Obesity Epidemiology

11Obesity and MortalityFrank B. HuThe relationship between body weight and mortality has long been a subject of debate. Inparticular, considerable controversy has surrounded the shape of the curve for the associationbetween body mass index (BMI) and mortality and the effects of moderate overweighton mortality. Epidemiologic studies have variously found J-shaped, U-shaped, andlinear relationships. 1 In some studies, overweight was associated with increased mortality,but in others, overweight individuals had no excess mortality or even slightly decreasedmortality as compared with those who were normal weight. In addition, wide variationsin the estimated numbers of deaths attributable to overweight and obesity have furtherfueled debate. 2,3 These divergent findings have not only caused a great deal of confusionamong the general public, but also been used by commercial interest groups to downplaythe importance of obesity as a public health issue. 4 Discrepancies in study findingsunderscore many methodological challenges in analyses of the relationship between BMIand mortality. These include reverse causation, confounding by smoking, overcontrol ofintermediate variables, effect modification by age, and imperfect measures of adiposity.In this chapter, we first discuss these methodological issues in analyses of obesity andmortality (Table 11.1). Then, we review recent literature on obesity and mortality, payingspecial attention to body fat distribution, intentionality of weight loss, and the relativeimpact of fatness and fitness. Finally, we discuss the impact of obesity on years of lifelost and life expectancy.Methodological Issues in Analyzing the Relationshipbetween Obesity and MortalityReverse CausationReverse causation—when a low BMI is the result of underlying illness rather thanthe cause—is a major concern in the analysis of the relationship between obesity andmortality 5 (Fig. 11.1A). Most people, even those who are obese, die after age 65. By thetime they reach older ages, many of those who were previously obese have already lostweight because of underlying diseases. Weight loss can result from the direct effects ofdisease on weight, or sometimes from conscious weight loss motivated by a diagnosisof serious illness. Many conditions that cause weight loss, such as chronic obstructivepulmonary disease (COPD) and depression, may remain undiagnosed for years.Because populations of lean individuals include smokers, healthy active people, and216