How many of us actually understand calories? When it comes to understanding calories many of us can be our own worst enemies. This is exacerbated by a food marketing designed to encourage us to eat more and a political system that does little to stop it. Dr Marion Nestle is a globally recognised expert in nutrition, food studies and public health and also a professor at New York university. In this radio interview Dr Nestle talks about her new book 'Why calories count: From science to politics' (co authored with Malden Nesheim) and explains what calories are, how they affect the body and why its so easy to eat more of them than we actually need.