Objective: To test whether and how human lactation and breastfeeding practices can adapt to fulfill infant energy requirements during exclusive breastfeeding for 6 months.
Methods: The First-Feed study was a longitudinal, observational field study to measure milk intake, energy intake (from the doubly labeled water method), anthropometry, and breastfeeding practices at 2 time points around 15 and 25 weeks of age. Fifty healthy exclusively breastfeeding mother-infant dyads were included from breastfeeding support groups in greater Glasgow, Scotland. Forty-seven completed (23 boys), and 41 were exclusively breastfed to 25 weeks of age.
Results: Milk intakes were higher than literature values (923 [SD: 122] g/day, n = 36; and 999 [SD: 146] g/day, n = 33) at both 15 and 25 weeks of age (both P < .001) and increased significantly between time points (mean increase: 61 g/day [95% confidence interval: 23-99]; P = .003). Infant growth was normal compared with World Health Organization Child Growth Standards, and energy intakes were adequate compared with references for energy requirements. Behavioral data indicated no evidence of strain on breastfeeding practices.
Conclusions: Results of this prospective study reveal that when mothers are well supported and follow the World Health Organization recommendation on breastfeeding, milk intakes are high and increase over time, and there is adequate energy intake, normal infant growth, and no marked changes in breastfeeding practices. This new empirical evidence on adaptations during exclusive breastfeeding should help health professionals promote the initiation, duration, and exclusivity of breastfeeding.