Millions of middle-aged people have been mistakenly led to believe they are not obese, according to an Italian study that looked at body fat instead of weight related to height.

Using a new, lower cut-off for obesity would give a truer picture of who is affected, the researchers say.

With age, muscle declines and fat builds up around organs in the waist region, often with no change in weight.

The challenge is to find a tool that easily screens for obesity.

The standard way of categorising people's weight is by calculating their body mass index, or BMI, which means dividing an adult's weight in kilograms by the square of their height in metres.

• 18.5-25 is said to suggest a healthy weight

• 25-29 that they are overweight

• 30 or above that they are obese

It is a quick and easy method, supported by the World Health Organization (WHO), and pretty accurate in most people most of the time - but cannot differentiate between fat, muscle and bone.

A study of 4,800 adults aged 40-80 years old , led by the University of Tor Vergata in Rome, and presented at the European Congress on Obesity, looked at an alternative - measuring body fat percentage.

Just 38% of the men and 41% of the women had a BMI above 30 - but when their body fat percentage was calculated using scans, 71% and 64% were found to be obese.

"If we continue to use the WHO standard for obesity screening, we will miss many middle-aged and older adults who are at risk for obesity-related diseases including type 2 diabetes, heart disease, and some cancers," co-author, Prof Antonino De Lorenzo said.

A BMI of 27 should be used to define obesity in future, the researchers said.

"Establishing this new BMI cut-off point in clinical settings and obesity guidelines will be beneficial to the potential health of millions of older adults," Prof De Lorenzo said.