The Body Mass Index – or BMI – has been used for the past half century as a standard measurement tool for weight and obesity. It’s calculated by dividing a person’s weight in kilograms by the square of their height in metres.

This produces a figure which is indexed on a spectrum of weight ranges. A BMI below 18.5 is considered underweight, above 25 is overweight, and above 30 is obese.

The latest episode of Debunks, a new podcast from Cosmos, investigates how useful the BMI actually is for assessing health.

Health advocacy bodies, health insurers and government departments all make reference to the BMI as being a globally recognised standard for weight classification.

Most – but not all – acknowledge that the tool is imperfect. Its simple arithmetic is based on a system devised by 19th-century Belgian mathematician Adolphe Quetelet, who was an early pioneer of the social sciences and the use of data to understand human trends.

The so-called ‘Quetelet Index’ was first described in 1832 as a means of trying to identify a calculation for the average man, first by conducting cross-sectional studies of infants and then adults.

The need to consider weight as an indicator for health, mortality and morbidity, saw scientists trial several measurements before settling on Quetelet’s formula and rebranding it as the BMI in 1972.

The problem? The BMI was largely based on studies of Anglo-Saxon populations. This is one of the biggest limitations often recognised by health groups. The Australian Department of Health, for instance, notes that a healthy BMI range is generally lower for people of Asian backgrounds, and higher for those of Polynesian backgrounds.

But ethnicity isn’t the only limiting factor. Age and pregnancy status also play a part. Even athletes with more lean muscle (which weighs more than fat) might also find the standard BMI doesn’t capture their health status accurately.

Diets and lifestyles have also shifted from the 19th century Belgian standard, and even from those of 50 years ago. Health professionals have long supported a shift away from the BMI being used as a rolled gold indicator of individual health, and medical professionals are beginning to take a wider view of patient health.

“There has been a recent change in the position from the [US] National Academies of Nutrition and Dietetics surrounding BMI and there are shifts in the guidelines around BMI for medical diagnosis,” Dr Emma Beckett, a molecular nutritionist at the University of Newcastle, tells Debunks.

The same goes for other measurements like waist-to-hip ratios and waist circumference. These metrics are often used by researchers conducting large population studies, but they don’t necessarily explain a person’s ‘health picture’.

“Because we measure them in so many of our research studies, people mistakenly believe they are the most important markers of health and it’s just not true. Health is so much more complicated,” Beckett says.

“The ‘normal’ [BMI] category is the one with the lowest health risks, but it doesn’t mean being in that category means you have no health risks and it doesn’t mean if you just get yourself into that category and change nothing else, there are no health risks.”

On the latest series of Debunks, a podcast from Cosmos and 9Podcasts, find out how weight – and measurements like the BMI – are much more complicated than they might seem.