Food, Medicine, and Signal: Mother's Milk Programs Infant Development

Milk is abundant at our local grocery store in the form of dairy products and infant formulas. This ubiquity of standardized milk in our modern environment has the potential for the general public, and even researchers, to think of milk one dimensionally. Milk is not just food though, milk is also medicine and signal. Hundreds, possibly thousands, of other “bioactives” are present in mother’s milk. Importantly, milk is not uniform across mothers or within a single mother across time. Maternal health, nutrition, culture, reproductive history, early-life conditions, and genes all contribute to differences in milk composition. Less understood are the consequences of that variation for infant development and fitness outcomes. Here I will present emerging research that addresses the magnitude, sources, and consequences of inter-individual variation of bioactive constituents in milk in the rhesus monkey. Long-term research at the Comparative Lactation Lab and the California National Primate Research Center has produced >1000 milk samples from hundreds of rhesus macaque mothers (Macaca mulatta) since 2005. Fats, proteins, sugars, minerals, hormones, bacteria, and other constituents in mother’s milk are highly variable and often differ between milk produced for sons and daughters. Milk exerts important influences on infant behavior, growth, and development and through those effects has consequences for transitions to adulthood and initiation of the reproductive career. Better understanding of variation in milk composition, especially for milk constituents linked to infant cognition, neurodevelopment, behavior, and metabolism, enhances an evolutionary biological perspective of parent-offspring dynamics. Moreover, biological and social scientific research on this topic can be directly translated into more personalized clinical recommendations and health optimization for mothers and their infants. Such research also highlights the importance of establishing infrastructure and institutional support for breastfeeding promotion and facilitation. Lastly, and quite importantly, a better understanding of the composition and function of milk informs the manufacture of more representative infant formulas. Milk matters.