I have been on a 400 calorie per day diet and exercise, i need to get back up to eating regularly without gaining it all back? i am 16 and female

Oooh..: Extreme calorie restrictive diets can be a double edged sword..If not done as part of a medically-supervised regimen they should be avoided. 400 calories is too low to meet any teenagers daily needs. If you eat too little kcals ur body thinks it is starving (b/c it is..) & starts to slow down ur metabolic rate and kcal needs, so you need to increase both ur cardio activity (gently)& kcal intake.