BMI is an 'imperfect' way to measure health. But replacing it is complicated

Vishal Gentle isn't the avid gym-goer he was before his daughter was born a few months ago.

But the new dad said he does what he can to stay active — carrying the baby up and down stairs to get a bit of a workout, lots of walking and using a stand-up desk. 

Gentle said he feels healthy, but according to the body mass index, or BMI, he's overweight.

"Honestly, I don't feel overweight. I mean, except a little bit near my waist. But like I'm fully active. I can do anything and everything," he said during an interview in his Toronto home. 

Gentle isn't alone when it comes to questioning if his BMI is an accurate way of measuring health. The BMI is under growing scrutiny — both for its usefulness as a measure of health and its problematic origins.

BMI is calculated by dividing weight in kilograms by height in metres squared. The result can be compared to the BMI chart, easily found all over the internet, social media and in doctors' offices, that instantly advises the user if their result is underweight, normal, overweight or obese.

Last month, the American Medical Association (AMA) formally recognized BMI's shortcomings and created a new policy advising caution when using it. 

In a news release in June, the AMA warned that the BMI is an "imperfect way to measure body fat in multiple groups given that it does not account for differences across race/ethnic groups, sexes, genders, and age-span."

It also notes that the BMI, which dates back to the 19th century, has a problematic foundation.

"Under the newly adopted policy, the AMA recognizes issues with using BMI as a measurement due to its historical harm, its use for racist exclusion, and because BMI is based primarily on data collected from previous generations of non-Hispanic white populations," the news release states.

The AMA encouraged clinicians to use other factors such as waist circumference, fat distribution and genetic factors when assessing a patient's health. 

The AMA's position is the latest in mounting criticism of the BMI. 

The inability to distinguish between mass created by fat, muscle and bone has been noted as a shortcoming by experts and health authorities, as is the lack of distinction for where fat is distributed on the body. 

They note abdomen fat can be a risk factor for conditions like diabetes and heart disease, while hip and thigh fat is less associated with health issues.