Doctoral Dissertation Improvement: More than just birth weight: a longitudinal study of the reproductive ecology of infant growth and development

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Recent epidemiological studies have highlighted the importance of fetal and infant development for lifelong physiology and health. While infant growth rates have been theorized to act in a programmatic manner determining future physiological function and adult chronic disease risk, intensive longitudinal investigations of infant somatic growth are rare and the relationship between somatic growth and the hormonal development underlying this growth are not well understood. Indeed, infancy is a sensitive period during which the tempo of growth and development is shaped in response to salient environmental variables. This shaping is mediated by the endocrine system, but the underlying mechanisms remain largely unexplored. This project examines the relationship between endocrine development and somatic growth during the first six months of life, a time previously documented to be an important developmental window for lifelong health and well-being, using novel methods of non-invasive hormonal collection. This study will evaluate: (1) the longitudinal patterns and relationship between sex steroids and somatic hormone levels; (2) the relationship between infant hormonal levels and postnatal body growth; and (3) the effects of environmental factors, such as feeding, illness and maternal characteristics on variations in sex steroids. To address these aims, weekly hormonal, twice weekly anthropometric and daily biobehavioral data will be collected on 36 healthy infants in Atlanta, Georgia.This research provides the first longitudinal documentation of infant sex steroid levels between birth and six months of age, contributing a major missing dataset in human growth. The intensive nature of the study permits investigation of the mechanisms by which body growth during infancy may contribute to adult reproductive function and health. Simultaneous somatic and endocrine measures during infancy will allow testing of the theoretical predictions of life history theory and reproductive ecology regarding the effects of local ecologies in shaping endocrine responses during development. These data expand the current realm of life history theory by examining the translation of the laws of life history theory, such as energy allocation and phenotypic plasticity, into the physiology of the normally growing infant.On a practical level, the results of this study will contribute to public health interventions aimed at improving infant growth in both developed and developing nations. The novel collection and extraction methods being used in this study for longitudinal investigation of hormonal development may have larger scale public health and clinical applications for the study of infant and child growth. Additionally, the non-invasive, and potentially field stable protocol, may allow researchers to extend mechanistic studies of infant growth and development to other non-Western populations, allowing elucidation of the mechanisms altering rates of growth in these populations. This mechanistic focus is critical amidst the rising incidence of infant and childhood obesity in the United States and worldwide and the growing concern over whether childhood obesity contributes to early maturation, increased adult morbidity, and the risk of developing reproductive cancers.