BMI may not highlight health risks linked to obesity in older people – study

Measuring someone’s height to waist ratio is a “much better indicator” of the health risks associated with carrying excess fat than body mass index (BMI), particularly in older people, according to researchers.

BMI does not highlight where fat is stored, and as people lose muscle as they age body fat percentage may go up despite the number on the scales coming down, experts said.

Using a tape measure to assess waist-to-height ratio may help pinpoint people who have more fat stored around their vital organs, which can put health at risk, they suggest.

BMI is calculated by dividing a person’s weight in kilograms by height in metres squared, while waist-to-height ratio divides waist circumference by height.

According to researchers, if the circumference of the waist is less than half a person’s height, it is “a good indicator” they are at a healthy weight.

While BMI may decline in old age due to muscle loss, waist-to-height ratio may continue to increase steadily and better reflect the risk of conditions such as heart disease and stroke.

Dr Laura Gray, of the University of Sheffield’s School of Medicine and Population Health, told the Press Association: “BMI doesn’t account for any muscle in your body.

“Weight could be fat weight or muscle weight, and it doesn’t distinguish between the two. That’s problematic.

“Quite often we’ll hear that athletes have a BMI over 30 and would be classed as living with obesity, but we know that they’re not. They don’t have excess weight.

“This study is the other side of that coin. Older people tend to lose muscle when they age, and that means that their body fat percentage is actually higher, but their weight is still dropped, so BMI starts coming down, but actually they’re getting less healthy.

“Waist-to-height ratio, because it measures round your waist, it’s giving us a more accurate measurement of visceral fat.”

Visceral fat – which is fat stored around the vital organs – has “more of an effect on how things work within your body”, according to Dr Gray.

The study, published in the International Journal of Obesity and led by researchers at the universities of Sheffield and Nottingham, analysed obesity trends between 2005 and 2021 using data from the Health Survey for England.

They found waist-to-height ratio provides a clearer picture of health risks with age compared to BMI.

High risk

Dr Gray said: “If we under-diagnose obesity in older adults because we’re relying solely on BMI, then this could mean we fail to identify large proportions of people who are at high risk and can benefit from healthcare interventions.

“People are more likely to have obesity the older they get, and that’s been the case for some time.

“As we’ve got an aging population there’s going to be a lot more older people as proportion of our society, that’s going to put a greater burden on the NHS.”

Dr Gray suggests the method may even save money.

“Tape measure is actually cheaper than a set of weighing scales,” she said. “It’s probably easier because it’s just a ratio, rather than squaring something like we do with BMI.”

The study also found the environment has a “significant influence” on obesity.

Dr Gray said: “As time has changed the different environments that people have lived in has influenced obesity and made the prevalence increase, but we didn’t find that there was any generational differences.

“It’s not that people have changed, it’s that society’s changing.”

Figures show that in 2023/24, 64.5% of adults in England were estimated to be overweight or obese, with 26.5% classed as living with obesity.

Age group

Dr Gray said the question of whether BMI is under or over diagnosing obesity depends on which age group you look at.

She told PA: “If we’re looking at older adults, then it’s suggesting that people might be healthy when they have excess fat. So it’s under diagnosing obesity.

“But in men in their 20s that tend to be a bit more muscular, then it could be over diagnosing obesity.

“BMI was originally developed to measure obesity in a whole population. So on an average, when you’ve got those people with low muscle and high muscle on average, it works quite well over a whole population. But it’s for individuals where it falls down a bit.”