This is a question no doubt many people are asking themselves around this time of year, when fad diets and gym memberships are at their peak. In this programme, Dr Xand van Tulleken and dietician Hala El-Shafie help six people go on the most hyped and written about diets on the market to see which works best. In this opening episode, Emilie and Harriet have two weeks to slim down for a photo shoot, while cousins Natasha and Chiquita go on a six week diet for Natasha's wedding, and brothers Gary and Wayne want to slim down for a darts competition due in four months. Xand also investigates how exposure to extreme temperatures can affect weight loss.