Researching the health of female and male athletes has seen a lot of progress over the last 50 years. Because of the clearest symptomatology, the effects of sports on reproductive health were first observed and explored. Now, a few decades later, we know that sport is not an etiologic factor that negatively affects athletes' health, but a lack of energy is. Low energy availability occurs when the body does not have enough energy to support the demands for training and normal physiological functioning. There are two main reasons for this - inadequate energy input or high consumption. Also, menstrual function is not the only health component that is affected. In 1992, women's sports triad was defined as a combination of menstrual dysfunction, eating disorder and osteoporosis. However, many female and male athletes were not diagnosed because they did not meet each of these three components. In the year 2000 the Triad was renamed into the relative energy deficit in the sport (RED - S), thus highlighting the complexity of the problem and the fact that male athletes can also be affected. RED - S syndrome is a disorder in the physiological functioning of an organism that can affect the metabolic rate, menstrual function, bone health, but also protein synthesis, cardiovascular health, and immune defence. Psychological consequences may precede syndrome but may also be the consequence of it. It is important to recognize the problem on time to make adequate preventive measures so that serious consequences would not develop. Also, it is necessary to educate doctors, athletes, trainers and others how to deal with it. Numerous tools and questionnaires for the detection of RED - S have been developed, as well as models for returning to sports if an athlete, due to a health hazard, has had to stop training.