Scientists have named a more accurate way to assess obesity in older people - and it's not BMI

The waist-to-height ratio has been shown to be a more accurate indicator of the risk of obesity and related diseases in older people than the usual body mass index (BMI). This is the conclusion reached by researchers from the Universities of Sheffield and Nottingham.

The findings are published in the International Journal of Obesity and are based on an analysis of Health Survey for England data from 2005 to 2021.

Why BMI is misleading

BMI has long been used as the primary measure of obesity, but it does not take into account body fat distribution and muscle mass levels. This is especially important for older adults, who experience a decrease in muscle tissue as they age.

As the study authors explain, because of this, BMI can create a false sense of "normal weight" while the risk of disease remains high.

"BMI is an understandable and familiar metric, but it can be misleading, especially for older people and those with reduced muscle mass," noted Dr Laura Gray from the University of Sheffield.

What's the advantage of waist-to-height ratio

The waist-to-height ratio (WHtR) better reflects the amount of visceral fat - the fat deposits in the abdominal area that most affect internal organ function.

Unlike BMI, which typically rises through middle age and then declines, WHtR continues to increase with age, more accurately reflecting the rising risks of cardiovascular disease, stroke and dementia.

A simple guideline that scientists suggest is:
waist circumference should not exceed half a person's height.

"This indicator is easy to measure by yourself - a tape measure is enough. If the waist is less than half your height, it's a good sign," Gray emphasised.

Obesity and age: a problem not only of personal choice

The study found that the rise in obesity in England is not only linked to individual lifestyle choices, but also to environmental changes - diet, availability of ultra-processed foods and lifestyle in general.

In addition, the risk of obesity increases with age, and an ageing population could lead to a further increase in disease if diagnosis is based on BMI alone.

Researchers warn: underestimating obesity in the elderly could lead to many people not getting preventative health care in time.

What this means for public health

The findings support recommendations from the UK's National Institute for Health and Clinical Excellence (NICE), which advises using waist-to-height ratio alongside BMI.

The authors emphasise that the common belief that there is a "protective role of overweight in old age" needs to be reconsidered - controlling waist circumference throughout life remains key.