There are some people who never seem to worry about what they eat, never diet or do any special exercise, and yet somehow manage to stay slim. Who are these people and how do they do it? This experiment follows 37-year-old Yemi and 41-year-old Anne Marie who appear to eat what they like and have no set exercise routines, yet never put on weight. Over a course of five days covert cameras follow them everywhere: at work, home and play. They are the envy of their friends. Is there something in their behaviour that's determining their size? Experts unpick every element of their lifestyle - from what they eat, to how much they fidget, to the amount of sleep they get. At a time when one in four people in Britain are obese, and billions of pounds are spent on fad diets, this fascinating documentary uncovers the simple secrets to being slim.