The important thing is to be in balance, so that resting metabolism stays high enough. Benardot and Thompson cite two studies that suggest that a person who has restricted his caloric intake develops a lower rate of energy expenditure, burning fewer calories. This is a normal adaptation by the human body to make up for the deprivation of calories. A fast-growth period requires high energy intake. An exercise enthusiast who is working to increase his muscle mass will have a higher caloric requirement.
Muscle growth is incompatible with severe caloric restriction.
Children, women and young athletes are particularly susceptible to calorie-deficit exercising. In addition to the energy requirements of heavy activity, children need extra energy just to grow. Restraint of energy intake in women contributes to ovulatory dysfunction, which ultimately relates to bone health, and puts them at risk for early osteoporosis. Young athletes who exercise heavily without proper energy intake risk altering their growth rate, and anemia is common in many. All these factors influence the total energy requirement.