Keith Frayn, professor of human metabolism at the university of Oxford, replies: Many people believe they have either a 'fast' or 'slow' metabolism. However, there is very little scientific evidence for this. If people who believe they can eat anything without putting on weight and those who think they put on weight just by looking at a cream bun are brought in for tests, the differences are very hard to pinpoint. In fact, the rate of 'basal metabolism' is determined almost entirely by the amount of non-fat tissue (muscle and internal organs) in the body.
If you are very thin it is almost certain that you eat less than you imagine or else you are more active than you think. Activity in this context might simply mean fidgeting and perhaps running up and down stairs when other people take the lift. If you are really keen to put on weight eat more and do strength exercises to build up muscles. But being thin within reasonable limits (obviously not to the degree seen in eating disorders) is generally a good thing in terms of health.