Relative energy deficiency in sport (REDs) not only affects female athletes, but male athletes as well. It results from severe and/or prolonged low energy availability, when energy intake no longer meets energy expenditure. In men, REDs may present as decreased performance, persistent fatigue, recurrent infections, mood disturbances, reduced libido, functional hypogonadism, impaired bone health, and an increased risk of stress fractures. Diagnosis is a medical decision based on a comprehensive clinical evaluation, as there are no specific biomarkers, and may be supported by structured tools such as the Relative Energy Deficiency in Sport Clinical Assessment Tool - version 2, developed under the auspices of the International Olympic Committee.