Body Mass Index and Mortality in a Very Large Cohort: Is It Really Healthier to Be Overweight?

Context: Controversy persists about optimal body weight. Many experts define "normal" (healthy) body mass index (BMI) as 18.5 to 24.9 kg/m2, 25 to 29.9 kg/m2 as overweight, and 30 kg/m2 or greater as obese. Obesity is subdivided into 30 to 34.9 kg/m2 (Grade 1), 35 to 39.9 kg/m2 (Grade 2), and 40 kg/m2 and above (Grade 3). Studies consistently show higher mortality for underweight and Grade 2 or 3 obesity, but results conflict for the overweight category and Grade 1 obesity. Objective: To study 30-year risk of death related to baseline BMI.

Design: Retrospective cohort study in a multiracial population of 273,843 persons using logistic regression with 7 covariates (sex, age, race-ethnicity, education, marital status, smoking, alcohol intake). Main outcome measures: Mortality risk by baseline BMI.

Results: With average follow-up exceeding 30 years, there were 103,218 deaths: 41,215 attributed to cardiovascular causes and 62,003 to noncardiovascular causes. Odds ratios (and 95% confidence intervals) for all deaths in BMI categories, with a BMI of 18.5 to 24.9 kg/m2 as the referent, were BMI below 18.5 kg/m2 = 1.1 (1.0-2.0), BMI 25 to 29 kg/m2 = 1.1 (1.1-1.2), BMI 30 to 34 kg/m2 = 1.5 (1.4-1.5), BMI 35 to 39 kg/m2 = 2.1 (1.9-2.3), and BMI 40 kg/m2 or higher = 2.7 (2.4-3.0). Disparities existed regarding age, race/ethnicity, cause of death, and interval to death.

Conclusion: Compared with persons with BMI defined as normal, persons who were underweight, overweight, and obese were at increased risk of death over 30 years.