The present study aimed to evaluate evidence on metabolisable energy consumption and pattern of consumption with age in infants in the developed world who were exclusively breast-fed, at around the time of introducing complementary feeding. We carried out a systematic review aimed at answering three questions: how much milk is transferred from mother to infant?; does transfer increase with the age of the infant?; and what is the metabolisable energy content of breast milk? Thirty-three eligible studies of 1041 mother-infant pairs reported transfer at 3-4 months of age, the weighted mean transfer being 779 (SD 40) g/d. Six studies (99 pairs) measured transfer at 5 months, with a weighted mean transfer of 827 (SD 39) g/d. Five studies (72 pairs) measured milk transfer at 6 months, reporting a weighted mean transfer of 894 (SD 87) g/d. Nine longitudinal studies reported no significant increases in milk transfer after 2-4 months. Twenty-five studies on breast-milk energy content were based on 777 mother-infant pairs. The weighted mean metabolisable energy content was 2·6 (SD 0·2) kJ/g. Breast-milk metabolisable energy content is probably lower, and breast-milk transfer slightly higher, than is usually assumed. Longitudinal studies do not support the hypothesis that breast-milk transfer increases markedly with age. More research on energy intake in 5-6-month-old exclusively breast-fed infants is necessary, and information on the metabolisability of breast milk in mid-infancy is desirable. This evidence should inform future recommendations on infant feeding and help to identify research needs in infant energy balance.
Infant nutrition: Breast feeding: Complementary feeding: Energy metabolismThe evidence base for many infant-feeding recommendations is currently limited (Michaelsen et al. 2000;Lanigan et al. 2001;Kramer & Kakuma, 2002). For example, a review on the possible adequacy of breast milk to meet energy and nutrient requirements up to the age of 6 months noted a lack of published evidence on infant energy intake with which to address this question and called for further research (Butte et al. 2002). We were also aware that many implicit but important assumptions (about breast-milk transfer, metabolisability and energy content, for example) have commonly been made in the literature, and these assumptions have been repeated and widely used in infant-feeding recommendations. The assumptions have not generally been formally tested, and it is widely recognised that more evidencebased approaches to the issue would be helpful (Michaelsen et al. 2000;Lanigan et al. 2001;Kramer & Kakuma, 2002).Before embarking on a new research project on energy balance in infancy, we had the opportunity to test the evidence base on critical energy-balance variables by conducting a systematic review of studies on exclusively breast-fed infants over the period when complementary feeding is usually introduced. 'Exclusive breast-feeding' is defined in WHO terms as feeding exclusively with breast milk (World Health Organization, 1995), wit...