The body mass index associated with lowest risk of death is going up

In a study appearing in the May 10, 2016 issue of JAMA, Børge G. Nordestgaard, M.D., D.M.Sc., of Copenhagen University Hospital, Herlev, Denmark and colleagues examined whether the body mass index (BMI) value that is associated with the lowest all-cause mortality has increased in the general population over a period of 3 decades.

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. Thus, the BMI associated with lowest all-cause mortality may have changed over time. This study included three groups from the same general population enrolled at different times: the Copenhagen City Heart Study in 1976-1978 (n = 13,704) and 1991-1994 (n = 9,482) 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.

The researchers found that the BMI value associated with the lowest all-cause mortality has increased by 3.3 over 3 decades from 1976-1978 to 2003-2013, from 23.7 to 27. In addition, the risk for all-cause mortality that was associated with BMI of 30 or greater vs BMI of 18.5 to 24.9 decreased from an adjusted hazard ratio of 1.3 to 1.0 over this 30-year period. "These latter findings were robust in analyses stratified by age, sex, smoking status, and history of cardiovascular disease or cancer."

The authors write that an interesting finding in this study is that the optimal BMI in relation to mortality is placed in the overweight category in the most recent 2003-2013 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). If this finding is confirmed in other studies, it would indicate a need to revise the WHO categories presently used to define overweight, which are based on data from before the 1990s."

Regarding the increase in the BMI value associated with the lowest all-cause mortality, the researchers write that "further investigation is needed to understand the reason for this change and its implications."