BMI Associated With Lowest All-Cause Mortality Has Increased

The optimal BMI for lowest all-cause mortality increased by 3.3, from 23.7 in 1976-1978 to 24.6 in 1991-1994 and to 27 in 2003-2013, according to a new study.

“Another interesting finding is that the optimal BMI in relation to lowest mortality is placed in the overweight category in the most recent 2003-2013 cohort,” said study author Børge G. Nordestgaard, MD, DMSc, chief physician in clinical biochemistry at Copenhagen University Hospital, Denmark.
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The researchers examined 3 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). They followed up with all participants from inclusion in the studies to November 2014, emigration, or death, whichever came first. Nordestgaard cites 100% complete data in Danish national registries with respect to all-cause mortality as a particular study strength.

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 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.

“The overall results surprised me–particularly how clear cut they were,” said Nordestgaard, who is planning future research to understand the reason for this change over the past 30-40 years. “Compared to in the 1970s, today’s overweight individuals have lower mortality than so-called normal weight individuals.”

—Mike Bederka

Reference:

Afzal S, Tybjærg-Hansen A, Jensen GB, Nordestgaard BG. Change in body mass index associated with lowest mortality in Denmark, 1976-2013. JAMA. 2016;315(18):1989-1996. doi: 10.1001/jama.2016.4666.