British scientists have developed a breathalyser 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 breathalyser 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."
"This is of great interest in sport studies and dietary studies to find out how people have worked out in the gym. That is an area we are trying to explore and we are trying to produce a monitor of how well you have burned off some body fat," the Telegraph quoted Hancock, as saying.
It works by using a detection method known as spectroscopy which measures the wavelengths of light that are absorbed by different molecules in a gas.
By shining an infrared laser through a complex series of mirrors they can detect even tiny changes in the levels of acetone when a person breathes into the breathalyser.
The scientists are also aiming to develop a breath test would allow diabetes diagnosis without having to give blood.