Energy deficiency is one of the serious nutritional concerns for individuals with high-volume exercise activity, and early detection and immediate treatments are crucial to prevent further problems for individuals. The terms “Athlete Triad” and “Relative Energy Deficiency in Sport” are commonly known for energy deficiency in athletes and exercise enthusiasts, both of which derive from “Low Energy Availability”. Energy availability considers the amount of energy available to sustain physiological function after subtracting energy for exercise as a priority from total dietary intake. Although measuring energy availability in a real-life situation is challenging, screening tools have been developed to avoid missing energy deficiency in athletes and exercise enthusiasts. This article introduces the outline of energy availability for registered dietitians/nutritionists who will possibly engage in nutritional support for athletes, to promote a better understanding of the concept.