Increasing obesity rates and corresponding public health problems are well-known, and disparities across socioeconomic groups are frequently reported. However, the literature is less clear on whether the increasing trends are specific to certain socioeconomic groups and whether disparities in obesity are increasing or decreasing over time. This knowledge sheds light on the understanding of the driving forces to the ongoing worldwide increases in obesity and body-mass index and gives guidance to plausible interventions aiming at reverting weights back to healthy levels. The purpose of this study is to explore long-term time trends and socioeconomic disparities in body-mass index and obesity among U.S. adults. Individual level data from ten cycles of the National Health and Nutrition Examination Survey between 1960 and 2008 are used to estimate adjusted time trends in the probabilities of obesity and severe obesity and in measured body-mass index for three racial/ethnical groups, for three educational groups, and for four levels of income, stratified by gender. Time trends in the probabilities of obesity and severe obesity are estimated by linear probability models, and trends at the 15th, 50th and 85th percentiles of the adjusted body-mass index distribution are estimated by quantile regression. Divergent time trends for the different socioeconomic groups are estimated by interaction terms between socioeconomic status and year. The results show that, with some exceptions, increases in both obesity, severe obesity and body-mass index are similar across the different racial/ethnic, educational and income groups. We conclude that the increase in body-mass index and obesity in the United States is a true epidemic, whose signal hallmark is to have affected an entire society. Accordingly, a whole-society approach is likely to be required if the increasing trends are to be reversed.
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