Increasing numbers of children and adolescents prefer undertaking physical exercise to overcome overweight or obesity. Children and adolescents are in the growth stage and require adequate nutrient supply. More calories and nutrients are required especially when they are engaged in physical exercise. Exercise is the only means to increase lean body mass and decrease body fat, but adequate nutrient supply is also essential. Lack of adequate nutrient supply causes muscle mass loss, menstruation irregularity, reduced bone density, fatigue, or frequent injury in children undertaking physical exercise. Here, I have introduced some guidelines on the nutrient requirement for child and adolescent athletes.