Abstract
Anthropological investigations of reproductive function have
theorized that childhood development serves as a "bioassay" of
environmental conditions, adaptively tuning the timing of
maturation and adult reproductive function to energy availability.
The relationship between somatic growth and hormonal development is
not well understood during the critical period of infancy when
growth is shaped in response to salient environmental variables.
This project seeks to fill this gap by extending reproductive
ecological models to infant growth and development.
The present research developed novel, non-invasive methods for
measuring fecal sex steroid levels in infants and investigated: (1)
sex-specific developmental trends in steroid production, (2) the
relationship between hormonal levels and growth and body
composition and (3) the effects of environmental factors, such as
feeding and maternal characteristics, on sex steroid levels. The
data used to address these aims came from a longitudinal sample of
32 infants followed weekly from age 1 week to 15 months. Growth and
body composition were measured using standard procedures.
Testosterone and estradiol were assessed from diaper samples using
methanol extraction and microradioimmunoassay techniques. Feeding
style, behavior and illness were measured through parental diaries
and questionnaires.
The results identified novel patterns of endocrine activity during
infancy. Sex steroid levels followed distinctive patterns in males
and females and contributed to inter-individual variance in body
size and composition. Relationships between sex steroids, body
composition and feeding measures indicated that energy availability
may also influence hormone levels.
These data provide the first frequent longitudinal documentation of
infant sex steroid levels. The intensive nature of the study
permitted investigation of sex steroid activation and growth,
providing a potential mechanism for epidemiological observations
that infant growth rate is linked to adult reproductive function
and health. The multiple methodologies used allowed for discussion
of the linkages between growth, energy availability and endocrine
development, an issue central to anthropological debates concerning
the determinants of growth and maturation. This mechanistic focus
is critical amidst the rising incidence of infant and childhood
obesity and the growing concern over whether childhood obesity
contributes to early maturation, increased morbidity, and the risk
of developing reproductive cancers.
Table of Contents
I. Introduction
Background and literature review
Study aims
Chapter summaries
II. Methods Development
Chapter 2: Noninvasive methods for sex steroid recovery in infants:
Fecal estradiol
Sample and methods
Extraction methods
Unmodified assay
Modified assay
Results
Extraction
Unmodified assay
Modified assay
Discussion
Chapter 3: Measurement of testosterone in infant fecal
samples
Sample and methods
Results
Discussion
III. Results
Chapter 4: HPG activation in infancy: developmental trends and
gender
Sample and methods
Results
Developmental trends
Sex differences
Discussion
Tables and figures
Chapter 5: HPG activation and infant growth
Sample and methods
Results
Sex differences in growth
Body size and hormone levels
Growth and sex steroids
Discussion
Summary of findings
Sex differences in growth
Sex differences in size and hormone levels
Growth and sex steroids
Tables and figures
Chapter 6: Feeding, hormones and body composition
Sample and methods
Results
Feeding and sex steroid levels
Feeding, hormones and sex steroids
Confounding effects of infant size and feeding
Other energetic markers
Discussion
Summary of findings
Energetic markers
Feeding and sex steroids
Sex differences in feeding and hormone levels
Feeding, sex steroids and growth
IV. Conclusion
Summary of main findings
Best statistical models
Proposed model
Possible mechanisms
Limitations
About this Dissertation
Rights statement
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