Body Mass Index (BMI) is an incomplete method of determining risk factors for type 2 diabetes, heart disease and all-cause mortality, according to new research.
The findings of a new study, published in the Annals of Internal Medicine, highlight that body fat percentage and physical fitness are superior to BMI in predicting these risks.
A study team led by Raj Padwal, MD, MSc, examined 49,476 women and 4,994 men who had been referred for bone mineral density testing. The average age of the cohort was 40 years.
4,965 women died during a median of 6.7 years and 984 men died over a median of 4.5 years. The researchers then used fully adjusted mortality models that included BMI and body fat percentage to conclude that low BMI and high body fat percentage was associated with increased mortality.
The researchers wrote: “Body mass index is widely used as a proxy for adiposity even though it more closely reflects lean mass than fat mass. Increasing BMI may therefore reflect higher fitness levels, greater metabolic reserve, and less cachexia – factors that are associated with greater survival – rather than increasing fat.”
Because BMI doesn’t take into account whole-body composition (fat, bone and muscle), Padwal’s team believe BMI is an incomplete method of determining a person’s mortality risk.
Study author William D. Leslie, a professor of medicine and radiology at the University of Manitoba, added: “Another problem with BMI: It can be skewed by muscle mass. The BMI is far from a perfect tool because it doesn’t differentiate between the person who has a spare tire around their waist from a person who is large because of lean muscle.”
Padwal explained that while previous studies have linked elevated BMI to higher survival, these studies did not account directly for adiposity (the state of being fat).
The researchers hope these findings could “help explain the counterintuitive relationship between BMI and mortality”, and encourage people to live healthily rather than pushing to reach a particular BMI target range.

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