Age, period and cohort effects on body mass index in New Zealand, 1997–2038

Abstract

Objective:To estimate the effects of age, period and birth cohort on observed trends, and to provide short‐ to medium‐term projections of population BMI in New Zealand. Methods:Data were obtained from New Zealand national health surveys covering the period 1997 to 2015 (n=76,294 individuals). A Hierarchical Age‐Period‐Cohort (HAPC) model and an Age‐Period model with interaction terms were specified for population groups defined by ethnicity and sex. Observed trends were extrapolated to estimate group‐specific BMI projections for the period 2015–2038; these were weighted by projected population sizes to calculate population‐wide BMI projections. Results:Population mean BMI increased from 26.4 kg/m2 (95%CI 26.2–26.5) in 1997 to 28.3 kg/m2 (95%CI 28.2–28.5) in 2015. Both models identified substantial, approximately linear, period trends behind this increase, with no significant cohort effects. Mean BMI was projected to reach 30.6 kg/m2 (95%CI 29.4–31.7; HAPC model) to 30.8 kg/m2 (95%CI 30.2–31.4; Age‐Period model) by 2038. Conclusions:BMI continues to increase in New Zealand. On current trends, population mean BMI will exceed 30 kg/m2 – the clinical cut‐off for obesity – by the early 2030s. Implications for public health:Unless prevented by comprehensive public health policy changes, increasing population obesity is likely to result in unfavourable economic and health impacts.