If, like this week’s Crowdscience listener Lili, you enjoy working out in the gym, you may have wondered where your fat disappears to when you exercise? The short answer is that we convert it to energy that powers a whole range of physical processes - from breathing to walking as well as lying down and doing nothing. But the science behind energy expenditure is a little more complicated than that. Presenter Anand Jagatia jumps on an exercise bike to have his metabolism measured and learns that he may be relying on an entirely different source of fuel as he works up a sweat. Is all that hard work worth the effort it involves? Recent research suggests there is a limit to the number of calories humans can burn and that engaging in physical activity is not always a sure-fire way to keep trim. So if working out is not the best way to lose weight, how about harnessing our own fat to tackle the complications of obesity? It used to be thought brown fat was exclusive to babies (and bears) but we now know adults have some of it too. Anand discovers that it appears to play a vital role in combatting a range of chronic diseases including hypertension and diabetes. Presenter: Anand Jagatia Producer: Marijke Peters Editor: Richard Collings Production co-ordinator: Jonathan Harris (Photo: Person squeezing their tummy. Credit: Getty Images)