life, significantly more than F infants who lost 4.3% (p<0.0001). These data are consistent with those previously reported for a smaller sample with mixed gestational ages showing no difference in total bilirubin production between bottle-fed and breast-fed infants (Pediatrics 65:1170-2,1982).

Conclusions: 1) The significantly greater weight loss in the B infants may be related to decreased milk intake compared to F infants in the first days of life. 2) The higher serum bilirubin levels seen in the B infants in the present study cannot be explained by higher bilirubin production rates, but more likely are due to increased enterohepatic circulation of bilirubin.