RED-S, or Relative Energy Deficiency in Sport, is a condition where an athlete consistently takes in less energy than their body needs to support both training and normal biological functions. When the gap between calories consumed and calories burned through exercise becomes too large, the body starts dialing down essential processes to conserve fuel. This affects far more than just weight or performance. It can disrupt hormones, weaken bones, suppress the immune system, and impair mental health.
The term was introduced by the International Olympic Committee to replace the older concept of the “Female Athlete Triad,” which focused narrowly on three issues in women: disordered eating, menstrual irregularity, and bone loss. RED-S broadened the framework to include all athletes, male and female, and to capture the full range of body systems that suffer when energy intake falls short.
How Low Energy Availability Works
The core problem in RED-S is low energy availability. Energy availability is what’s left over after you subtract the calories burned during exercise from the calories you eat. That leftover energy is what your body uses for everything else: repairing tissue, producing hormones, fighting infection, maintaining bone density, regulating body temperature, and keeping your brain sharp.
Researchers measure energy availability in calories per kilogram of fat-free mass per day. Above 45 calories per kilogram of fat-free mass, the body has enough fuel to maintain healthy functions and support weight stability. Between 30 and 45, physiological processes start to slow down. Below 30, the body enters a state of low energy availability where meaningful harm begins to accumulate. For context, an athlete with 60 kg of fat-free mass who drops below 30 cal/kg would be running on fewer than 1,800 calories after accounting for their training, leaving very little for the rest of their body’s needs.
Low energy availability doesn’t always stem from intentional dieting or an eating disorder. It can happen when training volume increases without a matching increase in food intake, when an athlete doesn’t realize how many calories their sport demands, or when busy schedules lead to skipped meals. The cause matters less than the result: the body doesn’t have enough fuel, and it starts making tradeoffs.
What RED-S Does to the Body
When the body senses a prolonged energy shortage, it begins shutting down or slowing functions it considers non-essential for immediate survival. The effects ripple across nearly every system.
The endocrine system is often the first to respond. The body reduces production of reproductive hormones, thyroid hormones, and growth-related hormones. In women, this commonly shows up as irregular or absent periods, one of the most recognizable early signs. The thyroid slows down too, reducing the body’s metabolic rate in an attempt to conserve energy. Athletes with RED-S often show suppressed levels of the active thyroid hormone T3, which helps regulate metabolism, energy, and body temperature.
Bone health takes a serious hit. Hormones like estrogen and testosterone play a direct role in maintaining bone density, and when their levels drop, bones weaken. This is why stress fractures are one of the hallmark injuries associated with RED-S. An athlete who keeps getting stress fractures despite reasonable training loads may be dealing with underlying energy deficiency.
The cardiovascular, gastrointestinal, and immune systems are also affected. Athletes with RED-S may experience a slower heart rate, digestive problems, and frequent illness. The immune system, which is energy-expensive to maintain, gets deprioritized when fuel is scarce.
RED-S in Male Athletes
Because RED-S was built on a framework originally designed around female athletes, many people assume it’s a women’s issue. It isn’t. Male athletes are affected too, but they’re often harder to identify because they lack the obvious warning sign that women have: a disrupted menstrual cycle.
In men, RED-S can cause low testosterone, decreased libido, reduced sperm count, and poor bone health. In adolescent male athletes, it can delay puberty and stunt growth. These symptoms tend to develop gradually, which makes them easy to dismiss or attribute to overtraining. A male athlete experiencing persistent fatigue, declining performance, or repeated injuries should consider whether energy intake is keeping pace with training demands.
Mental Health and Cognitive Effects
RED-S is not purely a physical condition. The brain is one of the most energy-hungry organs in the body, and it doesn’t function well on a deficit. Athletes with RED-S frequently experience irritability, difficulty concentrating, low mood, and impaired decision-making. These psychological symptoms can feed a destructive cycle: poor mood leads to disordered eating patterns or compulsive overtraining, which deepens the energy deficit, which worsens mental health further.
The psychological dimension also makes RED-S harder to recognize from the outside. Coaches and teammates may notice that an athlete seems “off” without connecting the dots to nutrition. The athlete themselves may attribute their mood changes to stress, poor sleep, or the demands of competition rather than recognizing that their body is running on too little fuel.
How RED-S Affects Performance
One of the cruel ironies of RED-S is that athletes often undereat in pursuit of better performance, whether to make a weight class, achieve a leaner physique, or simply because they believe lighter means faster. In reality, low energy availability undermines the very adaptations that training is supposed to produce. The body can’t build muscle effectively without adequate fuel. Glycogen stores, the primary energy source for high-intensity effort, stay chronically depleted. Recovery between sessions slows down, and the risk of injury climbs.
Over time, athletes with RED-S often notice that their training feels harder for worse results. Paces that once felt comfortable become a struggle. Strength plateaus or declines. Injuries that should heal in weeks linger for months. The performance decline can be subtle at first, which is part of what makes RED-S so insidious: by the time it’s obvious, the athlete may have been energy-deficient for months or even years.
Who Is Most at Risk
RED-S can occur in any sport, but certain athletic environments carry higher risk. Sports that emphasize leanness or aesthetics (distance running, gymnastics, figure skating, cycling, rowing) see higher rates, as do sports with weight classes like wrestling and boxing. Athletes who train at high volumes without nutritional guidance are particularly vulnerable, as are those in environments where “thinness culture” is normalized.
Young athletes face additional risks because their bodies need extra energy for growth and development on top of their training demands. An adolescent who is both growing and training intensively has very high energy needs, and falling short can have lasting consequences for bone density and hormonal development that persist well beyond their athletic career.
Recovery and Treatment
The fundamental treatment for RED-S is straightforward in concept: increase energy intake, reduce training load, or both. In practice, this can be challenging. Athletes may resist eating more or training less, particularly if the energy deficit is tied to disordered eating or deeply held beliefs about body composition and performance.
Recovery typically involves working with a sports dietitian to gradually increase caloric intake, often while temporarily reducing training volume to give the body a chance to restore normal function. For women, the return of a regular menstrual cycle is a useful marker that energy availability is improving. For men, improvements in libido, mood, and energy levels tend to signal recovery, though hormonal blood work can provide more objective confirmation.
Bone density lost during a period of RED-S can take years to fully recover, and in some cases, athletes never regain the density they would have had without the deficit. This makes early identification critical. The longer an athlete stays in a state of low energy availability, the more difficult and prolonged the recovery becomes.