Researchers at a Birmingham hospital have found people lose "significantly more weight" by measuring their progress using an alternative to the Body Mass Index.

As Britain's obesity problem reaches epidemic proportions, clinicians and politicians have been quick to quote the importance of a healthy BMI.

But Heartlands Hospital's weight management clinic has been testing an alternative, the Body Volume Index, which gives patients a better idea of where fat is stored on their body, not just their overall mass.

The project, part of the international Body Benchmark Study which began in 2005, monitored 53 men and women aged 25 to 55 for six months. They all had BMIs between 25 and 35, making them overweight or obese. A person's BMI is calculated by dividing their weight in kilograms by their height in metres squared, and a reading between 18 and 25 is considered healthy.

However this does not recognise factors such as body composition, age and ethnicity. Therefore a fit rugby player could be classed as obese while an average-build man with a beer belly might be rated as healthy.

Volunteers who took part in Heartland's study, run in conjunction with Edgbastonbased Select Research, were split into two groups.

Twenty of them were also given regular scans of their body, as a motivational tool.

Instead of being measured manually with a tape measure, a patient steps into a £25,000 photo booth-style scanner and is then blasted with white light to produce 3D images of where the fat is stored.

It carries out 20 readings a second to produce a comprehensive view of a person's body.

The 17 women and three men who were scanned regularly lost a total of 49.7kg (7st 9lb), including 63cm (24.8in) off their waists and 40cm (15.7in) off their thighs.

Dr Asad Rahim, the consultant endocrinologist who ran the two-year trial, said there was "a correlation" between those who were scanned and who lost the most weight.

"The scanner has certainly helped motivate some patients to manage their weight more effectively but there are also patients who were not scanned who still lost weight," added Dr Rahim.

"The problem with BMI at the moment is it cannot differentiate between a pot-bellied obese man and heavy but muscular athletes, like rugby players and body builders. They could both be classed as obese using the BMI scale.

"This new method can also help us to highlight potential health problems, such as diabetes, so this can be a diagnostic tool as well."

Heartlands' research will now be followed by a wider BVI programme, looking at a larger sample of adults and children, details of which are due to be confirmed next month.

Richard Barnes, project director of the Body Benchmark Study, said the long-term aim is to create a measurement database that could"result in substantial savings for the NHS".

He added: "BVI could be used in preoperative scheduling as neck volume can be extracted, as can abdominal volume, but it also has applications in prosthetics, physiotherapy, osteopathy and for GPs as a first stage assessment and motivation tool."