It is a simple trick don't let others define what you are and how you look. If you think you are fat and want to lose weight you can you don't need ask anyones permission for that. UNLESS you doing it for attention. Why don't you check you bmi? for example I am 168cm tall that means my weight shouldn't be over 68kg or less than 50kg i think and I am 56kg and I am happy with it. I am very healthy too.