If, like this week’s Crowdscience listener Lili, you’re an avid gymgoer, you may well have wondered where your fat disappears to when you exercise? Well, 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 pops 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. But is all that hard work worth the effort it involves? Recent research suggests there's a limit to the number of calories us humans can burn, and that doing physical activity isn’t a sure-fire way to keep trim. Even hunter-gatherers who walk 13,000 steps a day have the same metabolic rate as the average American. So if working out isn't 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, and it seems to play a vital role in combatting a range of chronic diseases including hypertension and diabetes. Presenter: Anand Jagatia Producer: Marijke Peters