Effect of Secular Trend, Age, and Length of Follow-up on Optimum Body Mass Index From 1985 Through 2015 in a Large Austrian Cohort

BACKGROUND: Obesity and its health consequences will dominate health care systems in many countries during the next decades. However, the body mass index (BMI) optimum in relation to all-cause mortality is still a matter of debate. MATERIAL AND METHODS: Data of the Vorarlberg Health Monitoring & Prevention Program (VHM&PP, 1985–2005) and data provided by the Main Association of Austrian Social Security Institutions (MAASSI, 2005–2015) were analyzed. Information was available on age, sex, smoking status, measured height and weight, and mortality. Generalized additive models were used to model mortality as a function of BMI, calendar time, age, and follow-up. RESULTS: In MAASSI (N = 282,216, 46.0% men), men and women were on average 2.7 years older than in VHM&PP (N = 185,361, 46.1% men). Average BMI was slightly higher in men (26.1 vs 25.7 kg/m(2)) but not in women (24.6 vs 24.7 kg/m(2)). We found an interactive effect of age and follow-up on the BMI optimum. Over age 35 years in men and 55 years in women, the BMI optimum decreased with length of follow-up. While keeping covariates fixed, BMI optimum increased slightly between 1985 and 2015 in men and women, 24.9 (95% CI, 23.9–25.9) to 26.4 (95% CI, 25.3–27.3), and 22.4 (95% CI, 21.7–23.1) to 23.3 (95% CI, 22.6–24.5) kg/m(2), respectively. CONCLUSION: Age and length of follow-up have a pronounced effect on the BMI associated with the lowest all-cause mortality. After controlling for age and length of follow-up, the BMI optimum increased slightly over 30 years in this large study sample.