摘要：Objectives. To estimate the percentage of excess death for US Black and White men and women associated with high body mass, we examined the combined effects of age variation in the obesity–mortality relationship and cohort variation in age-specific obesity prevalence. Methods. We examined 19 National Health Interview Survey waves linked to individual National Death Index mortality records, 1986–2006, for age and cohort patterns in the population-level association between obesity and US adult mortality. Results. The estimated percentage of adult deaths between 1986 and 2006 associated with overweight and obesity was 5.0% and 15.6% for Black and White men, and 26.8% and 21.7% for Black and White women, respectively. We found a substantially stronger association than previous research between obesity and mortality risk at older ages, and an increasing percentage of mortality attributable to obesity across birth cohorts. Conclusions. Previous research has likely underestimated obesity’s impact on US mortality. Methods attentive to cohort variation in obesity prevalence and age variation in obesity’s effect on mortality risk suggest that obesity significantly shapes US mortality levels, placing it at the forefront of concern for public health action. Among health professionals there is widespread agreement that high rates of obesity present serious public health challenges in the United States. Yet the ultimate toll that obesity will take on the health and longevity of Americans remains a hotly contested issue. Disagreement on this point is driven largely by contradictory findings about the obesity–mortality association, 1 as well as widely divergent estimates of the fraction of mortality in the US population that is attributable to overweight and obesity—also known as population attributable fractions (PAFs). 1–7 Without a broader consensus in the research community, policymakers will lack clear guidance on how to prioritize and appropriately respond to the US obesity epidemic. Our study helps resolve these debates by reassessing obesity–mortality associations and PAFs by using concepts and methods that better capture the complexity of the epidemic and its impact on US mortality. We note that a PAF is composed of 2 factors: (1) the individual-level association between body mass and mortality risk (i.e., the effect) and (2) patterns in US overweight and obesity prevalence (i.e., the population’s exposure to this effect). Early estimates of obesity-attributable mortality were criticized for failing to account for either variability (e.g., by age) in obesity’s effect on mortality (component 1 of the PAF) or the divergent rates of obesity across subpopulations (component 2 of the PAF). 7,8 More recent studies that attend to some of these issues have produced lower estimated fractions of US deaths attributable to overweight and obesity than found in earlier investigations. 5–7 Although corrective actions taken in recent studies were useful and necessary, our main contention is that they underestimated the effects of obesity on US mortality for 2 main reasons. First, survival models assessing mortality differences between the normal weight, overweight, and obese populations in recent studies failed to account for factors that potentially bias estimates of the obesity–mortality association. For example, older obese individuals are less likely than their normal-weight peers to participate in health surveys because of obesity-related health complications. Because previous studies have not accounted for this, associations between measures of high body mass (e.g., body mass index [BMI, defined as weight in kilograms divided by the square of height in meters], waist-to-thigh ratio) and mortality have been reported to be weaker than they likely really are. Because, in part, of such limitations, many existing studies have concluded that overweight and obesity confer only a modest increase in mortality risk. Second, PAF estimates for overweight and obesity as causes of mortality are usually derived from equations that use average rates of obesity to indicate exposure. 8 These population averages overlook substantial differences in obesity prevalence among persons belonging to different birth cohorts. 9–11 Although obesity is certainly influenced by the biological aging process (i.e., age effects) and broad societal changes that have transpired in recent decades (i.e., period effects), it is also influenced by birth cohort membership (i.e., cohort effects)—the so-called “third dimension” of the obesity epidemic. 11 Birth cohort membership is important because it represents the onset of exposure to obesogenic environments; newer birth cohorts tend to have earlier onset and, thus, higher rates of obesity than their predecessors. Although recent work suggests that birth cohort dynamics are key to understanding the future of US health and longevity, 10,11 existing PAF estimates for obesity as a cause of US mortality omit them from consideration. As a result, these studies may underestimate the impact of rising obesity prevalence on US mortality. In addition to these 2 main concerns, we also note that much previous research indicates that the obesity–mortality link diminishes with age, with some evidence suggesting that overweight may even provide a survival advantage at older ages. 12–18 However, recent studies have shown that weaker associations between obesity and mortality at older ages likely reflect confounding from cohort variation in mortality risk, 19 healthy participant effects (i.e., biases introduced by survey selection of healthy respondents), 20 or duration of one’s life spent obese. 21 In fact, contrary to current wisdom, the obesity–mortality association has been shown to grow significantly stronger with age after one accounts for these confounding factors. 22 Estimates of PAF for obesity as a cause of US mortality need to be updated in light of these recent discoveries. We address these problems in some previous research via analyses of a large, nationally representative data set composed of multiple birth cohorts that includes (1) previously unaccounted for confounders of the obesity–mortality association, and (2) cohort-specific estimates of obesity prevalence. Results from our models suggest that the obesity epidemic is worthy of its status as a leading public health concern in the United States.
