Body mass index and lactation performance.

Data from the world literature have been analysed in order to test whether low body mass index (BMI: kg/m2) is a useful indicator of functional impairment of lactation performance. Forty-one databases containing 1726 measurements have been identified as having reliable estimates of breast-milk quantity and/or quality. There is no detectable relationship between maternal BMI and the volume of milk produced by mothers when analysed according to the mean BMI of different populations, or of different subgroups stratified by BMI within populations. This conclusion holds even at BMIs < 18.5. The most remarkable feature of the data is the very high milk volumes produced by very thin mothers. It is accepted that the composition of breast milk is relatively unaffected by general undernutrition of the type that would be indicated by a low BMI with the possible exception of milk fat levels and hence the energy content. Analysis of the available data reveals studies in which there are weak, but significant, correlations between maternal BMI and milk fat. However, other studies show no association or even a negative relationship. Inter-country analysis fails to reveal any detectable association between BMI and milk energy. Milk energy levels seem adequate even at BMIs < 18.5. It is concluded that human lactation performance is extremely robust and that BMI does not provide a useful indicator of function at the levels studied so far. Lactation performance must become compromised when undernutrition is sufficiently severe, but it appears that this must occur only in famine or near famine conditions.