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

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.