Content, variability, and regulation of fatty acids in human milk

Fatty acids (FAs) are key determinants of the nutritional quality of human milk lipids. While milk fat concentration is highly variable even within milk produced by an individual woman, the relative FA composition is reasonably stable over time. These stable FA patterns result in large part from the utilization of maternal adipose tissue depots and other long-term storage pools as sources of milk FA, in addition to current dietary intake. Moreover, short-term fluctuations of the FA composition due to changing dietary intake are attenuated, whereas the effects of dietary changes or supplements show their full effects only after a considerable time delay. All milk FAs are subject to dietary influences, but the strongest effects of diet are seen in ω-3 long-chain polyunsaturated FA (LC-PUFA) percentage contributions. This is largely because their absolute amounts in milk are relatively small, and diet is their most important source. In contrast, for milk ω-6 LC-PUFA endogenous conversion from precursor FA is of considerable importance. In addition to maternal diet, duration of gestation, stage of lactation, maternal body mass index, and further factors influence milk FA composition, but the magnitude of these effects and interactions with further factors is uncertain as studies yield ambiguous results. There are several groups of FA, such as trans isomeric and branched-chain FA, which provide only small portions to total milk fat, but there are indications that they may influence infant health, which makes it worthwhile to investigate how they are influenced by diet and other factors.