Exercise is never going to be an effective way of slimming, unless you have the training schedule -- and the willpower -- of an Olympic athlete. "It's simple maths," says Professor Paul Gately, of the Carnegie Weight Management institution in Leeds. "If you want to lose a pound of body fat, then that requires you to run from Leeds to Nottingham, but if you want to do it through diet, you just have to skip a meal for seven days."
"In theory, of course, it's possible that you can burn more calories than you eat," says Dr Susan Jebb, head of nutrition and health research at the Medical Research Council, and one of the government's go-to academics for advice on nutrition. "But you have to do an awful lot more exercise than most people realise. To burn off an extra 500 calories is typically an extra two hours of cycling. And that's about two doughnuts."