Im 5'7 and 45 kgs and asian. i have friends who call me 'matchstick' and most of them are girls who are bigger?

Stop talking to these s. Those aren't your friends. You're lucky that you are so skinny. Do you know how much fat people pay for diet secrets, books, and tips? BILLIONS. F*ck 'em and go find yourself some friends who don't resemble Precious!