Having low dietary energy from excessive exercise or dietary restrictions leaves too little energy for the body to carry out normal functions such as maintaining a regular menstrual cycle or healthy bone density. Amenorrhea
Amenorrhea, defined as the cessation of a woman’s menstrual cycle for more than three months, is the second disorder in the triad. Weight fluctuations from dietary restrictions and/or excessive exercise affect the hypothalamus’s output of gonadotropic hormones. Gonadotropic hormones “stimulate growth of the gonads and the secretion of sex hormones.”  (e.g. gonadotropin-releasing hormone, lutenizing hormone and follicle stimulating hormone.) These gonadotropic hormones play a role in stimulating estrogen release from the ovaries. Without estrogen release, the menstrual cycle is disrupted. Exercising intensely and not eating enough calories can lead to decreases in estrogen, the hormone that helps to regulate the menstrual cycle. As a result, periods may become irregular or stop altogether.There are two types of amenorrhea. A woman who has been having her period and then stops menstruating for ninety days or more is said to have secondary amenorrhea. In the case of RED-S, the majority of secondary amenorrhea cases are attributed to functional hypothalamic amenorrhea (FHA), an adaptive mechanism to preserve energy for survival and vital processes rather than reproduction when energy balance is low. Primary amenorrhea is characterized by delayed menarche (the onset of menses during puberty). Delayed menarche may be associated with delay of the development of secondary sexual characteristics.
Relative energy deficiency in sport (RED-S) is a syndrome in which disordered eating (or low energy availability), amenorrhoea/oligomenorrhoea (in women), and decreased bone mineral density (osteoporosis and osteopenia) are present. It is caused by eating too little food to support the amount of energy being expended by an athlete, often at the urging of a coach or other authority figure who believes that athletes are more likely to win competitions when they have an extremely lean body type. RED-S is a serious illness with lifelong health consequences and can potentially be fatal.RED-S is the broader, more comprehensive name for what was formerly known as the female athlete triad (or simply the triad), which was a condition seen in females participating in sports that emphasize leanness or low body weight. As the non-menstrual components are also seen in males, the name was changed to the comprehensive term RED-S. Classification
Formerly known as the female athlete triad, RED-S is a syndrome of three interrelated conditions. Thus, if an athlete is suffering from one element of the triad, it is likely that they are suffering from the other two components of the triad as well.With the increase in female participation in sports, the incidence of a triad of disorders particular to women—the female athlete triad—has also increased. The female athlete triad and its relationship with athletics was identified in the 1980s as the prevalence increased during this period, and symptoms, risk factors, causes, and treatments were studied in depth and their relatedness evaluated.