Milk, Mleko, Leche: Variation in human milk bioactives across populations

Abstract

Human milk not only provides nutrition and passive immune protection to the infant during early life, but also contains a variety of bioactive components that actively shape the infant’s developing microbiota, metabolism, and immune system. Milk composition varies among mothers and is thought to be shaped by natural selection and ecological inputs from the mother’s environment in order to prepare the infant for the environment it is most likely to encounter during early life. Despite the wide variety of physical and cultural environments humans inhabit around the globe today, the majority of research on human milk composition has been conducted in urban, industrialized settings. As a result, relatively little is known about milk composition in diverse populations. This dissertation utilizes collaborative studies to explore how the composition of human milk varies among mothers from diverse geographic locations and with varying cultural and subsistence practices. First, the composition of innate and acquired immune proteins was compared across seven populations. Acquired immune protein composition was found to be more variable than innate immune protein composition across populations and subsistence strategies. This finding supports the idea that the immune protection provided by milk is shaped by the mother’s immunological exposures throughout her lifetime. Next, the concentrations of seven trace elements were compared in milk from four populations. Calcium was the most variable element investigated, while zinc concentrations did not differ among populations. Cultural practices contributed to higher milk iron concentrations in one population. Finally, the role of cortisol as a signal of maternal energetic condition was explored in two populations. Cortisol concentrations were higher in milk from primiparous mothers and mothers with lower BMI, but milk cortisol concentrations were not significantly related to maternal energy balance or parity. In well-nourished populations, milk cortisol concentrations alone may not be a reliable signal of maternal condition. These studies are among the first characterizations of human milk composition to include indigenous populations and populations practicing traditional subsistence strategies. Together, these results contribute to our understanding of how milk composition varies across diverse populations.