Scientists now know why some people can be fat but fit.

Swedish medical researchers have pinpointed a gene which helps to keep fat in place in the human body’s fatty tissue.

Fat is constantly produced, broken down and rebuilt by the body and the protein adiponutrin takes part in the process.

But Lund University researchers found some overweight people have a variant of the adiponutrin gene that causes amounts of this protein to be lower than normal.

"Adiponutrin is supposed to constitute a kind of corset that keeps fat in its place in fatty tissue. If the protein doesn’t do its job after a sugar-rich meal, fatty acids leak into the blood instead.

"The high content of fat in the blood then affects the cardiovascular system, the liver, muscles, and pancreas," wrote Associate Professor Martin Ridderstrale in the international journal Diabetes.