New year resolutions often include eating more healthily, doing exercise and trying to shift some of the extra weight put on over Christmas. Yet research suggests the vast majority of people who do lose weight ultimately end up putting nearly all of it back on. So why is it so difficult? Madeleine Finlay speaks to health journalist and ex-neuroscientist David Cox on the science of metabolism, and what it means for our health. Help support our independent journalism at theguardian.com/sciencepod