Adults Classified As Overweight May Have Lower Mortality Risk Than Normal Weight Adults
EBM Focus - Volume 8, Issue 2
The question of optimal weight from a health perspective is of great interest to clinicians and patients alike. Results from studies evaluating mortality risks that are based on body mass index (BMI) may be difficult to reconcile with each other due to differing definitions of BMI categories used across studies. A recent systematic review of 97 prospective cohort studies assessed all-cause mortality risk in a population of more than 2.88 million persons using standard of weight classifications as defined by the World Health Organization. Normal weight is defined as BMI 18.5-24.9 kg/m2 and overweight as BMI 25-29.9 kg/m2. Obesity was stratified to 3 classes: class 1 includes BMI 30-34.9 kg/m2, class 2 is BMI 35-39.9 kg/m2, and class 3 is BMI ≥ 40 kg/m2.
During follow-up ranging from 3-42 years, there were more than 270,000 deaths. Compared to normal weight, mortality was significantly reduced in persons who were classified as overweight (hazard ratio 0.94, 95% CI 0.91-0.96) in analysis of 93 studies. There was no significant difference in mortality comparing normal weight to class 1 obesity, but mortality was increased in persons with class 2 or 3 obesity (BMI ≥ 35 kg/m2) (hazard ratio 1.29, 95% CI 1.18-1.41) in analysis of 32 studies. In subgroup analyses of persons ≥ 65 years old, being overweight was again associated with reduced mortality compared to normal weight (hazard ratio 0.9, 95% CI 0.86-0.95), and there were no significant differences in mortality comparing normal weight to any obesity category (JAMA 2013 Jan 2;309(1):71).
Previous research has suggested that mortality is lowest for BMI 22.5-25 kg/m2 and somewhat higher at the lower extreme of “normal” as well as at higher levels (Lancet 2009 Mar 28;373(9669):1083). The reduction in mortality associated with being overweight in the current study may be due, in part, to the wide BMI range considered normal by the WHO classification. Nevertheless, these data suggest that categorizing people as overweight or obese by BMI alone may not provide a valid assessment of health risk.