No adult would admit to being stumped by the prep-school joke, “Which weighs more, a kilo of feathers or a kilo of lead?” But the mainstream medical fraternity applies the same absurd premise every day when it uses body mass index (BMI) to determine if someone is overweight. Because just as a kilo of feathers weighs the same as a kilo of lead (you weren’t stumped, right?), a muscle-bound bodybuilder of 90kg will weigh the same as someone who isn’t as active.
And because BMI is based on weight and height, if the average Joe and the athlete are the same height, then at, say, 5’10” (178cm), they will have exactly the same BMI of 28, forcing the family doctor to label them both as “overweight”, which of course is absurd.
Body mass index was derived in 1832 by a Belgian statistician, Adolphe Quetelet. He established that, aside from growth spurts after birth and during puberty, our weight increases in proportion to the square of our height. So, in the main, BMI should be a good guide to how much each of us should weigh. And for most of us, it probably is.
But BMI is outdated because weight itself is outdated. We talk about people being overweight when what we really mean is they are ‘over fat’. Just about everyone looking to lose weight is actually looking to lose fat. The only people who need to lose weight are athletes competing in a particular weight class for their sport, such as boxing.
The UK’s obesity crisis is not about people being too heavy, it’s about people being too fat. (And here’s a sobering thought – in 2015/16 more than half-a-million hospital admissions recorded obesity as a factor.)
Weight is nothing more than your relationship with gravity and you cannot change the force that keeps your feet on the ground. What you can change, however, is your body composition – how much of you is fat and how much is muscle.
What most of us seek is a body composition that is low in fat (for good health, aesthetics and low risk of obesity-related diseases) and an ‘optimum’ amount of muscle (for strength, performance or a favourite sport).
The problem with BMI is it blind to the individual elements of body composition and, like weight, wraps everything up into just a single number.
It’s also not very culturally sensitive. You can imagine that, in 1832, Monsieur Quetelet didn’t have the same ethnically diverse population around him to establish his hypothesis, so BMI thresholds have since been tweaked for different ethnicities. In 2014, a University of Glasgow study of half a million Britons found the incidence of diabetes among Whites with a BMI of at least 30 (the threshold defined as ‘obese’) was the same for South Asians with a BMI of as little as 22 and for Chinese with a BMI of as low as 24. Twenty-two and 24 are both well inside the “normal weight” zone on the Anglo-Saxon-derived BMI.
So if we are to replace weight and BMI, what with?
Perhaps the most obvious candidate is body fat percentage and with any body composition test, it’s the single number that just about everybody wants to know.
But that is exactly the problem – it’s a single number. Body fat percentage is just one number trying to take account of two components – fat and muscle.
To see where it falls short, meet Bruno, who weighs 80kg with a body fat percentage of 20%. It’s easy to work out he carries 16kg of fat.
After six months in the gym Bruno has reduced his body fat to 19%. Instantly we conclude that Bruno has made very slow progress, dropping body fat by less than a kilo in six months.
The truth is Bruno made awesome progress – he put on 4kg of pure muscle and not a gram of fat.
You see? Bruno reduced his body fat percentage without any reduction in fat. That is the problem with single numbers – they mask the detail.
A much better way of assessing body composition is to measure fat and lean mass separately with (Mr Quetelet would be pleased) a fat mass index (FMI) and lean mass index (LMI). Essentially these indices tell you how much fat and muscle you carry relative to your height.
The huge advantage of FMI and LMI are that each index is completely independent of the other; your fat mass index is unaffected by muscle, and your lean mass index is not influenced by fat. Accordingly a change in either index is a guarantee of a change in, respectively, fat or muscle. That is not the case with weight, BMI or body fat percentage.
To measure and track your FMI and LMI reliably, you will need an accurate assessment of your body composition. The most reliable and precise method is a DEXA body scan, which until recently was the preserve of sports science universities and research labs. Now, however, you can get a body scan with specialist companies like Bodyscan (www.bodyscanuk.com), which has two centres in London
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