By Rachel Rasker, ABC
Photo: 123rf
If you had a health check-up recently, you might have been told that you're underweight, overweight or obese based on your body mass index or BMI.
But the BMI was never intended as a diagnostic tool.
At best, experts have called it "inaccurate"; at worst, it's been branded "nonsense".
Yet the BMI has become embedded in our healthcare system, with those scoring too high or low excluded from accessing essential services such as medications and surgery.
Now, there's a new formula on the block.
Why are we still using BMI if we know its flaws?
The body roundness index (BRI) is gaining traction as a possible replacement for the BMI, with a recent study of more than 30,000 US adults finding a correlation between BRI and mortality risk.
But health professionals have their hesitations, so don't expect to see it in your GP's office just yet.
The controversy around the BMI
Your BMI is calculated by taking your weight in kilograms and dividing it by your height in centimetres squared.
It's quick and easy to measure, and is intended to calculate someone's health risk based on body size.
It does this by sorting you into one of three categories:
- a BMI under 18.5 is classed as "underweight"
- 18.5 to 24.9 is "healthy", and
- over 25 is "overweight" or "obese".
But the index is often criticised as inaccurate, especially if your body is different to that of a 19th-century European man (the original model for the BMI).
It's biased against certain ethnicities, and - given it's based on your total weight - doesn't account for muscle mass, with athletes such as rugby star Ilona Maher classed in the higher "unhealthy" categories.
University of Sydney health and obesity expert Dr Nick Fuller said industry experts "worry about [the BMI's] accuracy", as well as our healthcare system's "fixation on using it as a single indicator of health".
Enter: the BRI
The body roundness index is also calculated with a formula, albeit a more complicated one than the BMI.
The most common way uses a person's height and waist circumference.
Dr Fuller said this waist measurement can more accurately indicate someone's body fat distribution.
"Fat stored around the stomach [means] your risk of disease is much higher" compared to when it's stored around the hips, thighs or buttocks.
This is because fat around your abdominal area is an indicator of the amount of visceral fat you have.
"That's the dangerous fat, the type of fat that's lying deep inside the belly, that surrounds the organs and prevents them from doing their job," Fuller said.
Terri-Lynne South, who is a GP, dietitian and obesity expert with the Royal Australian College of General Practitioners, said "we've known for some time that a single waist circumference measure is a much better idea of that increased visceral risk [than the BMI alone]".
Is the BRI a better alternative to the BMI?
The BRI's reliance on waist circumference, which is part of its appeal, is also its main drawback.
"It's relying on consistent and reliable waist measurements being taken," Fuller said.
"Believe it or not, waist circumference can be quite challenging to take. You do need to standardise that measure … because otherwise you can get a huge discrepancy."
He said measuring technique could vary between practitioners and could be even less accurate for those attempting to take their BRI at home.
Dr South agreed: "Line up 10 different people and ask them to measure a waist, you get 10 centimetres of difference [between them]."
Despite its shortfalls, the BMI is based on an objective number, which anyone can check at home with a simple set of scales.
So while the BRI "does seem on the surface to be a better tool, it's probably a bit too early to advocate for replacement just yet", South said.
"It needs a lot more research, and we need to see it moving into agreed-upon clinical guidelines."
Is the BRI more inclusive than the BMI?
While the BRI could deliver more accurate results for a wider range of body types, a waist measurement may feel uncomfortable or invasive for people who've experienced weight stigma in the healthcare system.
South said the name "body roundness index" isn't ideal either, suggesting "body risk index" may be more appropriate.
Given potential inaccuracies when measuring a waist at home via a telehealth, the BRI may also exclude regional and remote communities without easy access to health services, those who can't afford routine check-ups, and people who don't feel welcome in health settings.
"It can take a lot of courage for a person living in a larger body to even seek care … because of that shame and stigma and discrimination they face," South said.
Why don't we switch to the BRI now?
Even if the BRI was an ideal measure, with a plethora of research to back it, overhauling an entire medical system built on the BMI would be a significant challenge.
The Royal Australian College of GPs continues to recommend the BMI to its doctors, "but I do think the tide is slowly changing", South said.
"It's still used as a tool, but we're getting more of the message [out there] that it's a screening tool rather than a diagnostic tool."
For Fuller, "there's a lot of limitations of the body mass index, but then there's also limitations with the body roundness index … we don't want to be just relying on one calculation.
"We've got to start using a battery of different tests to assess a person's overall health."
He said BMI, waist circumference, blood pressure, blood tests, cardiovascular and diabetes risk factors should all form part of the equation.
"We don't just want to move from one crude measure to another."
-ABC