Basically you need to increase your calorie intake, you can do this in a healthy way by eating more carbohyrdrate such as potato, rice and bananas. However if you are naturally thin you may find it doesn't make much difference. I was anorexically thin as a teenager and early twenties but I ate like a horse ( I daren't miss a meal or I lost weight).
Mind you I hit my thirties and the weight went on and stayed and right now I'd be very happy to lose a bit!



