Claire MacEvilly, nutritionist at the Medical Research Council's Human Nutrition centre in Cambridge, disagrees with US studies showing that the much-touted grapefruit diet - advocates believe the fruit to be an appetite repressant - which has been around since the Seventies, actually works. "There is no miracle ingredient in a grapefruit," says MacEvilly. "These people simply knew they were on a diet, and probably ate more healthily."