The public health impact of the obesity epidemic is generally recognized as negative but there is disagreement on the nature and magnitude of the effect. This disagreement is based in part on divergent estimates of the link between obesity and mortality (the effect) and in part on varied estimates of the patterns of obesity prevalence within the population (the exposure). These two factors are incorporated into the “population attributable fraction” (PAF), which is the percentage of mortality in the population that can be attributed to obesity. Estimates of the patterns of obesity prevalence have been criticized for not accounting for biases in the population data.
A study published in the October, 2013, issue of the American Journal of Public Health tried to address these problems of earlier studies. The authors analyzed the obesity-mortality link in 717,568 men and women who participated in 19 consecutive editions of the National Health Interview Survey between 1986 and 2004 and linked to the National Death Index through 2006. The analysis was stratified by race and gender and covariates included education level, household income, marital status, residence region, age and the time of the survey. Unlike many other studies, the authors also controlled for birth cohort missed in earlier studies.
The birth cohort groups reflect the fact that the members of a cohort born during a particular time period experience a different set of environmental and cultural influence at a specific age than the members of another cohort born during a different time period. For example, after controlling for cohort among white men, at age 66 grade 1 obesity (BMI 30-35) was found in this study to account for 3.4% of deaths for men born between 1915-1919, 4.5% of deaths for men born 1925-1929, and 5.8% deaths for men born 1935-1939. This appears to reflect the fact that those born later were exposed to the obesogenic environment and culture at an earlier age and experienced a greater life exposure.
The “take away” messages of this study include: (1) The prevalence of obesity varies not only with age but also by birth cohort. This indicates that obesity will contribute to an increasing fraction of mortality in more recent birth cohorts. (2) Contrary to previous thought that obesity contributed a diminishing percentage of mortality with age, or even a protective effect with age, the impact of obesity on mortality increases significantly with age. And (3) obesity accounted for about 18% of all deaths between the ages of 40 and 85 in the time period from 1986 and 2006.
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