RED-S stands for Relative Energy Deficiency in Sport.

It happens when an athlete is not taking in enough usable energy to cover both:

normal body functions, such as hormones, immunity, bone repair, metabolism, and growth; and training demands, such as running, cycling, gym work, sport practice, or competition.

So the issue is not just “eating too little.” It is low energy availability: after exercise energy expenditure, the body does not have enough left for health and recovery. The International Olympic Committee describes RED-S as impaired physiological functioning caused by relative energy deficiency, affecting systems such as metabolic rate, menstrual function, bone health, immunity, protein synthesis, and cardiovascular health.

It was originally discussed mostly as the Female Athlete Triad: low energy availability, menstrual dysfunction, and low bone density. The newer RED-S concept is broader because it also affects male athletes and involves many more systems than menstruation and bone health alone.

Common signs include persistent fatigue, frequent injuries, stress fractures, poor recovery, declining performance, low mood, feeling cold, disrupted sleep, low libido, repeated illness, and, in women, irregular or missing periods. In men, it can show up as low testosterone, reduced morning erections, poor recovery, mood changes, and reduced strength or endurance.

mportant point: RED-S can happen even when the athlete looks fit, lean, or “healthy.” It is common in endurance sports, aesthetic sports, combat sports, cycling, running, gymnastics, dance, rowing, and any sport where low body weight is rewarded. Recent sport coverage has highlighted under-fuelling as a growing concern in cycling and other endurance sports.

For a young athlete or teenager, RED-S should be taken especially seriously because it can affect growth, puberty, bone development, and long-term health.