If I were writing about 200 years ago, the title of this column would be Should I worry about my Quetelet Index?. That’s what BMI, the body mass index, was known as when the statistician and sociologist Adolphe Quetelet devised it in the 1830s. But Quetelet never intended the measure as a means of medical assessment. Two centuries later, plenty of experts will tell you that it doesn’t exactly work as advertised either. So is it time to throw it out altogether, or can it still tell us something useful about our health?
To get the basics out of the way, your BMI is simply your mass (in kilos) divided by the square of your height (in metres), expressed in the units kg/m2. According to the NHS, a calculation in the healthy weight range is between 18.5 and 24.9 – though this can differ slightly among ethnic groups. Obviously, this doesn’t work as well as a measure for children, old people or pregnant women, but where things get trickier is among groups with higher muscle mass and bone density, who can often be classed as “obese” or “overweight”, even when they are sporting a well-defined six pack.
“As with any tool, it’s only as good as its interpretation and it does have flaws,” says Emil Hodzovic, a doctor and health coach. “Muscle is more dense than fat, so in theory you can have a high BMI and still not be obese according to your body fat percentage – though it’s also worth noting that people can have a ‘normal’ BMI and be obese according to body fat if they have low muscle. It also doesn’t take sex differences into account.”
So BMI’s pointless as a measure if you go to the gym a lot? Not so fast. “A lot of people who lift weights tend to use ‘muscle’ as an excuse for having a higher BMI even if they also have excess body fat – which is the actual risk factor,” says Hodzovic. “One US study based on more than 13,000 people suggests that if you are obese according to BMI, you are most likely obese according to body fat percentage as well.” This means that if your BMI goes over (or under) the NHS-approved “healthy” range, it’s probably worth investigating further.
The real concern is body fat: having too much, particularly around the waist and internal organs, is a risk factor for cardiovascular disease, diabetes and several cancers. “A lot of the other ways of measuring body fat are more involved – for instance, a Dexa scan [a test of bone density] or skin calipers,” says Hodzovic. “In terms of simplicity and speed, and to prompt further action, whether that’s an immediate lifestyle change or further testing, it could be argued that BMI has a place. The problem is that it’s been overvalued or used in isolation and misinterpreted historically by health professionals, and that does need to change.”
The prescription? Use your BMI as an easy reference point – but if you’re telling yourself that you’re OK despite being over the acceptable range, perhaps check your waist circumference with a tape measure (the British Heart Foundation has a handy guide) or do a body fat test using smart scales or hydrostatic, or underwater, weighing. If you are concerned, your doctor should be able to check other markers of health, including blood pressure and blood work, to give you a more complete picture about what might be going on. If you are over the recommended BMI upper limit, it won’t hurt to take stock of what you are eating in a typical day.
“Eat less processed food, drink less alcohol, focus on getting protein and vegetables at every meal, build a movement habit and get enough sleep,” says Hodzovic. “The key is consistency over intensity, so focus on building long-term habits rather than trying to turn your life upside down overnight.” Do all this, and you won’t need to worry too much about which measures of health are right or wrong – you’ll be doing the best you can for your body either way.
