BMI was never intended to be a way to measure a healthy weight. Photo / 123rf
The method used to diagnose obesity has serious limitations, says Helen Chandler-Wilde. A more accurate tool already exists.
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 predict a person's risk of all manner of health conditions, from anorexia to diabetes and Covid-19.
But is it doing more harm than good? Research now suggests it is an outdated and ineffective way to measure how your weight affects your health. In April, the UK's women and equalities committee of MPs called for BMI to be scrapped, describing it as a "dangerous" obesity strategy which triggers eating disorders, and has led to "a rise in body image anxiety". Many experts argue that it is a blunt tool, a rough guide which is flawed because it does not distinguish between fat and muscle and on an individual level it can see healthy people misdiagnosed as overweight or obese, and vice versa.
It's now established that body shape, and particularly how big your waist is, is a strong predictor of health problems. A new metric to measure obesity, 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.
I've long been suspicious of BMI. Being in the overweight category never felt accurate to me: I've got a narrow-ish waist and above average amount of muscle from vigorous exercise. My BMI is 25.1, which is classed as overweight, but according to ABSI I am in fine fettle. I type my numbers into an online calculator and score 0.067, which puts me in the healthiest category, with a "very low" risk of health complications resulting from excess body fat.
BMI was never intended to be a way to measure a healthy weight. It was invented by a mathematician in the mid 19th century to describe the growth spurts that happen after birth and puberty. ABSI was designed from the outset 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 waist circumference alone.
Including your waist circumference helps to estimate where you store fat and 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 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 more than 13,000 people, to determine how well BMI diagnosed obesity as defined by the World Health Organisation: having more than 35 per cent body fat for women, or 25 per cent for men.
While 31 per cent of women and 21 per cent 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 the state of 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.
"BMI is a crude measure for fat distribution, which is more related to actual obesity," says Dr Mengmeng Ji, an obesity researcher at the University of Illinois at Urbana-Champaign.
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 BMIs with specific measurements of health such as insulin resistance, blood pressure and cholesterol levels. Nearly half of those whose BMIs diagnosed them as overweight were metabolically healthy by those measures.
Critics also point out that BMI was designed and validated mainly on white men, and body compositions and corresponding thresholds for disease risk can vary according to gender and ethnicity. For people with South Asian heritage, a healthy BMI is considered as between 18.5 and 23, rather than up to 25 for people with a European background. There are problems, too, in the way that BMI is applied. The report from the parliamentary women and equalities committee found that health problems in overweight and obese people are often not investigated or diagnosed as well as the same complaint in a "normal" weight person, with doctors too often simply blaming problems on their size. They also found issues with how GPs use BMI to diagnose eating disorders, with some patients told that they were too heavy to get help — an illogical and dangerous thing to tell someone with anorexia or bulimia.
Ji says she expects BMI to dwindle and ABSI to get more popular, though she says that the latter is still not perfect: "If you really want to use a number to show health or excessive fat you still need to use a machine to detect fat percentage and specifically where it is," she says.
Sir David Haslam, a GP, obesity expert, and former chairman of the National Institute for Health and Care Excellence in London, agrees that any one index will never be enough to measure someone's state of 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, a good online calculator can be found at: fatcalc.com/absi
For a more basic reading, you can measure your waist with a tape measure. According to the NZ health guidelines, a healthy waist circumference is 80cm or less for women or 94cm or below for men, whatever your age. Be aware that this doesn't take your height into account, so it won't be as accurate if you are very tall or very short.
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