Body mass index (BMI) is one method of calculating body fat percentage, and is a handy and widely used tool to give a quick impression of your health.
You can work out your BMI in a couple of seconds by dividing your weight in kilograms by your height in metres squared. For example: I weigh 80kg and I'm 1.75m tall, so my BMI works out to 26.
But BMI has well-known limitations, because it doesn't take into account whether your weight comes from fat, muscle, or that baby you might be carrying. A score of 26 technically puts me in the overweight category for men, and I'm not overweight (here's a recent pic of me; I'm on the right).
Nor does BMI indicate where fat is stored on your body. Fat carried around your midsection, aka "visceral adopise tissue" to doctors and "skinny-fat" to us regular people, is associated with a higher risk of disease and early death. But you can have a "healthy" BMI and still have dangerously high levels of tummy fat.
A far more precise way to measure body fat is via a DXA (or DEXA) scan — but though it's considered the gold standard, it's a lot less practical and affordable than a simple calculation like BMI.
There are several calculations that predict body fat, and researchers from Leeds Beckett University in England set out to determine which is the most accurate.
For their study, published in PLOS One, they pitted five common metrics against one another — BMI, waist circumference, waist-to-hip ratio, waist-to-height ratio, and waist/height0.5 — and compared them to a DXA scan.
Ultimately, waist-to-height ratio proved best at predicting both whole body fat percentage and visceral adipose tissue.
It's simple to measure: divide your waist by your height (make sure to use the same units of measurement for each). My waist is 0.79m and my height is 1.75m, for a waist-to-height ratio of 0.45.
This chart from Wikipedia indicates how waist-to-height ratios stack up:
|Female college swimmer||0.4240|
|Male college swimmer||0.4280|
|Female at increased risk||0.4920|
|General healthy cutoff||0.5000|
|Risk equivalent to BMI of 25||0.5100|
|Males at increased risk||0.5360|
|Risk equivalent to BMI of 30||0.5700|
|Substantial risk increase||0.5820|
According to the Leeds Beckett study, cut-points for predicting whole body obesity were 0.53 in men and 0.54 in women. The cut-point for predicting abdominal obesity was 0.59 in both sexes.
Dr Michelle Swainson, a senior lecturer in exercise physiology who led the study, said adults and children should generally try to keep their waist circumference to less than half their height.
It's not like waist-to-height ratio is a newly discovered method of classifying obesity — other studies have indicated it's the best way of predicting body fat and the risks that come with it.
However, waist-to-height is not routinely used by health professionals, and the Leeds Beckett team believes their study should change that.
Waist-to-hip ratio, commonly used by doctors and in the fitness industry, was found to be pretty bad at predicting obesity.
“The conventional measurement of obesity used by GPs is BMI," said Swainson in a statement. "Although there are benefits to this method, there is concern that a lot of people are being classified as obese by BMI when they are not or are being missed by this classification when they need to be referred for help."
Waist-to-height is a better marker because it predicts not only total body fat but also abdominal fat.
“Carrying fat around the abdominal area has been shown to be an independent predictor of all-cause mortality in men and women," she said. "Put simply, it is more important, especially for cardio-metabolic conditions, that your belt notch goes down than the reading on the scales.”