Over the last 3 decades, the BMI value associated with the lowest risk for all-cause mortality has increased in the general population, researchers reported in the Journal of the American Medical Association.
“Previous findings indicate that while average BMI has increased over time in most countries, the prevalence of cardiovascular risk factors may be decreasing among obese individuals over time,” they wrote. “Thus, the BMI associated with lowest all-cause mortality may have changed over time.”
The researchers noted, however, that there is a lack of data addressing this topic, while the studies that do exist may be limited by their designs.
To determine whether the BMI value associated with the lowest all-cause mortality has increased, the researchers evaluated 3 cohorts from the general population in Denmark enrolled at different time periods: the Copenhagen City Heart Study from 1976 to 1978 (n=13 704) and 1991 to 1994 (n=9482) and the Copenhagen General Population Study from 2003 to 2013 (n=97 362). Participants were followed to November 2014, emigration, or death.
During follow-up, the number of deaths totaled 10 624 in the 1976 to 1978 cohort (78% cumulative mortality; mortality rate, 30 per 1000 person-years), 5025 in the 1991 to 1994 cohort (53% cumulative mortality; mortality rate, 16 per 1000 person-years), and 5580 in the 2003 to 2013 cohort (6% cumulative mortality; mortality rate, 4 per 1000 person-years).
With the exception of cancer mortality, results suggested a U-shaped association between BMI and all-cause, cardiovascular, and other mortality.
Over 3 decades, the BMI value associated with the lowest all-cause mortality increased by 3.3, from 23.7 in the 1976 to 1978 cohort to 27.0 in the 2003 to 2013 cohort.
Additionally, the risk for all-cause mortality associated with higher BMI decreased over 30 years. In the 1976 to 1978 cohort, the multivariable-adjusted hazard ratio (HR) was 1.31 (95% CI, 1.23-1.39) for BMI of 30 or greater vs 18.5 to 24.9. The HRs were 1.13 (95% CI, 1.04-1.22) in the 1991 to 1994 cohort and 0.99 (95% CI, 0.92-1.07) in the 2003 to 2013 cohort.
“These latter findings were robust in analyses stratified by age, sex, smoking status, and history of cardiovascular disease, or cancer,” the researchers wrote.
Interestingly, the researchers noted, BMI in the overweight category appeared to be optimal in relation to mortality. These values were 23.7 (95% CI, 23.4-24.3) in the 1976 to 1978 cohort, 24.6 (95% CI, 24.0-26.3) in the 1991 to 1994 cohort, and 27.0 (95% CI, 26.5-27.6) in the 2003 to 2013 cohort.
“This finding was consistent in both the whole population sample (optimal BMI, 27) and in a subgroup of never-smokers without history of cardiovascular disease or cancer (optimal BMI, 26.1),” they wrote. “If this finding is confirmed in other studies, it would indicate a need to revise the [World Health Organization] categories presently used to define overweight, which are based on data from before the 1990s.”
Although the study had several strengths, including the fact that the 3 samples were drawn from the same population in Copenhagen, the researchers noted that it also had limitations, such as different follow-up time within the cohorts and the overlapping of the 1976 to 1978 cohort and the 1991 to 1994 cohorts.
Regarding their findings, the researchers concluded, “Further investigation is needed to understand the reason for this change and its implications.”