The effect of age on the shape of the BMI-mortality relation and BMI associated with minimum all-cause mortality in a large Austrian cohort

BACKGROUND: It is unclear if the body mass index (BMI) associated with minimum all-cause mortality is constant throughout adult life or increasing with age. METHODS: We applied multivariable fractional polynomials to the data of the Vorarlberg Health Monitoring and Prevention Program to quantify the BMI associated with minimum mortality over age. The analysis included data of 129 904 never-smoking women and men (mean age: 45.4 years) who were followed for a median of 18.6 years. RESULTS: Optimum BMI in women increased with age, lying within the normal BMI category (according to the World Health Organization definition) from the age of 20 years (23.3 kg m(-2), 95% confidence interval (CI): 22.2-24.3) to the age of 54 years and in the lower half of the overweight category from the age of 55 years onwards, reaching 26.2 kg m(-2) (95% CI: 25.1-27.3) at the age of 69 years. In men, optimum BMI increased slightly from 23.7 kg m(-2) (95% CI: 22.1-25.2) at the age of 20 years until the age of 59 years, reaching a BMI of 25.4 kg m(-2) (95% CI: 24.8-26.0) and decreased afterwards to 22.7 kg m(-2) (95% CI: 20.9-24.6) at the age of 80 years. CONCLUSIONS: Our results indicate that BMI associated with minimum all-cause mortality changes with age and that patterns differ by sex. Sex-and age-independent BMI recommendations might therefore be inappropriate. Further studies using flexible methods instead of predefined categories are necessary to revise BMI recommendations.