BODY MASS INDEX AND MORTALITY IN A VERY LARGE COHORT: IS IT REALLY HEALTHIER TO BE OVERWEIGHT?



Arthur L. Klatsky, M.D., Northern CA Kaiser Permanente Division of Research, Oakland, CA, USA

Purpose: To study risk of death in relation to body mass index (BMI) in 273,843 free-living persons.

Background: Despite substantial published literature controversy persists about the optimal level of body weight. The overall BMI-mortality risk is J-shaped, with underweight persons and obese persons at increased risk. Many experts define “normal” BMI as 18.5-24.9 kg/m2, with 25-29.9 kg/m2 as “overweight” and =30 kg/m2 as “obese”. Obesity is subdivided into 30-34.9 kg/m2 (grade I), 35-39.9 kg/m2 (grade II) and =40kg/m2 (grade III). Studies consistently show higher mortality for grade II-III obesity, but results conflict for the “overweight” category and even grade I obesity.

Methods: We used logistic regression with 8 covariates including smoking; the BMI referent was 18.5-24.9 kg/m2.

Results: With average follow-up of >30 years, there were 103,218 deaths, 41,215 attributed to cardiovascular (CV) causes and 62,003 to non-CV causes. Hazard ratios (HR) and [95% confidence intervals] for all deaths in relation to BMI were: <18.5=1.1 [1.0-2.0], 25-29=1.1 [1.1-1.2], 30-34=1.5 [1.4-1.5], 35-39=2.1 [1.9-2.3], and =40=2.7 [2.4-3.0]. Increased risk of persons with BMI below 18.5 kg/m2 was concentrated in non-CV deaths; for CV deaths these persons had a HR of 0.7 (0.6-0.7). For the overweight and grade 1 obesity categories, the HRs were 1.4 and 1.8 for CV deaths; for non-CV deaths these HRs were 1.0.

Conclusions: These data show the importance of examining causes of death when considering risks associated with underweight, overweight and obesity. For risk of CV death it is better to be thin.