Josh Bassett, 34 and from London, is a perfect size.
Josh says he plans to do more exercise
For his 6ft (1.8m) frame, his body weight of 12st 9lb (80kg) is ideal, according to the calculation the government and doctors use to assess a person's condition.
But the body mass index (BMI) sum does not take into account things like gender or the proportion of body fat to muscle.
And a novel scan being used at the Hammersmith Hospital, in west London, by Medical Research Council scientists shows fat can be lurking deep inside the body.
The MRI scan used by Professor Jimmy Bell and colleagues shows how much fat is coating vital organs like the heart, liver and pancreas.
This "hidden" fat puts people at greater risk of conditions such as heart disease and type 2 diabetes.
Mr Bassett's scan showed he had about four litres of fat inside him - experts say about one litre is the ideal amount to aim for.
He said: "I never thought I was a particularly fat person.
"To find out there was a lot more going on inside was worrying really."
Mr Bassett said he tended to eat healthily, except for the occasional meal out, and did not really need to exercise to stay trim.
"I play football every two to three weeks, but that's it.
"I have always thought exercise was about making yourself look better rather than necessarily making yourself much more healthy inside."
But he said having the scan had made him see things in a different light.
"Seeing the amount of fat around my organs might make me exercise more. Scans like this bring it home to you.
"I've joined a gym and have bought a bike with a view to cycling to work."