They have identified a single genetic mutation responsible for cyclist Tom Staniford's condition, which prevents him storing fat under the skin.
Despite being super-lean, the former national paracycling champion has type 2 diabetes, a disease usually linked to being overweight. He also has fused bones in his hands and feet.
Scientists have diagnosed MDP syndrome, an extremely rare and complex condition, after tracking down its genetic origins.
MDP syndrome is only known to affect eight people in the world, including 23-year-old Mr Staniford.
He and other patients were found by researchers at the University of Exeter to have an abnormality in the POLD1 gene that gives rise to a defective enzyme crucial to DNA replication.
Mr Staniford hopes the new diagnosis will allow him to be more accurately classified for events and help his chances of success. He is bidding to participate in the Rio 2016 Paralympic Games.
"In some ways, identifying the syndrome behind my symptoms shouldn't be important – a name is just a name, after all – but it is reassuring to know that there are other people with the condition and that we can lead relatively normal lives," he said.
"What could prove crucial, though, is enabling me to be properly classified in competitions so that I am not competing at an unfair disadvantage against others. I hope to be able to compete for Great Britain in the 2016 Paralympics and this finding could make a real difference to my chances."
Mr Staniford was born at normal weight, but throughout his childhood and teenage years lost all the fat around his face and limbs. His hearing also deteriorated during childhood, and he now wears hearing aids.