Body mass index (BMI) is the commonly used measure for checking if a person is obese. It divides an adult’s weight by their height squared to determine if their weight is healthy. But BMI has flaws. It can’t tell the difference between fat and muscle mass and the latter actually weighs more than the former. In practice this means a professional rugby player would usually be classified as obese if they took a BMI test. However, we’d know they actually have more muscle than fat.
Since BMI can’t distinguish between muscle (which is good) and fat (which is bad), test results can’t accurately predict if someone’s at risk of an early death. The good news is there is an alternative measure.
Professor Rickayzen and Professor Mayhew’s research shows that waist-to-height ratio (WHtR) is more accurate than BMI for measuring the mortality risk associated with obesity and is therefore a better predictor of the number of years someone will lose if they are obese. It’s simple to measure too – you just need a piece of string. Not surprisingly given that being overweight now affects two-thirds of the adult population, Professor Rickayzen and Professor Mayhew’s work on WHtR has received lots of media attention since being published in 2014.
What did we explore and how?
Professor Rickayzen, collaborating with Professor Mayhew and Margaret Ashwell OBE of Ashwell Associates, collected data from several medical resources. These include the Health Survey for England and the Health and Lifestyle Survey. The researchers then calculated the years of life lost at the ages of 30, 50 and 70 for males and females based on obese measures of WHtR, compared to someone having a good WHtR. They made the same calculations using BMI since this is so widely used.
The research led to three big discoveries. The first is that WHtR is a more accurate and easier way than BMI of determining if someone is at risk of an early death through obesity. Rather than dividing an adult’s weight by their height squared, WHtR compares a person’s waist size with their height. The second discovery is that people should aim to keep their waist to half of their height. The third discovery is that it’s possible to quantify the numbers of years of life that people stand to lose through being obese. For example, a 30 year old male with a waist circumference which is 80% of his height would be expected to lose over 16 years of life compared to someone whose waist is half their height.
The researchers published their findings in a 2014 report called ‘Waist-to-height ratio is more predictive of years of life lost than body mass index’. It comes with a simple, easy-to-remember message: “keep your waist to half of your height.” To test this, you can cut a piece of string at your height. Fold it in half, then see if it can fit around your waist.
Benefits and influence of this research
Since publishing the research, its reach has gone beyond the academic community. Professor Mark Baker of the National Institute for Health and Care Excellence (NICE) revealed that he wished to use the research in the organisation’s guidelines on how to avoid obesity.
The research has attracted media attention that includes a front cover and inside article in the Sunday Times in 2014, an interview on Sky News in 2017, a BBC documentary in 2018 and an article in the Mail on Sunday in 2019.
The research’s message is simple: keep your waist to half your height. It’s easy for anyone to understand, regardless of their age, gender, country or culture. And governments from all over the world can share the message with little effort.
Anything that gives obese people a more accurate measure of their mortality risk than BMI is a good thing. WHtR could inspire people to change their exercise and dieting patterns, and ultimately help them to live for longer. All they need to get started is a piece of string.