The Life History Significance of Human Breast Milk: Immune and endocrine factors as indicators of maternal condition and predictors of infant health and growth

Abstract

This dissertation explores the relationships between maternal energetic condition, four bioactive compounds in milk, and infant health and growth outcomes through the lens of human life history theory. Research was conducted among the Toba of Barrio Namqom in Formosa, Argentina. This is among the first of studies to apply a life history biology lens to the dynamics of cortisol, insulin-like growth factor 1 (IGF-1), lactoferrin, and secretory immunoglobulin A (sIgA) in human milk. First, the role of maternal energetic condition as a predictor of the concentration of these four compounds in milk is explored. Several interesting relationships emerge, including significantly lower milk cortisol and IGF-1 among women with higher body mass index, and significantly higher milk cortisol among primiparous mothers. Next, the relationships between infant symptoms of illness and the two milk immunofactors, lactoferrin and sIgA, are investigated. Lactoferrin is found to exhibit a positive association with symptoms of illness, and sIgA shows a negative association. Finally, associations between concentration of milk bioactives and infant growth rate are tested, as well as associations between infant illness and growth rate, and maternal energetic condition and growth rate. Milk IGF-1 is found to positively associate with infant linear growth rate. Maternal parity is found to negatively associate with linear growth rate. First-born status is associated with significantly greater gains in length and mass over a four-month period. These and other results are discussed through the lens of life history biology and avenues for future research at the intersection of life history biology and public health are identified.