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The big problem with measuring Body Mass Index (BMI)

It has long been used to screen for obesity and assess health trends, but experts warn BMI doesn’t tell the whole story.
Woman sad on the couch
BMI is less reliable for women compared to men. Image: Getty

Most people know that good health hinges on more than a single number, yet there’s one figure that’s been punching above its weight for over 100 years.

Body Mass Index, or BMI, is calculated by taking a person’s weight and dividing it by their height squared. Doctors commonly use BMI to quickly categorise people as underweight (BMI less than 18.5), overweight (25 to 29.9), obese (30 or more), or somewhere in between (18.5 to 24.9).

In the research world, BMI is considered statistical gold and has been deemed the cheapest and easiest way to track and study obesity in the population.

BMI values are also commonplace in many other settings, including wellbeing apps, insurance forms and classrooms. They’ve even caused a stir within the fashion industry, with several countries, including Spain, Italy and France, using BMI cut-offs to ban ultra-thin models from catwalks and advertising.

However, health experts and advocates say that the popular measure is inherently flawed and harmful. So, is BMI a scam? And what should we use instead?

BMI only tells part of the story

Where BMI fails is that it doesn’t provide the full picture of a person’s health. While research shows that a higher BMI is linked to developing type 2 diabetes, high blood pressure, heart disease and some cancers, BMI alone can’t predict health risks.

“A person can have a ‘normal’ BMI but still have high blood pressure, high cholesterol and insulin resistance – all of which can increase heart disease risk,” says Professor Garry Jennings, Chief Medical Advisor at the Heart Foundation.

In addition, BMI doesn’t distinguish between fat and muscle. So, it can overestimate body fat in people with a muscular build, such as athletes, and underestimate body fat in older people who tend to lose muscle with age.

Female patient talking to GP doctor
Doctors commonly use BMI to quickly categorise people. Image: Getty

It also doesn’t consider differences across sex or ethnicity. For example, BMI is less reliable for women compared to men, particularly as body composition shifts with pregnancy and after menopause.

“A lot of my patients who are going through menopause will say to me, ‘I’m not doing anything different compared to 10 years ago, but I’m getting a little thicker around the middle’. It’s important to be mindful of that change,” says Dr Terri-Lynne South, Obesity Management Chair at the Royal Australian College of General Practitioners.

Professor Jennings adds that abdominal fat is more strongly linked to heart disease than fat distributed in other parts of the body.

Body fat is also influenced by ethnicity. For example, some South Asians and Asians may have a higher risk of disease at a lower BMI compared to other populations. As a result, Japan, China and India have all set lower BMI cut-offs for obesity compared to the international standard.

Problematic origins

BMI’s limitations are hardly surprising given it was only ever intended as a way to study populations, not individuals – and was validated in mostly Caucasian men.

In 1832, Belgian astronomer and statistician Adolphe Quetelet developed the formula to describe and study the average European man. More than a century later, US physiologist Ancel Keys built on this work and coined the term “body mass index” in his 1972 study, drawing on data from mostly European and American men.

In June 2023, the American Medical Association called out the outdated measure, citing “its historical harm” and “its use for racist exclusion” as reasons to move away from BMI as the sole indicator of health in clinical practice.

BMI fuels weight bias and stigma

Focusing on BMI alone to measure a person’s health stigmatises individuals and leads to weight discrimination, says Dr Fiona Willer, a practising dietitian and lecturer in nutrition and dietetics at the Queensland University of Technology.

“BMI is simple, and that’s why it gets away with so much. But you can’t assess someone’s health based on what they look like or their BMI.

“We’ve all heard of the person who went to the doctor about an ear infection and came out with weight loss advice. ‘You need to lose weight’ is often a cover for ‘I don’t know what’s wrong with you’. It’s not a good way to do healthcare,” Dr Willer says.

Woman with unbuttoned jeans
Body image issues, particularly among women, are still an issue for many Australians. Image: Getty

At the 2022 International Congress on Obesity, experts from the University of Sydney and Melbourne’s Austin Health spoke up about the dangers of BMI being used in the wrong way. Unrealistic body weight goals can “fuel weight bias and stigma, demotivate patients and promote eating disorders such as binge eating,” they wrote.

BMI cut-offs can also prevent someone from accessing health services, such as IVF or surgery.

“BMI is used as a gatekeeping measure for health services, and that’s weight discrimination,” Dr Willer says.

Australia’s body image crisis

Body image distress is a serious and growing problem, particularly among young Australians and women. It can lead to low self-esteem, anxiety, depression and self-harm, and be detrimental to a person’s life-long relationship with food and exercise.

According to the Butterfly Foundation, nine in 10 Australians aged 12-18 have concerns about their body image.

A 2021 Australian study found that, across all age groups, roughly half of the participants felt very distressed or preoccupied with their body image, despite many of them falling into the ‘normal’ BMI range.

Almost two-thirds of people who experience an eating disorder in Australia are women and girls. LGBTQIA+ youth have a greater risk than other young people.

New mums, who are faced with intense pressure from society to lose “baby weight”, are also at an increased risk of harm. A 2017 review of 12 studies found a link between higher body weight, body dissatisfaction and anxiety among first-year postpartum mums.

“We have this culture of thinness as an ideal, especially for women, which we are slowly moving away from, but we’re not there yet,” says Dr South.

Dr South says BMI still has a role in population statistics but interpreting it as a measure of individual health can be harmful to patients. She says the peak bodies are also working toward a less stigmatising definition of obesity.

Woman stretching after a run
Focusing on health instead of using weight as the sole measure is key say experts. Image: Getty

In another sign that the tide is turning, “BMI”, “diets”, and “calories” were among hundreds of terms officially removed from Australian classrooms earlier this year after tireless campaigning from body positivity advocates.

The Australian Curriculum, updated in February, advises teachers to avoid activities that cause unintentional harm to students, such as calculating their BMI and using food diaries.

Health, not weight

Experts say no single measure is perfect, but a combination of patient-centric care and other measurements will provide a clearer picture of a person’s health.

“Measuring waist size can provide insight into abdominal fat levels, which are strongly linked to heart disease and metabolic issues,” says Professor Jennings.

Diabetes Australia also suggests measuring waist circumference, since carrying extra weight around the waistline is among the risk factors for developing type 2 diabetes.

Other alternatives to BMI include waist-to-hip ratio, DEXA scans and skinfold measurements to assess health and body composition. Regardless of BMI, the Heart Foundation recommends a Heart Health Check, which is available through your GP.

Dr South says that waist-to-height ratio is another practical alternative to BMI – but cautions against replacing one simple measure with another. She suggests talking to a doctor to have an overall assessment instead.

Dr Willer recommends that anyone who has concerns about their weight see their GP or an accredited practising dietitian.

“Good health is supported centrally by good nutrition,” she says. “It’s important for health practitioners to look at a person’s life experience and their relationship with food and their body, rather than relying on a weight measurement alone.”

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