Crucially, BMI doesn't distinguish between fat and muscle and studies have found it unreliable. So should we do away with the measurement once and for all?
For decades, official guidance has decreed that the way to find out if you’re a healthy weight is by measuring your BMI.
The calculation takes into account your height and weight, and then tells you whether you're underweight, healthy, overweight or obese.
The BMI measurement was first created in the 1800s by a Belgian mathematician called Adolphe Quetelet. By the late 1900s, it had been adopted by governments across the world as a way for people to work out if they were over- or underweight.
But increasingly researchers and health experts are speaking out to say BMI isn’t the perfect measure of health we once thought.
Last year, a study by UCLA concluded that tens of millions of people who had overweight and obese BMI scores were in fact perfectly healthy.
But they also found that 30 per cent of people with “healthy” BMIs were in fact not healthy at all based on their other health data.
So what’s going on?
There are a myriad of reasons why BMI isn’t as accurate a measure of health as many of us were brought up to believe, but one of the main ones is that BMI can’t distinguish between fat and muscle.
A pound of fat takes up a much larger volume on the body than a pound of muscle, even though they of course weigh the same. (Muscle is about 18 per cent more dense than fat.)
As a result, many incredibly fit, athletic, muscular people are labeled “overweight” by their BMI.
“I’m overweight and nearly obese accord to my BMI,” personal trainer Tom Mans explains to The Independent.
He believes BMI is only of any use when looking at large populations of people to get an idea of trends. “It should not be used on a individual basis,” he says.
Many PTs believe BMI measurements are too broad and it assumes one size will fit all, when this simply isn’t the case.
David Valentine-Jones of Sculpt Health & Fitness gives the example of two female clients of the same age, weight and height:
“One of them is very active with a low percentage of body fat and a high percentage of muscle mass, the other sedentary with a high percentage of body fat and a low percentage of muscle mass.”
Both women can come out with the same BMI, despite having drastically different body compositions.
Another problem with BMI is that it doesn’t measure or take into account where on the body fat is carried. One person could have skinny legs and carry a lot of fat around their middle, whilst another could have the same amount of fat spread around their body.
Both may have the same BMI, but the former person would be at a higher risk health-wise.
Visceral fat around your organs is particularly dangerous, but it’s possible to have a lot and still come out with a healthy BMI.
This has led many people to argue that a far more important measurement is your hip to waist ratio. According to the World Health Organisation, this should be no more than 0.85 for women and 0.9 for men.
It’s crucial to look at where your fat is placed, as too much around the midsection is cause for concern.
The UCLA study gives the example of a man with a healthy BMI of 22 but too much fat round his middle according to his waist to hip ratio - he had an 87 per cent higher mortality risk than a man with the same BMI but a healthy waist to hip ratio.
Many people have come up with their own equations as alternatives to BMI over the years - in 2013, mathematician Nick Trefethen, Professor of Numerical Analysis at Oxford University, proposed that a better calculation is to multiply your weight by 1.3, and then divide that by your height to the power of 2.5.
The calculation tends to favour taller people, but it still has shortcomings.
Most experts like to use a combination of measures when assessing health, including body fat percentage, waist to hip ratio, BAI (body adiposity index), physical measurements, weight and tracked strength or cardiovascular improvements.
“This gives a much more accurate picture of a client’s starting point and ongoing progress in relation to their health, fitness and personal goals,” Valentine-Jones explains.
Phil Chant, the founder of Bodyscan, believes what we should really be doing is measuring our fat and lean mass separately with a fat mass index (FMI) and a lean mass index (LMI) - “essentially, how much fat and muscle you carry relative to your height,” he explained to The Independent.
“The key advantages to FMI and LMI are that each index is completely independent of the other, and a change in either index is a guarantee of a change in, respectively, fat and muscle. That is not the case with weight, BMI or body fat percentage.”
So should we forget our BMI entirely?
There are some cases where BMI can be useful, namely when assessing underweight people who are trying to recover from eating disorders.
“For access to certain treatment areas of eating disorders a BMI is required to provide a cut off point of entry,” Harley Street nutritionist Rhiannon Lambert explained to The Independent.
“I find this is a good tool for those looking to work with clients in recovery to ensure they are a stable BMI but not so good when people urgently need help and their BMI is not low enough to get the care they need.”
Overall though, Lambert, like most fitness and health experts, believes BMI is more often a problem than a reliable measure of health.
“I think other methods should be used to measure an individual's health as it often rides on so much more than a number,” she says, pointing out that labelling can be unhelpful for many people too.
If you want to know whether you’re in good health or not, there are endless aspects to consider. But it’s probably best not to see your BMI as the be all and end all - there’s a lot more to it than that.