The WHO recommends exclusive breast-feeding (EBF) until 6 months of age⁽ Footnote ^{1 )}. EBF indisputably has many advantages for mother and infant, but a systematic review of cross-sectional studies has found only sparse data on energy transfer from mother to baby after age 5 months⁽ Footnote ^{2 )}, and has suggested uncertainty as to whether sufficient adaptive changes take place during EBF to accommodate increasing infant energy needs during EBF to 6 months⁽ Footnote ^{3 )}. Longitudinal studies have failed to find significant changes in breast milk volume or energy content after 4 months of lactation⁽ Footnote ^{2 )}. Most studies have used test weighing combined with milk sampling, which may interfere with breast-feeding routines, and provide unrepresentative estimates of the energy content of breast milk, so there is a need for more evidence using isotopic techniques. In the present investigation isotopic techniques have been used in cross-sectional and longitudinal studies in order to identify the adaptive changes that occur to make EBF possible for 6 months. Data from the cross-sectional study are presented here.

The metabolisable energy content of breast milk was examined in 21 infants at 12.3 (sd 0.6) weeks of age⁽ Footnote ^{4 )}. A dose-to-infant of doubly-labelled water was administered through a feeding tube, and disappearance rates of Deuterium and Oxygen-18 was measured from daily urine samples for the subsequent 7 days, as described previously⁽ Footnote ^{4 )}.

Mean breast milk intake was 879 (sd 121) ml/d and the metabolisable energy density content was 2.59 kJ (0.62 kcal)/ml breast milk (Table).

This cross-sectional study suggests that the metabolisable energy density of breast milk may be lower than literature values, which are commonly found to be 2.9–3.0 kJ/ml (ranging from 2.6–3.3 kJ/ml)⁽ Footnote ^{2 )}. Increasing infant energy needs during EBF could be met by increased breast milk energy density and/or milk volume. Alternatively, infant growth rate may decrease, delaying the point at which breast milk energy supply becomes insufficient. To investigate this it is necessary to conduct a longitudinal observational study on the energetics, growth and body composition of healthy EBF infants from 4 to 6 months of age, using the doubly-labelled water method, and such a study is underway.