Study: BMI may not increase mortality independently of other risk factors

Body mass index (BMI) may not increase mortality independently of other risk factors in adults, according to a new study published this week in the open-access journal PLOS ONE by Aayush Visaria and Soko Setoguchi of Rutgers University, US.

The prevalence of overweight and obesity has risen dramatically over the last 25 years, and it is well-established that elevated BMI can contribute to several cardio-metabolic conditions. However, studies that have analyzed the association between BMI and all-cause mortality have been inconsistent. Most US studies have used data from the 1960s through 1990s and have included predominantly non-Hispanic White adults.

In the new work, the researchers retrospectively studied data on 554,332 US adults from the 1999-2018 National Health Interview Survey and the 2019 US National Death Index. BMI was calculated using self-reported height and weight and participants were divided into 9 BMI categories for the analysis. Information on demographics, socio-behavioral factors, comorbidities and healthcare access was also available. On average, participants were 46 years old, 50% female and 69% non-Hispanic White. 35% of those included in the study had a BMI between 25 and 30, which is typically defined as overweight, and 27.2% had a BMI above or equal to 30, typically defined as obese.

Over a median follow-up of 9 years and a maximum follow-up of 20 years, the researchers observed 75,807 deaths. The risk of all-cause mortality was similar across a wide range of BMI categories. For older adults, there was no significant increase in mortality for any BMI between 22.5 and 34.9, which extends into the BMI categories typically considered obese. For younger adults, there was no significant increase in mortality for any BMI between 22.5 and 27.4. Overall, for adults with a BMI of 30 or over, there was a 21% to 108% increased mortality risk attributed to their weight. The patterns observed in the overall population remained largely the same in men and women and across races and ethnicities.

The authors conclude that further studies incorporating weight history, body composition and morbidity outcomes are needed to fully characterize BMI-mortality associations, but say that BMI in the overweight range is generally not associated with increased risk of all-cause mortality.

The authors add: "Our study highlights the increasing reservations of using BMI alone to drive clinical decisions. There is no clear increase in all-cause mortality across a range of traditionally normal and overweight BMI ranges; however, that is not to say that morbidity is similar across these BMI ranges. Future studies will need to assess incidence of cardio-metabolic morbidities."