A scan can spot which people harbour dangerous levels of fat around their vital internal organs, scientists say.
Fat appears white on the MRI scan
Hammersmith Hospital, in west London, is currently the only hospital in Europe using the MRI scan.
Its scientists say 40% of the population have "bad" fat around the heart, liver or pancreas, even though many appear thin.
They warn it is possible to be slim and yet still be at risk of conditions like diabetes because of "hidden" fat.
Evidence suggests the precise location of fat has more of a bearing on health than simply being overweight.
For example, people who have too much weight around their middle, often called an "apple" shape, have a greater risk of developing heart disease and type 2 diabetes than those who are pear shaped and carry the weight around the hips.
While doctors can check whether a person is a healthy weight for their height by calculating their body mass index or BMI (weight in kilograms divided by height in metres squared), they cannot see the dangerous hidden fat.
Lead researcher Professor Jimmy Bell, a Medical Research Council scientist, explained: "One of the problems with BMI is it gives you the wrong idea of how much fat you have.
"It's about where you have the fat.
"Fat is good in general, but when you have too much of it or fat in the wrong places it is bad.
"Someone can look really thin and have a normal BMI but have seven litres of fat inside them when they should ideally only have one litre."
Litres of internal fat
The MRI scan he uses can look inside the body and see if there is any dangerous fat lurking around the internal organs.
Once doctors are aware someone has dangerous levels of hidden fat, they can work out the right combination of exercise and healthy eating needed to shift it, Professor Bell said.
He explained: "We are trying to understand what factors - genetic and environmental - make people put on fat internally and then determine what people need to do to get rid of this fat.
"When you diet you don't lose the right type of fat. If you exercise you lose more of the bad fat.
"If people are only going to do an hour of exercise a week, let's make sure that hour is spent doing the right exercise to shift the important internal fat."
Professor Bell says many others are interested in Hammersmith Hospital's findings.
A Food Standards Agency spokeswoman said BMI was a useful indicator, but stressed it was not appropriate for everyone.
She said: "It's not an appropriate measurement for children, younger or older people...or people who have well developed muscles, such as body builders.
"Body shape is also an important consideration."