Change in Body Mass Index Associated With Lowest Mortality in Denmark, 1976-2013

Importance

Research has shown a U-shaped pattern in the association of body mass index (BMI) with mortality. Although average BMI has increased over time in most countries, the prevalence of cardiovascular risk factors may also be decreasing among obese individuals over time. Thus, the BMI associated with lowest all-cause mortality may have changed.

Objective

To determine whether the BMI value that is associated with the lowest all-cause mortality has increased in the general population over a period of 3 decades.

Design, Setting, and Participants

Three cohorts from the same general population enrolled at different times: the Copenhagen City Heart Study in 1976-1978 (n = 13 704) and 1991-1994 (n = 9482) and the Copenhagen General Population Study in 2003-2013 (n = 97 362). All participants were followed up from inclusion in the studies to November 2014, emigration, or death, whichever came first.

Exposures

For observational studies, BMI was modeled using splines and in categories defined by the World Health Organization. Body mass index was calculated as weight in kilograms divided by height in meters squared.

Main Outcomes and Measures

Main outcome was all-cause mortality and secondary outcomes were cause-specific mortality.

Results

The number of deaths during follow-up was 10 624 in the 1976-1978 cohort (78% cumulative mortality; mortality rate [MR], 30/1000 person-years [95% CI, 20-46]), 5025 in the 1991-1994 cohort (53%; MR, 16/1000 person-years [95% CI, 9-30]), and 5580 in the 2003-2013 cohort (6%; MR, 4/1000 person-years [95% CI, 1-10]). Except for cancer mortality, the association of BMI with all-cause, cardiovascular, and other mortality was curvilinear (U-shaped). The BMI value that was associated with the lowest all-cause mortality was 23.7 (95% CI, 23.4-24.3) in the 1976-1978 cohort, 24.6 (95% CI, 24.0-26.3) in the 1991-1994 cohort, and 27.0 (95% CI, 26.5-27.6) in the 2003-2013 cohort. The corresponding BMI estimates for cardiovascular mortality were 23.2 (95% CI, 22.6-23.7), 24.0 (95% CI, 23.4-25.0), and 26.4 (95% CI, 24.1-27.4), respectively, and for other mortality, 24.1 (95% CI, 23.5-25.9), 26.8 (95% CI, 26.1-27.9), and 27.8 (95% CI, 27.1-29.6), respectively. The multivariable-adjusted hazard ratios for all-cause mortality for BMI of 30 or more vs BMI of 18.5 to 24.9 were 1.31 (95% CI, 1.23-1.39; MR, 46/1000 person-years [95% CI, 32-66] vs 28/1000 person-years [95% CI, 18-45]) in the 1976-1978 cohort, 1.13 (95% CI, 1.04-1.22; MR, 28/1000 person-years [95% CI, 17-47] vs 15/1000 person-years [95% CI, 7-31]) in the 1991-1994 cohort, and 0.99 (95% CI, 0.92-1.07; MR, 5/1000 person-years [95% CI, 2-12] vs 4/1000 person-years [95% CI, 1-11]) in the 2003-2013 cohort.

Conclusions and Relevance

Among 3 Danish cohorts, the BMI associated with the lowest all-cause mortality increased by 3.3 from cohorts enrolled from 1976-1978 through 2003-2013. Further investigation is needed to understand the reason for this change and its implications.