Relative energy, in the context most people search for, refers to Relative Energy Deficiency in Sport, commonly known as RED-S (or REDs). It’s a syndrome that develops when an athlete consistently takes in too few calories to cover both their exercise demands and the basic biological functions their body needs to perform, like maintaining hormones, building bone, and supporting immune health. The International Olympic Committee formally defined the condition in 2014, and it affects both male and female athletes across a wide range of sports.
How Energy Availability Works
The core concept behind RED-S is “energy availability,” which is the amount of dietary energy left over for your body’s normal functions after you subtract the calories burned during exercise. It’s calculated with a simple formula: take your total calorie intake for the day, subtract the calories you burned exercising, then divide by your fat-free mass (essentially your body weight minus fat) in kilograms. The result is expressed in kilocalories per kilogram of fat-free mass per day.
For women, optimal energy availability is at least 45 kcal/kg of fat-free mass per day. For men, the threshold is around 40 kcal/kg. When energy availability drops below 30 kcal/kg of fat-free mass, it’s considered clinically low, and that’s where serious health problems start. The range between 30 and 45 for women (or 30 and 40 for men) is considered subclinical, meaning the body is under strain but may not yet show obvious symptoms.
To put this in practical terms: if a female athlete with 50 kg of fat-free mass eats 2,500 calories and burns 700 calories exercising, her energy availability is 36 kcal/kg. That falls in the subclinical zone, not yet dangerous but not enough to keep all her body’s systems running optimally.
From the Female Athlete Triad to RED-S
The understanding of this problem has evolved significantly over the past five decades. In the 1970s, researchers first identified “athletic amenorrhea,” the loss of menstrual periods in female athletes. By the 1990s, this was grouped with low bone density and disordered eating into what was called the Female Athlete Triad. But the Triad framework was too narrow. It focused only on women and only on three specific outcomes.
In 2014, the International Olympic Committee broadened the concept into RED-S, recognizing that low energy availability disrupts far more than just menstrual function and bone health, and that it affects men too. The 2023 IOC consensus statement, backed by over 170 original research publications since 2018, further refined the understanding. Key updates include the growing recognition that low carbohydrate availability compounds the problem, that RED-S overlaps with overtraining syndrome, and that mental health is deeply intertwined with the condition.
What RED-S Does to the Body
When the body doesn’t have enough energy left after exercise to run its internal systems, it starts rationing. The effects span nearly every physiological system. Hormonal disruption is one of the earliest signs. In women, this typically shows up as irregular or missing periods. In men, researchers have identified reduced testosterone levels and decreased signaling from the hormones that regulate reproductive function, though the relationship is complex. A study of 60 competitive male endurance athletes found that while hormone levels often stayed within normal clinical ranges, bone density was significantly associated with energy availability status.
Bone health is a major concern for both sexes. Low energy availability impairs the body’s ability to build and maintain bone, increasing fracture risk. The immune system weakens, making athletes more susceptible to illness. Metabolic rate slows as the body tries to conserve energy. Psychological effects are also common, including mood disturbances, impaired concentration, and increased irritability. All of these together create a vicious cycle: the athlete’s health declines, performance drops, and the temptation to train harder or eat less only makes things worse.
Who Is Most at Risk
RED-S is remarkably common. The risk of low energy availability ranges from 14% to 63% depending on the sport, with athletes in sports that emphasize leanness facing the highest rates. Among gymnasts, nearly 45% show signs of low energy availability. The rate is about 33% in soccer players, 22% in ballet dancers, and 20% in volleyball players.
Male athletes are not spared. Studies of adolescent athletes found low energy availability in 56% of males compared to 51% of females. Among cross-country runners, 42% of males and 29% of females fell below the clinical threshold of 30 kcal/kg of fat-free mass per day. Endurance sports carry particular risk for both sexes, with about 25% of male and 31% of female endurance athletes affected.
The causes aren’t always intentional. Some athletes deliberately restrict food to make weight or look a certain way, but many simply can’t keep up with the caloric demands of their training. A distance runner adding extra mileage or a swimmer doubling practice sessions can slip into low energy availability without realizing it, simply because appetite doesn’t always match expenditure.
Screening and Diagnosis
Because RED-S can be difficult to detect, especially when athletes don’t recognize their own symptoms, screening tools have been developed. The most widely used for women is the Low Energy Availability in Females Questionnaire (LEAF-Q), a 25-question survey covering three areas: injuries, gastrointestinal symptoms, and reproductive function. A total score of 8 or higher out of 25 flags an athlete as at risk. The questionnaire asks about things like missed training days due to injury, bloating unrelated to menstruation, whether periods have stopped for three or more consecutive months, and whether menstrual patterns change with increased exercise.
The 2023 IOC consensus introduced an updated clinical tool called the REDs CAT2, which uses a four-color traffic-light system to categorize severity and risk. This replaced an older three-color system that was too broad in its middle category, lumping together athletes with minor symptoms and those close to needing removal from sport. The new system gives clinicians a more precise way to determine whether an athlete can safely continue training, needs modifications, or should stop competing until they recover.
How RED-S Affects Performance
Athletes often resist the idea that eating more could make them faster or stronger, but the performance consequences of RED-S are well documented. When energy availability drops too low, the body can’t adequately replenish its glycogen stores, the primary fuel source for high-intensity exercise. Muscle repair slows. Reaction time and decision-making suffer as the brain receives less of the glucose it depends on. Chronic low energy availability over weeks to months leads to measurable declines in strength, endurance, and coordination.
Even short, severe energy deficits lasting just a few days can trigger problems. The body responds quickly to energy shortages by dialing down processes it considers non-essential, and many of those processes, like tissue repair and immune surveillance, are exactly what athletes need to train effectively and stay healthy.
Recovery and Carbohydrate Availability
Recovery from RED-S centers on restoring energy availability above 45 kcal/kg of fat-free mass for women and above 40 for men. This typically means increasing food intake, sometimes reducing training volume, or both. Different body systems recover on different timelines. Metabolic rate and energy levels tend to improve within weeks. Menstrual function in women can take months to return. Bone density recovery is the slowest, often requiring a year or more of sustained adequate nutrition.
One of the key emerging findings from recent research is that low carbohydrate availability plays a compounding role alongside overall low energy. Even if total calorie intake is borderline adequate, failing to consume enough carbohydrates around training sessions can trigger some of the same hormonal and metabolic disruptions seen in full RED-S. This is particularly relevant for athletes following low-carb or ketogenic diets while maintaining heavy training loads.