Weight loss from anorexia causes menstrual cycles to stop (amenorrhea) and weight gain restores menstrual function. A minimum weight is necessary for menstrual cycles to start or resume in cases of undernutrition, which is indicated by the weight of normal girls at menarche (first period) and age 18. Those with anorexia resume periods at a heavier weight than at menarche. Anorexia and bulimia affect menstrual cycles through weight loss and nutritional imbalance disrupting the level of stored energy needed for ovulation and periods.