After Sunday lunch with your family, do you feel ready to burst? The temptation is to put your running shoes on and start running around the block to burn off all the calories you’ve eaten. Bad idea! All that effort on a full stomach, even worse if it’s cold outside, can block digestion, so – nutritionists point out – only sugar gets burnt, which is not very useful for keeping in shape. Much better is a long walk (at least an hour) that also mobilizes fat deposits.