1 . This is xxfitness , so it ' s only for women . But since you ' re young , I ' ll help . 2 . You need a certain amount of calories a day to allow your body to function properly , your basebasal metabolic rate . If you do not feed your body , it will consume energy stored in fat and then muscle . If caloric denial goes on too long , the body feeding off of muscles gets to the point if heart damage and cardiac distress . 2 . You ' re very young , and 70 ish kg is a very reasonable weight to be , especially if you are 15 year old male , which means you will likely grow several more inches . 3 . If you actually were very overweight , which you are not , losing weigh too fast is not sustainable . You ' ll drop a lot of weight on a crash diet , then gain it back or some of it back when you begin to binge as you almost surely will . That kind of restriction is not sustainable . 4 . Please consider seeing a therapist . Your school almost surely has a counselor on staff and the shirt post you ' ve written reeks of an eating disorder .