Background: 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.
Objectives: 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.
Materials and Methods: 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.
Results: 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.
Conclusion: 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
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