Assessment of longitudinal and cross-sectional effects of age on adult obesity in an Iranian population: results of a large population-based cohort study

The risk of obesity commonly changes with age, which is a longitudinal (aging) effect. Moreover, the individuals who enter the study at the same age have similar living conditions that may influence their obesity risk in a particular way; this is a cross-sectional effect. Both cross-sectional and longitudinal effects of age should be considered for a better understanding of the effect of age on obesity and the related factors.

The present study aimed to (i) assess both the cross-sectional and longitudinal effects of age on obesity and (ii) determine how obesity changes with age in the target population using a Marginal Logistic Regression (MLR) model.

The current study made use of the information of individuals who had participated in the Isfahan Cohort Study. Participants were a large group of Iranian adults over 35 years of age who lived in the central region of Iran in 2001. Repeated measurements were obtained in 2001, 2007, and 2013.

From 2001 to 2013, the percentage of obesity in men and women raised from 13% and 31% to 18% and 44%, respectively. Both cross-sectional and longitudinal effects of age were significantly associated with the odds ratio of obesity. There was a rise in the probability of obesity for individuals aged 35-60 years at baseline and a decline for the older ones. The odds of obesity had about a 2% increase (on average) per year, regardless of the baseline age.

The obtained results pointed to a difference between the cross-sectional and longitudinal effects of age on the probability of obesity in the target population. The high and rising prevalence of obesity was a serious public health issue among participants, especially women, aged 35-60 years. The assessment of changes in obesity in a population-based study provides opportunities to target subpopulations that need more care and attention in public health interventions.