Rising to the challenges and opportunities of life course epidemiology

Abstract

This edition of the International Journal of Epidemiology has four papers1–4 and accompanying commentaries5–7 that can be conveniently clustered under the heading of life course epidemiology. In the concluding chapter of ‘A life course approach to chronic disease epidemiology’,8 Diana Kuh and I raised several emerging and common themes that we felt needed to be addressed by future research. These were (i) understanding heterogeneity, (ii) going beyond repeat measures to understand trajectories, (iii) the role of accelerated postnatal weight and height gain and (iv) the use of life cohort cohorts and less conventional designs. All of these topics are addressed to some degree by these publications. The papers by Strachan1 and Power2 are some of the first to come out from the recent follow-up of the 1958 Birth cohort and these researchers and their colleagues are to be congratulated on the valuable addition these new data will make to other existing studies such as the 1946 National Survey of Health and Development.9 Their work re-examines the relative importance of exposures in early and later life by comparing the influence of geography, region of birth or examination amongst migrants, and personal characteristics, childhood and adult socioeconomic position. Each paper alone presents fairly clear conclusions though reconciling their findings across both papers remains more of a challenge. Both childhood and adult socioeconomic position were in general associated with cardiovascular risk factors, in some cases the earlier measure had if anything a bigger effect. There was no evidence of any critical or sensitive period or any interaction and the most parsimonious model was a simple accumulation of effect model.10,,11 This is consistent with previous studies and the observation that childhood socioeconomic position still predicts adult cardiovascular disease, though with a moderately attenuated effect after adjustment for adult socioeconomic position.12 This is also consistent with an ‘early origins’ hypothesis though it does not restrict exposures to just the pre-natal period as post-natal growth, childhood and adolescent development may also be relevant, as highlighted by the Guatemalan paper.4 However, the accompanying 1958 Birth cohort paper appears to contradict this conclusion. It identifies only current adult region of examination as a predictor of cardiovascular risk factors, other than height, which was predicted by region of birth and claims to negate the early life hypothesis in terms of its importance in explaining regional differences in risk factors. Clearly, as Geoffrey Rose highlighted over 20 years ago,13 what determines an individual's risk may not be the same as what determines the risk between populations. Hence within each population, poorer individuals both in childhood and adulthood have a worse risk profile, but this may have little effect on the mean values between areas. However, given the known ecological associations between area-deprivation and cardiovascular risk, this requires more careful consideration.14 One possible explanation is that the effects of early environment are simply less important than that of later environment and hence after mutual adjustment they cease to add any explanatory power. This would not refute the early origins per se, but simply argue that the relative importance of early environment has been overstated. However, as Nybo Anderson points out5 what does region of birth actually mean? Let us consider body mass index as our outcome of interest. Firstly there is marked regional heterogeneity in the non-migrant group. This could be explained by both variations in genetic15 and/or environmental risk factors. The former, however would not explain the marked effect of region of examination for migrants, whose genetic predisposition is more likely to be reflected by their region of origin. However, how have dietary habits and/or physical activity levels altered over time between areas? Recent data from the ALSPAC cohort16 show the large effects of physical activity on fat mass. Physical levels in children are likely to have reduced over the last 50 years, but what is less clear is whether hetereogeneity at a regional level has changed. If activity levels were not only higher in the past, but more similar across regions than they are today, the effects of current region would naturally be stronger even though the relative effects of activity on obesity for individuals may be the same at both time periods.

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