Natural selection and demographic transition in a Zapotec-speaking genetic isolate in the Valley of Oaxaca, southern Mexico

Secular trends are associated with the demographic and epidemiologic transitions in an indigenous community in Oaxaca, Southern Mexico

OBJECTIVE

To test the hypothesis that secular changes in body size and age at menarche are related to the demographic and epidemiologic transitions in an indigenous community in Oaxaca, southern Mexico.

METHODS

Data were derived from surveys of a Zapotec-speaking community conducted between 1968 and 2000. Segmented linear regressions of height, weight, BMI and recalled age at menarche on year of birth in cohorts of adults born before and after the demographic transition were used to evaluate secular changes. Corresponding comparisons of body size (MANCOVA controlling for age) and age at menarche (status quo, probit analysis) were done for samples of children and adolescents born before and after the epidemiological transition.

RESULTS

Height and weight increased in adults born after the demographic transition (mid-1950s), and especially in children and adolescents born after the epidemiological transition (mid-1980s). Age at menarche also decreased significantly in women born after the demographic transition, but at a more rapid estimated rate in adolescents born after the epidemiological transition. Secular gains in body weight were proportional to those for height among children and adolescents, but adults, males more so than females, gained proportionally more weight.

CONCLUSIONS

The secular trend in height in adults of both sexes was associated with the decade of the demographic transition in the mid-1950s. Significant secular gains in size attained and age at menarche occurred in children and youth born after the epidemiologic transition which likely reflected improved health and nutritional conditions since the mid-1980s.

The development and experience of epidemiological transition theory over four decades: a systematic review

BACKGROUND

Epidemiological transition (ET) theory, first postulated in 1971, has developed alongside changes in population structures over time. However, understandings of mortality transitions and associated epidemiological changes remain poorly defined for public health practitioners. Here, we review the concept and development of ET theory, contextualising this in empirical evidence, which variously supports and contradicts the original theoretical propositions.

DESIGN

A Medline literature search covering publications over four decades, from 1971 to 2013, was conducted. Studies were included if they assessed human populations, were original articles, focused on mortality and health or demographic or ET and were in English. The reference lists of the selected articles were checked for additional sources.

RESULTS

We found that there were changes in emphasis in the research field over the four decades. There was an increasing tendency to study wide-ranging aspects of the determinants of mortality, including risk factors, lifestyle changes, socio-economics, and macro factors such as climate change. Research on ET has focused increasingly on low- and middle-income countries rather than industrialised countries, despite its origins in industrialised countries. Countries have experienced different levels of progress in ET in terms of time, pace, and underlying mechanisms. Elements of ET are described for many countries, but observed transitions have not always followed pathways described in the original theory.

CONCLUSIONS

The classic ET theory largely neglected the critical role of social determinants, being largely a theoretical generalisation of mortality experience in some countries. This review shows increasing interest in ET all over the world but only partial concordance between established theory and empirical evidence. Empirical evidence suggests that some unconsidered aspects of social determinants contributed to deviations from classic theoretical pathways. A better-constructed, revised ET theory, with a stronger basis in evidence, is needed.

Physical activity in youth from a subsistence agriculture community in the Valley of Oaxaca, southern Mexico

Observations of activities of contemporary subsistence agricultural communities may provide insights into the lifestyle of youth of 2 to 3 generations ago. The purpose of this study was to document age- and sex-associated variation in household activities and daily steps walking to school of youth 9-17 years in an indigenous subsistence agricultural community in Oaxaca, southern Mexico. Activities during leisure were also considered. A cross-sectional survey of a rural Zapotec-speaking community was undertaken, and respondents included 118 boys and 152 girls, aged 8.7-17.9 years. Household and leisure activities were documented by questionnaire and subsequent interview. Household activities were classified by estimated intensity for before and after school and on the weekend, and an estimate of METS per day accumulated while doing chores was derived. Number of steps from home to school was estimated. Contingency table analysis and MANCOVA controlling for age was used to evaluate results. Household activities tended to cluster at light and moderate intensities in girls and at moderate to moderate-to-vigorous intensities in boys. Estimated METS per day in approximately 2 h of chores differed significantly by sex. Secondary school girls expended significantly more METS per day in chores than primary school girls, but there was no difference by school level in boys. The daily round trip from home to school was approximately 2400 steps for primary students and approximately 2700 and approximately 3100 steps for secondary boys and girls, respectively. Television viewing and participation in sports were major leisure activities for boys and girls. Daily household chores, walking, and leisure activities suggest moderately active and moderately-to-vigorously active lifestyles in girls and boys, respectively, in this indigenous subsistence agricultural community.

Epidemiologic transition in an isolated indigenous community in the Valley of Oaxaca, Mexico

The objective of the present study is to analyze age-specific mortality in a rural indigenous community in the throes of a secular increase in size in the Valley of Oaxaca, southern Mexico, over 30 years, 1970-1999. Variation in mortality by age group was analyzed over time for evidence of an epidemiological transition. The seasonal rain pattern in the Valley of Oaxaca (83% from May through September) was evaluated for its relationship with mortality in wet and dry months. Mortality and causes of death changed markedly over the 30-year interval. Infant and preschool mortality, overall mortality, and causes of death changed from the 1970s through the 1990s. Prereproductive deaths (<15 years) predominated in the 1970s and were largely due to gastrointestinal and respiratory diseases, with periodic outbreaks of measles. Deaths of adults 65+ years predominated in the 1990s and were largely due to degenerative diseases usually associated with aging. The marked changes in age and causes of death over the three decades (epidemiologic transition from Stage I to Stage II) occurred concurrently with significant secular increases in body size in children, adolescents, and young adults, highlighting improved health and nutritional conditions in the community which is in early Stage II of the demographic transition. The demographic transition to Stage II is a leading indicator (15-25 years lag) for the onset of the secular trend, while the epidemiologic transition to Stage II is a predictor that the secular increase is in process in the study community.