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Body mass index: why practitioners are relying on it less when looking at a patient’s health

Someone's feet standing on a weighing scale with a tape measure on the floor in front

Body mass index has long been used by doctors as the standard method for measuring health – and often still is. But earlier this month, the American Medical Association (AMA) adopted a new policy that cautions medical professionals from using the body mass index (BMI) as a stand-alone clinical tool during patient consultations.

In 2022, England’s National Institute for Health and Care Excellence (Nice) also made similar changes to obesity guidelines, recommending that waist-to-hip ratio be used alongside BMI.

It’s hardly surprising this new policy recommendation has been made given the widespread recognition in recent years that BMI has many limitations – particularly when used to make judgements about body weight, and, importantly, health.

Such issues are perhaps expected considering the origins of the BMI and its intended purpose. Body mass index was created in 1832 by Belgian mathematician Adolphe Quetelet. The Quetelet index, as it was originally called, was designed as a tool to study health in populations – not individuals.

The unintended use of the BMI to classify a person’s weight status came in 1995, after the World Health Organization published what we now consider the standard BMI criteria. Interestingly, the subsequent misuse of BMI was an indirect consequence of this formal categorisation, as even the World Health Organization explicitly stated in this report that BMI should always be interpreted alongside other determinants of health.

BMI is calculated by taking a person’s weight in kilograms and dividing it by their height in metres squared. The result is then used to categorise a person’s weight status. A BMI of 18.5-24.9 is considered healthy weight, while a BMI of 25.0 – 29.9 is considered overweight, and over 30 indicates obesity.

But while BMI is a quick and easy way to get a general picture of a person’s weight status, there are many limitations with only using this to determine a person’s health.

First, BMI doesn’t account for body composition – the proportion of fat, muscle and bone a person has. This is important to know because excess body fat is what may increase our risk of certain health conditions. This means that people who are muscular, such as athletes, may have high BMI values despite having low body fat. This could lead to an incorrect assumption that they’re unhealthy.

BMI also doesn’t consider where a person stores their body fat. This is important, as body fat stored in certain areas may carry greater health risks.

F‌or the full article by Dr James KingProfessor David Stensel, and Dr Dimitris Papamargaritis (University of Leicester) visit The Conversation