Imagine trying to work out if you are dangerously overweight without once hopping on to the bathroom scales.
That’s what a new method for determining body size aims to do, using a formula that divides your height by your waist size.
Called the body roundness index (BRI), it can reveal how much fat you have inside you, no matter what your build – and is claimed to be a huge step up from the crude results given by the body mass index (BMI) charts used by doctors for years.
And research suggests that as well as being far more accurate, the complex mathematical formula behind the BRI can also determine how likely it is this fat will lead to your premature death from serious obesity-related conditions such as heart disease, diabetes and cancer.
The BRI was devised more than a decade ago by researchers in the US, who were looking for more accurate ways to assess unhealthy weight gain. Dr Diana Thomas, a mathematician who came up with the new formula, told recently how she was looking in the mirror one day and thought: ‘I’m not a cylinder – I’m kind of more of an egg. But how do I capture that?’
For decades, doctors have been monitoring health using the BMI chart, which uses weight and height measurements to produce a reading that bluntly states if we are a healthy size, underweight or obese – a healthy BMI range is between 18.5 and 25.
But the system has long been acknowledged as flawed, as a muscle-bound athlete might be classed as obese simply because muscle is more dense than fat.
At the other end of the scale, a slim person with little muscle but with a stomach paunch – indicating a build-up of visceral fat – could be classed as healthy.
Visceral fat is the unhealthy type that accumulates around organs, such as the liver, heightening the risk of conditions such as type 2 diabetes and heart disease by disrupting the normal balance of glucose and insulin in the body.
Dr Diana Thomas, a mathematician who came up with the new formula called the body roundness index (BRI) which reveals how much fat you have inside you, no matter your build
In short, BMI assumes that if your weight is excessive then so are your levels of visceral fat.
With BRI, however, you tap your height and waist measurements into the BRI calculator – you can find one online. It then uses a complex formula to produce a score that reflects either how slim you are from head to toe, or whether you are more rounded in the middle due to visceral fat.
BRI scores range from 1 to 16 – those close to the lower end are likely to be underweight and in danger of developing thinning bones, infertility and a weakened immune system.
Those rated 3.5 to 7 are likely to be a healthy weight, with little excess abdominal fat. Anyone scoring over 7 is in the danger zone for obesity and poor health.
The BRI attracted interest after a study in the Journal of the American Medical Association Network Open, published in June, found that it accurately predicted which middle-aged men and women were at risk of dying from obesity-related conditions.
Scientists at Beijing University studied almost 33,000 adults who took part in a 20-year health study. They used volunteers’ data on height and waist size to work out each one’s BRI, before comparing this with deaths from all causes.
The results showed the risk of dying prematurely among those with the highest BRI was 49 per cent higher than in those in the lower range.
And a study published in the journal Lipids in Health and Disease in 2023 showed the BRI was also a good indicator of who is most likely to develop bowel cancer.
Researchers from China studied more than 53,000 people and found those with a high BRI score were up to five times more likely to fall victim to the disease than those with a low score.
Originally the BMI was devised to advise on public health strategies – not for quantifying an individual’s health – but was popularised as such in the US during the 1970s. A 2019 study published in the BMJ Open Sport and Exercise Medicine journal found many top England rugby players – according to their BMIs – qualified as obese.
Tom Sanders, a professor of nutrition and dietetics at King’s College London, told Good Health: ‘Body fat itself is not harmful – it is where it is stored that matters. Subcutaneous [just below the skin] fat on the hips and thighs is mainly harmless.’
But Professor Sanders questions whether the BRI is any more accurate an indicator for health than a tape measure. ‘The simplest and most robust measure of unhealthy fat is waist circumference,’ he says. ‘This is what the International Diabetes Federation recommends instead of BMI to calculate the risk of type 2 diabetes.’
To measure your waist properly, breathe out, relax your stomach and place the tape measure just above the belly button.
The NHS says anything above 37in (94cm) in men and 31.5in (80cm) in women is considered a high risk for ill health. The threshold is lower in men of African Caribbean, South Asian or Chinese origin – around 35.5in (90cm).
A paper published earlier this month in Current Problems in Cardiology suggested both BRI and BMI may not be as good as waist circumference at predicting who will develop type 2 diabetes.
Mike Lean, a professor of human nutrition at Glasgow University, says: ‘Stick with the measure that clearly works – waistline size.’
