Educational differences in adult body mass index: age-period-cohort analysis using cohort data from 2000 to 2022

Lower socioeconomic position is linked to higher body mass index (BMI) across the life course, but the contributions of age, period, and cohort in this association are less understood. This study examined age, period, and cohort effects on educational differences in BMI over a 22-year follow-up. We used repeated Helsinki Health Study survey data from 2000 to 2022. In Phase 1 (2000-02 for the 'ageing cohort', aged 40-60, n = 8960; and 2017 for the 'employee cohort', aged 19-39, n = 5898), all participants were employees of the City of Helsinki, Finland. We calculated participants' BMI from their self-reported height and weight in each period and divided participants' educational attainment into high and low. The final sample comprised 11 636 women and 3037 men. We used generalised additive mixed models for the age-period-cohort analyses. Educational differences in BMI mostly widened until late midlife and then stabilised. Younger participants and more recent cohorts had the steepest increases in BMI over periods, and their educational differences moderately narrowed. However, BMI increased in all cohorts, educational groups, and both genders over periods. Although the educational differences in BMI were clear among earlier cohorts, the overall patterns in their BMI trajectories were highly similar between the educational groups. Since age, period, and cohort each affected the development of educational differences in BMI over time, they should be considered when estimating future trends in socioeconomic inequalities in BMI and when planning policy actions to tackle these inequalities.