The purpose of this case study was to longitudinally elucidate energy availability from the dietary and training records of a Japanese female athlete (adult competitive swimmer) with disordered eating and amenorrhea during the competitive season. As a result, the subject’s energy availability was below 30 kcal/kg FFM/day throughout approximately three months of the study period. This indicates a continuous mismatch between the subject’s total energy intake (diet) and energy expended in exercise (i.e., low energy availability: leaving inadequate energy to support the functions required by the body to maintain optimal health and performance), which is clearly linked to amenorrhea. In order to return physiological function to normal while continuing athletic activities (i.e., ensuring sufficient energy availability), the subject needs to increase total energy intake through appropriate intake of each energy-providing nutrient (especially carbohydrates).