Obesity Epidemiology

OBESITY AND MORTALITY 229on mortality. The Aerobics Center Longitudinal Study (ACLS), 53 which measured physicalfitness using a maximal treadmill exercise test, found that low fitness conferred higher riskof mortality compared with fatness, and that fitness eliminated the excess risk associatedwith fatness in men. These results, however, were not consistent with a Lipid Research ClinicsStudy analysis that found that both fitness and fatness were independent risk factors formortality, and that being fit did not abolish the relationship between obesity and mortality. 54Because physical activity is the primary modifiable determinant of fitness, we examinedthe combined effects of physical activity and obesity on mortality in the NHS. Resultsshowed that excess adiposity (reflected by BMI or WHR) and lower physical activitywere independent predictors of overall and cause-specific mortality. 7 The adverse effectsof body fatness on mortality were consistent in both lower and higher physical activitycategories, and being physically active did not mitigate the mortality risk associated withbody fatness. This study demonstrated that regardless of a woman’s physical activity levels,even modest weight gain during adulthood significantly increased mortality risk. Italso showed that the benefits of physical activity were not limited to lean women. Amongthose who were overweight or obese, physically active women tended to have lower mortalitycompared with sedentary women. Women who were both lean and physically activehad the lowest risk. Thus, maintaining a healthy weight and being physically active areboth important for longevity. Even among those who are physically active, it is importantto minimize weight gain during adulthood.Classifying lean people based on physical activity levels offers a useful way to differentiatebetween active and inactive members of the lean group. Compared withthe normal weight group in the HPFS cohort, there was increased mortality amonglean inactive men but not among lean active men. 24 The lean inactive group probablyincluded many participants with preexisting or subclinical chronic diseases that had ledto decreased physical activity levels. Excluding of subjects with existing diagnoses ofchronic diseases, and eliminating deaths that occur early in the follow-up period can helpreduce, but not completely eliminate, biases due to reverse causation. Cross-classificationof physical activity and BMI can also be a useful way to separate healthy, lean groupsfrom the unhealthy ones.Obesity, Years of Life Lost, and Life ExpectancySeveral recent studies have estimated years of life lost (YLL) associated with obesityor the impact of obesity on life expectancy in the general population. As a way to measurethe health effects of obesity, YLL offers the advantages of simplicity and intuitiveappeal. However, statistical methods used to estimate YLL, or life expectancy reductionassociated with obesity, have varied across studies, as have the estimates. Using a cohortlife table derived from the Framingham Heart Study, Peeters et al. 55 estimated that overweight40-year-old female nonsmokers lost 3.3 years and that overweight 40-year-oldmale nonsmokers lost 3.1 years of life expectancy. In comparison, obese 40-year-oldfemale nonsmokers lost 7.1 years and obese 40-year-old male nonsmokers lost 5.8 years.The YLL associated with obesity was similar to that observed with smoking.Fontaine et al. 56 estimated the expected number of YLL due to overweight and obesityusing data from the 1999 U.S. life tables and NHANES I and II epidemiologicfollow-up studies. Overall, a BMI of 33 kg/m 2 at age 40 was associated with a loss oflife expectancy of 2 to 3 years. The estimated YLL associated with severe obesity (BMI> 45) was 13 years for white men aged 20 to 30 years, representing a 22% reduction inexpected remaining life span. The corresponding YLL for severely obese white womenwas 8 years. The YLL estimates were substantially lower in blacks than in whites.