I used to weigh 57 kilos but now i am 54 kilos, how can i put on weight fast and build muscle? i am 18 years old and female, i am also 5'7".

Diet and muscle mass: To gain weight you have to eat more calories than burned. Do this in a healthy way though. Don't choose junk or fast food. Keep a well balanced diet and make sure you are getting all the nutrients you need. Weight lifting and other muscle building exercises can help you increase your body mass. Muscle size is partly genetic. Likewise, some people are naturally thin, based on their genetics.