Background It is not known whether the effort expended by preterm infants for feeding by bottle and breast is different. We hypothesized that resting energy expenditure (REE) for feeding by both sources is similar.

Design A randomized study of 19 preterm >30 weeks thriving and thermally stable infants who were fed 160 mL/kg/day of their mothers’ expressed breast milk by bottle and by direct breastfeeding. REE was measured for 20 minutes after feeding. Breast milk quantity was evaluated by pre and post feeding weighing. REEs by bottle and breastfeeding were compared by the paired t-test.

Results Mean gestational age at birth was 33±1.4 weeks (range 30–35). Birthweight was 1859±227 g (1480–2210). Chronological age was 22±9 days, and weight was 2147±188 g. There was no significant difference in REE between bottle and breastfeeding (68.3±6.4 vs. 67.5±6.8 Kcal/kg/day, respectively, p = 0.7). Duration of feeding was significantly longer at the breast (20.1±7.9 min) than by bottle (7.8±2.9 min; p<0.001). Regression analysis of the difference in REE between bottle and breastfeeding as the dependent variable and duration of feeding as the independent one was not significant (p = 0.8). Assuming that there is a true disparity in REE, we would need 1136 infants to detect a significant difference (α = 0.05, power = 0.8).

Conclusion There was no significant difference in REE when infants were breast or bottle fed. Longer feeding time at the breast did not increase REE. We speculate that it is safe to recommend breastfeeding after 36 weeks corrected gestational age without incurring energy loss.