Waist-to-height ratio is a more accurate and reliable indicator of age-related obesity risk than body mass index (BMI), according to new research, with scientists warning that reliance on BMI alone could lead to under-diagnosis among older adults.

As the UK population continues to age, obesity and related diseases are placing increasing pressure on the National Health Service (NHS). Researchers say broader use of the waist-to-height ratio as a screening tool could help identify at-risk individuals earlier and enable timely preventive care.

Unlike BMI, which does not differentiate between fat and muscle mass, the waist-to-height ratio provides a better measure of visceral fat — the fat stored around the abdomen that has a greater impact on vital organs and overall health.

The study, conducted by researchers from the Universities of Sheffield and Nottingham, analysed obesity trends in England between 2005 and 2021 using data from the Health Survey for England. The team examined BMI alongside other waist-based measures, including the waist-to-height ratio, to understand how obesity patterns have changed over time.

Co-authors Dr Laura Gray from the University of Sheffield and Dr Magdalena Opazo-Breton from the University of Nottingham explored whether these trends were driven primarily by ageing, environmental factors, or generational differences.

Gray, from the University of Sheffield’s School of Medicine and Population Health, said, “BMI is a really well-known and well-understood measure. However, it could be misleading people, particularly older people and those with less muscle mass, into thinking they are not at risk of obesity.”

“Waist-to-height ratio is also a lot easier for people to understand and remember than more complex BMI categories, particularly in children. People can easily check their own risk with a measuring tape. If your waist is less than half your height, that’s a good indicator you’re at a healthy weight,” she said.

Gray noted that the findings suggest rising obesity levels are largely driven by long-term environmental changes rather than individual behaviour alone, highlighting the need for broader systemic action by policymakers to address the food environment.

She also pointed out that the likelihood of obesity increases with age, meaning an ageing population could further raise overall prevalence.

“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,” she said.