RED-S in Endurance Sport: Signs of Low Energy Availability Athletes Should Not Ignore
Relative Energy Deficiency in Sport (RED-S) describes impaired health and performance associated with problematic low energy availability. It can affect athletes of any sex, size or level. You do not have to look underweight to be underfuelled.
Low energy availability occurs when dietary energy is insufficient after accounting for exercise demands. It can be accidental, driven by schedule and appetite, or linked with disordered eating.
Updated 25 August 2026. This is general sports nutrition information, not individual medical advice.
signs are wider than performance
Possible signals include persistent fatigue, recurrent injury or illness, menstrual disruption, reduced libido, low mood, impaired concentration, declining performance, poor recovery and bone stress injury. No single symptom diagnoses RED-S.
The danger is normalising the pattern as commitment. A training block that repeatedly removes sleep, food and recovery is not evidence of toughness.
carbohydrate availability matters
The 2023 IOC consensus highlights the growing role of low carbohydrate availability within the broader energy picture. Fasted sessions and weight-loss phases can become risky when layered onto high training volume without adequate compensation.
Fueling key sessions and restoring energy across the day support both performance and health. A gel during training cannot fix an overall energy deficit, but refusing fuel during every session can contribute to it.
what to do
Speak with a sports physician and accredited sports dietitian, particularly for bone stress injury, menstrual changes, rapid weight loss, disordered eating, persistent fatigue or declining performance. Treatment is individual and may require training modification.
Our whole-food versus sports-food guide explains why performance products and complete daily nutrition have different roles.
the bottom line
Relative Energy Deficiency in Sport (RED-S) describes impaired health and performance associated with problematic low energy availability. It can affect athletes of any sex, size or level. You do not have to look underweight to be underfuelled. Turn the principle into a plan, test it under relevant training pressure and keep clear notes on what actually works.
For a food-first option providing 30g carbohydrate and 250mg sodium per pouch, see 2THRV Whole Food Fuel Gel. Product does not replace a complete daily diet or individual medical care.
frequently asked questions
can male athletes get RED-S?
Yes. RED-S affects male and female athletes.
can someone have RED-S at a normal body weight?
Yes. Body weight alone does not show whether energy availability is adequate.
is one fasted run dangerous?
Not automatically. Risk comes from context, frequency, total intake, symptoms and the wider training load.
research and guidance
whole-food endurance fuel