Age, period and cohort effects on body weight and body mass index in adults: The Doetinchem Cohort Study

Abstract

Objective: To study the development of body weight with ageing, in a general adult population, taking into account possible period and cohort effects. Design: A prospective cohort study with 11 years of follow-up. At baseline and after 6 and 11 years, body weight and height were measured. Setting: The Doetinchem Cohort Study, consisting of inhabitants of Doetinchem, a town in a rural area of The Netherlands. Subjects: In total, 4070 healthy men and women aged 20-59 years at baseline. Results: Increase in BMI with ageing was less profound based on cross-sectional data than based on longitudinal data. More recent-born cohorts had a higher BMI at a given age than cohorts who were born earlier. Increase in mean BMI with ageing was observed in all age groups and was similar for groups with a different educational level. Highest increase in BMI over 11 years was observed in the youngest group, aged 20-29 years at baseline (2?2 [95 % CL 2?0, 2?3] kg/m 2 ), and lowest increase in the oldest group, aged 50-59 years at baseline (1?1 [1?0, 1?3] kg/m 2 ). Conclusions: Findings of the present study using longitudinal data suggest that increase in BMI with ageing is underestimated in all age groups by studying crosssectional data only. Further, weight gain is present in all educational levels and does not stop at middle age. Keywords Adults Ageing Body weight Cohort LongitudinalIt is often observed that levels of BMI and prevalence of obesity increase with age until age 60-70 years and decline thereafter. These observations are usually based on crosssectional studies (1)(2)(3)(4)(5) . However, such age-BMI relationships may result from mixed effects of ageing per se, characteristics of subsequent birth cohorts and selective survival. Studies based on successive cross-sectional surveys show that younger birth cohorts have a higher mean BMI at the same age compared to older cohorts (2,(6)(7)(8)(9)(10) , and that the prevalence of overweight and obesity increases over time, independent of age (11)(12)(13)(14)(15) . Increases in BMI with ageing in cross-sectional studies may be the result of age and cohort effects, while increases in BMI with ageing in longitudinal studies may include period effects. Age effects on body weight and BMI are due to behavioural and physiological changes within individuals (16) that occur as a result of ageing, e.g. the lowering in energy requirement at rest with ageing. Period effects are due to population-wide changes in behaviours and other exposures among individuals independent of ageing (16) during the study period and affect all birth cohorts simultaneously, e.g. a change in legislation during the study period that promotes active transport. Cohort effects are influences of populationwide exposures that took place before the study period, e.g. secular differences in environment that may affect behaviour during later life. Disentangling age, period and cohort effects on BMI is crucial to assess future trends in BMI and accompanying health disorders (17) . Evaluation of age, period and co...

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