Relative Energy Deficiency in Sports, RED-S, caused by reduced energy availability can lead to a variety of symptoms. Both male and female athletes on all performance levels and ages are at risk of developing RED-S.Case 1 describes a patient with a low energy availability due to a disturbed energy balance with increased training intensity. She presented herself with an amenorrhea, one of the possible symptoms in women. Case 2 describes a patient with fatigue due to low energy availability, caused by an underlying eating disorder. Distincting RED-S from an eating disorder is difficult due to overlapping symptoms, but important given the difference in treatment. Altogether, RED-S can disrupt various body functions, therefore early recognition is desirable. Early recognition may prevent short- and long-term effects and reduce recovery time. Increasing awareness and knowledge on this topic is of great importance.