Relative energy deficiency in sport (RED - S).

Relative Energy Deficiency in Sport (RED-S) refers to a condition in which energy imbalance leads to impaired physiological function of multiple organ systems and expands on the diagnosis previously known as the Female Athlete Triad. Researchers attribute the medical complications of RED-S to low energy availability, in which energy availability is defined as dietary energy intake minus exercise energy expenditure divided by fat-free mass. This article reviews the history of this diagnosis, the changing terminology, and the reasons for the expansion.