BMI (which stands for body mass index) charts have been used as an indicator of whether we are underweight, a healthy weight, overweight or obese for decades – the system was devised in the 19th century and coined in a medical paper in 1972. But it’s also long been a topic of debate, because it works purely on weight and height measurements (with age taken into account too) and doesn’t distinguish between fat and muscle.
The simple measurement is your weight in kilograms divided by height in metres, and divided by height again. Below 18.5 is classified as underweight, over 25 is overweight and over 30 means you’re in the obese category.
What is BMI helpful for?
BMI is currently recommended by the National Institute for Health and Care Excellence (NICE) and the Royal College of Paediatrics and Child Health. Being overweight or obese is associated with a higher risk of major conditions like heart disease, stroke and type 2 diabetes, and BMI scales can help identify if someone is more likely to be at risk.
“BMI is a straightforward and cost-effective way of accurately measuring a person’s weight status. It is used widely across the world, which helps to make valuable international comparisons,” says Professor Louis Levy, head of nutrition science at Public Health England.
It provides invaluable insight into a general overview of the population as a whole – figures released by the World Health Organisation (WHO) last week showed that childhood obesity has increased 10-fold in the past four decades. It’s worth noting, BMI in children is measured slightly differently to adults though (using BMI centile on a child growth chart). World Obesity Federation data shows that obesity is steadily climbing in adults as well, and by 2025, it’s predicted that 41% of UK adults will be obese.
What are the disadvantages of the BMI system?
In short, people are categorised based purely on measurements, without taking into account things like their build and fitness levels, which may be very important.
If someone has a lot of muscle (which weighs more than fat), the BMI index will be overestimated by the calculation – so a fit, strapping rugby player could have relatively low body fat levels but a high BMI, for example. Dr Clare Morrison, GP at online pharmacy and doctor MedExpress, says: “While a BMI can be used as an indicator of obesity, other factors need to be considered. Muscle weighs more than fat and so a person who is muscle-building, who does not have any excess body fat, could potentially have the same BMI as an obese individual.”
Luke Hughes, from Origym Personal Training, says: “Many people interpret BMI as an absolute to their health, but for a large percentage of the population, the figures are extremely distorted, especially as we move into our more polarised society with regards to our physical activity levels. The scale does not take into consideration the placement of where body fat settles on the body.”
While athletes will probably be well aware of how healthy they are regardless of BMI, where fat appears on someone’s body is important to determining health and risk of disease. A 2014 study by the Mayo Clinic in the US found that people with a normal BMI but a large belly (ie. those who carry most of their body fat around the middle) were more likely to die younger or from weight-associated illnesses.
Measuring waist circumference
The NHS website says a healthy waist circumference for men is less than 94cm (37 inches) and less than 80cm (32 inches) for women. Your middle is a particularly dangerous place to carry excess fat, compared to say, carrying it on your bottom or thighs. This visceral fat makes you more likely to develop heart disease or diabetes.
As the BMI index doesn’t correspond to the same degree of weight in different individuals, Public Health England recommend that it’s not the only method you use to work out whether or not you’re overweight or obese.
“Other measures such as waist circumference are rough guides that can also reflect weight status and risk of ill health,” Professor Levy says.
The University of Wolverhampton’s Professor Alan Nevill, who specialises in biostatistics for health, sport and exercise, has written a paper on the need to redefine age and gender-specific overweight and obese BMI index cut-off points. He says that although monitoring BMI is sensible and will help identify people who are overweight, in most cases, he believes waist circumference (divided by height, square-rooted) would be a more accurate measurement.
“Waist circumference would be a much more sensitive measure for people who are carrying too much fat and are much more sensitive to metabolic risk. There’s very little [chance] muscle could interfere with a waist circumference measurement,” he says.
BMI may also be more or less relevant depending on your age. “It’s quite easy to show that younger people with a BMI over 30 don’t have as much fat as older people over 30,” Professor Neville says. “While younger people will carry a higher proportion of muscle in their 20s than someone in their 50s.”
“I’ve done calculations that show some people in their 20s with a BMI of 33 or even 34 in the case of males, have the same fat as the people in their 40s [with the same BMI]. And someone [in their 50s] with a BMI of 29 could actually be at more risk than someone with a BMI of 32 who is younger.”
Measuring visceral fat
But how easy is it to switch from BMI as the single population measure for obesity to something more accurate? Well, undertaking waist circumference measures at population level could draw variability concerns compared to BMI and it may not be as efficient.
Other methods for measuring body fat are available, but they’re more expensive. Mohammed Hudda, research fellow in medical statistics at St George’s, University of London, says: “More detailed measures of body fat are now being made more available, such as bioimpedance scales, which measure the percentage of body fat.”
The DEXA (dual-energy X-ray absorptiometry) Body Scan, claims to be the most accurate way of measuring visceral fat – it reveals the amount and distribution of fat and muscle mass, and rates the result for your age, height and gender. It’s not as cheap as a quick calculation though; a scan with a consultation is £159 (bodyscanuk.com).