Fad diets have been with us for generations, but the the truth is that any regime that focuses on excluding whole food groups should be approached with caution, says Tim Spector, Professor of Genetic Epidemiology at King's College London and author of the bestselling books The Diet Myth and Spoon-Fed. For this reason, he says, intermittent fasting (also known as the 5:2 diet) is the only weight-loss diet he'd truly recommend.