At some point or another, you’ll have worked out your BMI. Body mass index, calculated by dividing your weight in kilograms by your height in metres squared, is the formula used to diagnose obesity, and by clinicians and researchers to predict a person’s risk of all manner of health conditions, from anorexia to diabetes and Covid-19.
ut is it doing more harm than good? A large body of research now suggests it is an outdated and ineffective way to measure how your weight affects your health. Many experts argue that BMI is a blunt tool, a rough guide which is flawed by the fact it does not distinguish between fat and muscle. So while it is useful in epidemiological research, on an individual level it can see healthy people misdiagnosed as overweight or obese, and vice versa.
It’s now widely established that body shape, and particularly how big your waist is, is a strong predictor of health problems. A new metric to measure obesity called ABSI, or a body shape index — which takes into account age, sex, weight, height and waist circumference — is looking like a more effective tool. In May, a study by the universities of Glasgow and Newcastle found that measuring ABSI alongside BMI was a better predictor of people’s risk of bowel, lung and liver cancer.
From the beginning, BMI was never intended to be a way to measure a healthy weight. It was invented by a mathematician in the mid 19th century as a way to describe the growth spurts that happen after birth and puberty. ABSI, on the other hand, was designed from the outset as a way to predict disease risk.
Its inclusion of waist circumference is important, because while research suggests that fat on your bottom or thighs may be neutral or even beneficial for health, fat around your middle is more dangerous, and is strongly linked to type 2 diabetes, dementia and heart disease.
Studies have shown that ABSI is much better at predicting the risk of diseases and early death than either BMI or just waist circumference alone. Including your waist circumference not only helps to estimate where you store fat, but also your ratio of fat to muscle.
For example, if your weight remains the same, but you start lifting weights and building muscle, then your waist size is likely to drop as you lose fat from around your middle. However, your BMI would remain the same, despite you becoming healthier.
A 2008 study by the Mayo Clinic looked at the height, weight and body fat percentages of a group of over 13,000 people to determine how well BMI diagnosed obesity as defined by the World Health Organisation: having over 35pc body fat for women, or 25pc for men. While 31pc of women and 21pc of men in the study were classed as obese by their BMI, more than twice that number were obese by their proportion of body fat. In other words, BMI only managed to spot half of the obese people in the study, and gave the other half a false sense of security about their health.
Researchers concluded that the accuracy of BMI is limited “particularly for individuals in the intermediate BMI ranges”. In short: it is very accurate for those at the high end of the spectrum, but in the middle it’s a lot patchier. The study also found that the accuracy of BMI is worse as you get older, you may stay the same weight as when you were young, but with less muscle and more fat.
Another study published in 2016 compared 40,000 people’s BMI with specific measurements of health such as insulin resistance, blood pressure and cholesterol levels. Nearly half of those whose BMI diagnosed them as overweight were metabolically healthy by those measures.
Sir David Haslam, a GP, obesity expert, and former chair of the National Institute for Health and Care Excellence in the UK, says that any one index will never be enough to measure someone’s health. “ABSI is edging forward from just plain and simple BMI, but nevertheless I object to a number telling me how healthy my patient is when there’s so much more to consider,” he says.
He says that while a BMI of 24 might put someone in the “normal” category, he looks out for other symptoms like being pale or short of breath, which could be markers of poor metabolic health.
To calculate your ABSI, visit: fatcalc.com/absi
(© Telegraph Media Group Ltd 2021)
Telegraph Media Group Limited [2021]