I'm sure it's not a myth, but equally I'm sure that most people who claim to have a high metabolism just don't eat as much as they think. Throughout most of my teenage years I would eat 5-6 good meals a day, with snacks inbetween, and see no aesthetic or weight change. Yet when it came up, lots of my friends said that they must have a high met. aswell, citing an extra bag of crisps as "way too much food"