Body mass index (BMI) may not increase mortality independently of other risk factors in adults, according to a new study that stressed the need for incorporating other factors.

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 analysed the association between BMI and all-cause mortality have been inconsistent.

To understand, a team from Rutgers University in the US retrospectively studied data on 554,332 US adults.

Of these, about 35 per cent had a BMI between 25 and 30, which is typically defined as overweight, and 27.2 per cent had a BMI above or equal to 30, typically defined as obese.

Over a median follow-up of nine 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 and in 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 per cent to 108 per cent 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.

Further studies incorporating weight history, body composition and morbidity outcomes are needed to fully characterise BMI-mortality associations, said researchers in the paper published in the open-access journal PLOS ONE.

BMI in the overweight range is generally not associated with increased risk of all-cause mortality, they said.

"Our study highlights the increasing reservations about 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 the incidence of cardio-metabolic morbidities," they added.