Metabolizable Energy Content of Breastmilk Supports Normal Growth in Exclusively Breastfed Icelandic Infants to Age 6 Months

Birna Thorisdottir*, Tinna Odinsdottir, Geir Gunnlaugsson, Simon Eaton, Mary S. Fewtrell, Adriana Vázquez-Vázquez, Ronald E. Kleinman, Inga Thorsdottir, Jonathan CK Wells

*Corresponding author for this work

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Background: Neither the global population nor individual countries have reached the World Health Organization (WHO) target of ≥50% of infants exclusively breastfed (EBF) until 6 mo. This may partly be because of the perceptions of insufficient milk and energy supply to meet rapid growth and development needs. Objectives: In a longitudinal observational study, we aimed to determine whether breastmilk energy content is sufficient to support growth during EBF until 6 mo. Methods: A sample of 27 EBF infants was dosed with doubly labeled water (DLW) at 5.6 mo to measure body composition, breastmilk intake, energy intake, and the metabolizable energy (ME) content of their mother's breastmilk over the following week. Z-scores were calculated for anthropometry using WHO reference data and for fat-free mass (FFM) and fat mass (FM) using United Kingdom reference data. Results: Anthropometric z-scores from birth indicated normal weight and length growth patterns. At ∼6 mo, the mean ± standard deviation (SD) FFM z-score was 0.22 ± 1.07, and the FM z-score was 0.78 ± 0.70, significantly >0. In the 22 infants with acceptable data, the mean ± SD measured intake of breastmilk was 983 ± 170 g/d and of energy, 318 ± 60 kJ/kg/d, equivalent to 75.9 ± 14.3 kcal/kg/d. The mean ME content of breastmilk was 2.61 kJ/g [standard error (SE) 0.1], equivalent to 0.62 kcal/g (SE 0.02). Mothers were positive toward breastfeeding, on paid maternity leave (planned mean 10 mo), and many (56%) had received specialized breastfeeding support. Conclusions: The evidence from this study confirms that when mothers are motivated and supported without economic restraints, breastmilk intake and the energy supplied by breastmilk to EBF infants at 6 mo of age is sufficient to support normal growth patterns. There was no evidence of constraint on FFM, and other studies show that high FM in EBF infants is likely to be transient. These data further support the recommendation for EBF ≤6 mo of age for body composition. This trial was registered at as NCT02586571.

Original languageEnglish
Pages (from-to)468-475
Number of pages8
JournalAmerican Journal of Clinical Nutrition
Issue number2
Publication statusPublished - Aug 2023

Bibliographical note

Funding Information:
This research was funded by the Icelandic Research Fund ( 196157-053 ) and the University of Iceland Research Fund (to Inga Thorsdottir). All research at Great Ormond Street Hospital NHS Foundation Trust and UCL Great Ormond Street Institute of Child Health is made possible by the National Institute for Health and Care Research (NIHR) Great Ormond Street Hospital Biomedical Research Centre. The views expressed are those of the author(s) and not necessarily those of the NHS, the NIHR, or the Department of Health.

Publisher Copyright:
© 2023 The Authors

Other keywords

  • body composition
  • breastfeeding
  • exclusive
  • growth
  • human
  • infant
  • milk
  • nutrition


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