BMI as main mortality factor in overweight adults sparks research discussion

06 Jul 2023 --- Body Mass Index (BMI) may not increase mortality independently of other risk factors in obese adults, according to researchers from Rutgers University, US.

Published in Plos One, the study reveals how previous US studies analyzing the association between BMI and all-cause mortality had been inconsistent and used data between 1960-1990.

“Our study highlights the increasing reservations about using BMI alone to drive clinical decisions,” say the study authors, Aayush Visaria and Soko Setoguchi.

“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.”

UK-based researchers have called for clearer messaging on the disease of obesity to help the public and policymakers better understand it and to improve prevention and treatment. They highlight that the disease centers on mechanisms that regulate dietary intake, not on body weight or BMI, which is how people generally define obesity. 

Beyond BMI
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, details the researchers. 

The current study’s authors say further studies incorporating weight history, body composition and morbidity outcomes are needed to characterize BMI-mortality associations. BMI in the overweight range is generally not associated with an increased risk of all-cause mortality.A BMI of 30 is still associated with a third of the US population.

The researchers 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. 

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. 

In summary, for adults with a BMI of 30 or more, a 21-108% increase in mortality risk was attributed to their weight. The patterns observed in the overall population remained largely the same in men and women.

Experts react
Several members of the scientific community have responded to the findings of the study. One response argues that the paper offers convincing evidence contesting the validity of BMI as a marker of health.

“BMI only reflects body weight adjusting for height, as a proxy for ‘excess’ weight. From a health perspective, it is assumed that excess weight directly reflects excess body fat, which is associated with increased health risk,” says Dr. Adam Collins, associate professor of Nutrition at the University of Surrey.

“The observation that within the BMI range of 22.5 to 30 kg per m2, no change in mortality is likely a consequence of the relatively weak association between BMI and adiposity within this BMI range. As the authors acknowledged, differences in lean mass between individuals are a potential critical confounder without measures of body composition.”

BMI still an indicator?New research weakens the association between BMI and all-cause mortality.
Dr. Katarina Kos, senior lecturer in diabetes and obesity at the University of Exeter, UK, states that the report of a somewhat lower risk of dying in the overweight BMI range is surprising and strongly contrasted with the 21-108% increased risk of mortality for men and women with a BMI above 30 kg per m2.

“This is a US study and data is based on the US body shape, and the weight data and reported illness were obtained from individual’s self-reports. These shortcomings will, to some extent, be reflected in the data and make interpretation difficult. It is not possible to extrapolate how applicable the observed trend is to the British population.”

According to Dr. George Savva, a statistician at Quadram Institute Bioscience, UK, the paper shows no associations with mortality for those who might be slightly overweight (BMI between 25 and 27.5) compared to those at a ‘healthy’ weight (BMI 20-24.9).

“Overall, the results reveal a typical U-shaped relationship between weight and mortality, with those at the extremes of high and low BMI having the highest death rates in many groups, and a smaller effect for those closer to a ‘healthy’ weight,” says Savva. 

“It’s important to remember that a BMI of 30 or more still includes around a third of the US population and more than one in four UK adults, and so even if the excess risk is restricted to this group as the paper suggests, it still potentially affects a very large number of people, particularly in middle age.”

By Inga de Jong