Does Body Mass Index Adequately Convey a Patient's Mortality Risk?

Abstract

WHAT ADULT WEIGHT BEST ADVANCES HEALTH, minimizes the risk of chronic disease, and promotes longevity? This question has engaged the interest of the public, health care professionals, and a wide range of clinical investigators. The consequences of answering this question have profound health, social, and economic implications for individuals, communities, and the population as a whole. A Metropolitan Life Insurance Company (MLIC) statistician revealed an association between longevity and body weight in 1942. Lifespan was longest when body weight was maintained at the same level as 25-year-old adults with similar height and frame size. The initial 1942 MLIC ideal body weight tables were later revised to desirable weight tables in 1959 and again in 1983 to height and weight tables. Obesity was considered present when a person’s weight exceeded his or her desirable weight by 20%. This approach was popular among researchers. However, the MLIC tables were complex, had technical limitations, and were not easily applied when evaluating patients in the clinic or when screening individuals in field settings. Because body weight increases with height, there is a need to establish normal body weight as a function of height. This is accomplished by the body mass index (BMI; calculated as weight in kilograms divided by height in meters squared), which increases with greater adiposity. In 1985, a National Institutes of Health consensus panel defined overweight-obesity as a BMI of 27.8 or greater for men and 27.3 or greater for women. The BMIs at these levels are approximately 20% above the MLIC desirable weights. The panel recommended weight loss for people whose BMIs exceeded these thresholds. In 1997, a World Health Organization Consultation on obesity defined preobesity (overweight) as a BMI of 25 or greater and class (grade) 1 obesity as a BMI of 30 or greater, class 2 as a BMI of 35 or greater, and class 3 as a BMI of 40 or greater. One year later, a National Heart, Lung, and Blood Institute (NHLBI) expert panel recommended a similar BMI-based definition of overweight and obesity. Total mortality has a U-shaped relationship with BMI. Mortality rates sharply increase at BMIs of less than 18.5 and of greater than 30. In this issue of JAMA, Flegal and colleagues report their findings from a systematic review and meta-analysis of associations between standard NHLBI BMI categories for overweight and obesity and all-cause mortality. Using rigorous study selection criteria and statistical methods, and based on a sample of more than 2.88 million individuals with more than 270 000 deaths, they confirmed significantly increased all-cause mortality hazard ratios (HRs) relative to normal weight (defined as a BMI of 18.525) for overall obesity (grades 1, 2, and 3 combined; HR, 1.18 [95% CI, 1.12-1.25]) and grades 2 and 3 obesity (HR, 1.29 [95% CI, 1.18-1.41]). Higher all-cause mortality was not observed in individuals with grade 1 obesity. Mortality was significantly lower among those who were overweight (HR, 0.94 [95% CI, 0.91-0.96]) compared with normal weight individuals. The findings remained consistent even after adjusting for smoking status, preexisting disease, or weight and height reporting method (self or measured). The present investigation extends a 2005 study by Flegal et al based on the National Health and Nutrition Examination Survey that did not find an increased mortality risk in overweight persons. Among others with similar findings, McGee et al found no increase in all-cause mortality for overweight males (relative risk, 0.965 [95% CI, 0.922-1.009]) and females (relative risk, 0.968 [95% CI, 0.925-0.987]) in a 2005 meta-analysis including 388 622 individuals with 60 374 deaths. Orpana et al estimated the relationship between allcause mortality and BMI in a nationally representative sample of 11 326 Canadian adults and found a relative mortality risk of 0.76 (95% CI, 0.58-0.99) in overweight nonsmokers. Based on these observations, are the concerns about overweight as currently defined unfounded? Answering this question requires consideration of BMI as a health risk phenotype. Body mass index accounts for about two-thirds of the between-individual variation in total adiposity. Body mass index does not account for sex, race, age, and fitness differences in fat mass even at the same body weight. Race and age both influence the associations among disease risk, mortality, and BMI. Moreover, fat distribution varies widely