British scientists have developed a breath analyzer that tells how much fat you are burning off at the gym. The device is being built to pinpoint the moment when a sweaty session on the treadmill finally starts to pay off by detecting when the body has used up its supply of food energy and switches to breaking down fat instead.

Exercise machines currently estimate when people have entered the “fat burning zone”.The breath analyzer works by picking up minute changes in the levels of a molecule called acetone in people’s breath, which is given off when the body starts to burn fat. Gus Hancock, whose company Oxford Medical Diagnostics has developed the machine, said, “Acetone is a molecule that is produced by people who are burning fat rather than food.”