If you’ve ever pushed through a training block only to find yourself feeling drained, losing focus, or dealing with nagging injuries, you might have brushed it off as “just part of being an endurance athlete”.
But what if it’s something more? What if these signs are your body’s way of telling you it’s not getting enough fuel?
RED-S, or Relative Energy Deficiency in Sport, is more common than many athletes realize. It’s not just about missing a meal here and there—RED-S can affect your hormones, bones, performance, and even your mental health.
What is Relative Energy Deficiency in Sport (RED-S)?
RED-S is a syndrome that occurs when there’s low energy availability (LEA)—meaning the energy you consume through food isn’t enough to cover both your training needs and basic physiological functions like breathing, hormone production, or maintaining bone health.
Originally deemed as the Female Athlete Triad (disordered eating, menstrual dysfunction, and low bone density), RED-S broadens the lens by recognizing that athletes of all genders can experience a wide range of health and performance issues due to inadequate fueling.
Key systems affected by RED-S include:
- Metabolic rate
- Menstrual function
- Bone health
- Immunity
- Cardiovascular health
- Psychological well being
- Muscle strength and endurance
Causes of RED-S in Endurance Athletes
You don’t have to have an eating disorder to experience RED-S. For many endurance athletes, it’s an unintentional result of under-fueling.
Long training sessions, high mileage weeks, and busy lives can make it tough to consistently meet your energy needs.
Common causes of RED-S include:
- Underestimating caloric needs during peak training
- Skipping meals due to time constraints
- Restrictive diets (for example low-carb, intermittent fasting)
- Body image concerns or disordered eating habits
- Lack of recovery nutrition post-workout
- GI distress that reduces food intake
Athletes often pride themselves on discipline, but RED-S is not about willpower—it’s about energy balance.
Common RED-S Symptoms in Athletes
The signs of RED-S can be subtle at first, but over time, they become harder to ignore.
Here are some of the symptoms of RED-S.
Physical symptoms of RED-S include:
- Chronic fatigue and sluggishness
- Frequent injuries, especially stress fractures
- Decreased coordination or performance dips
- Increased frequency of illness
Hormonal and reproductive symptoms include:
- Irregular or absent periods in female athletes
- Low libido or reduced testosterone in males
- Difficulty building or maintaining muscle
Psychological symptoms include:
- Mood swings, irritability
- Difficulty concentrating
- Low motivation or burnout
If these RED-S symptoms feel familiar, it’s time to take a closer look at your fueling and recovery habits and get support from your doctor or a registered dietitian.
Understanding RED-S Syndrome Beyond Female Athletes
Despite its roots in the Female Athlete Triad, RED-S syndrome affects men, women, and non-binary athletes. Male endurance athletes, in particular, are often underdiagnosed due to the lack of visible markers like menstruation.
Here’s how RED-S manifests in male athletes:
- Low bone mineral density
- Low testosterone
- Erectile dysfunction
- Poor training adaptation
It’s essential to move beyond the gendered lens and recognize that RED-S is a systemic issue tied to energy availability, not gender identity or body type.
How RED-S Impacts Performance and Long-Term Health
RED-S doesn’t just affect your day-to-day training—it can have profound implications for your athletic career and your future health.
Some performance impacts of RED-S include:
- Reduced endurance capacity
- Slower recovery times
- Plateaued or declining performance despite training efforts
Long-term health risks of RED-S include:
- Osteoporosis and poor bone health
- Infertility or reproductive issues
- Increased cardiovascular risk
- Disordered eating patterns becoming entrenched
Left unchecked, RED-S can derail your goals—and even your ability to participate in the sport you love.
Diagnosing and Identifying RED-S
There is no single test to diagnose RED-S. Instead, it’s a clinical diagnosis made through a combination of history, symptoms, lab work, and sometimes bone scans.
Some signs of RED-S that warrant evaluation include:
- Loss of menstrual cycle for 3+ months
- Recurrent stress fractures
- Lack of performance gains despite consistent training
- Significant drop in mood or energy
An experienced sports dietitian and a physician (often with experience in sports medicine or endocrinology) can help navigate this process and rule out other causes.
Nutrition Strategies to Prevent and Begin to Recover from RED-S
The cornerstone of RED-S prevention and recovery is adequate fueling. That means getting enough calories in total—and the right timing, types, and distribution of macronutrients.
Here are some key strategies you can implement:
- Prioritize carbohydrates: Your body’s preferred fuel source during endurance efforts
- Include protein at each meal to support muscle repair and satiety
- Don’t fear fat: Essential for hormone production and energy
- Eat early and often, especially around training
- Fuel during long sessions (sports drinks, gels, bars, etc.)
- Recovery nutrition should happen within 30–60 minutes post-exercise
- Hydrate with electrolytes to support performance and avoid dehydration
If you’re unsure what you should be eating, check out my blog post Foods That Can Increase Endurance: An Athlete’s Guide to Sports Nutrition.
Recovery from RED-S: What Athletes Need to Know
Recovering from RED-S is not a quick fix—it’s a gradual, deliberate process that involves not only physical healing but often a shift in mindset around food, body image, and rest.
Recover may involve increasing total daily caloric intake, reducing training volume and/or intensity temporarily, restoring menstrual function or hormone levels, regular check-ins with a dietitian or therapist, and learning to trust hunger cues again.
Most importantly, recovery means understanding that more training, restriction, and control is not always better. Sometimes, less is the key to more performance.
How FlyNutrition Can Support Athletes with RED-S
If you suspect you might be dealing with RED-S—or you’re simply unsure if you’re fueling enough for the demands of your training—The Flight Collective is here to help.
This membership was built for endurance athletes who want expert guidance, a supportive community, and evidence-based resources without the pressure of dieting or weight loss.
Inside The Flight Collective, you’ll get:
- Monthly expert workshops on topics like low energy availability, recovery nutrition, and performance fueling.
- A library of on-demand resources covering everything from shifting your food mindset to better benefit health and performance to elevating your supplement routine.
- Nutrition office hours to ask questions and get feedback in real time.
- A community of like-minded athletes who are navigating the same challenges and wins.
- Supportive, non-diet guidance rooted in HAES and performance, not weight or appearance.
Whether you’re deep in recovery or trying to prevent RED-S before it starts, The Flight Collective gives you the tools, support, and knowledge to keep your body healthy and your training sustainable.
Fueling is the Foundation of Performance
RED-S is a serious, but preventable and treatable condition. If you’re experiencing unexplained fatigue, frequent injuries, or hormonal disruptions, don’t ignore the signs. You deserve to feel strong, resilient, and capable—not constantly running on empty.
Want to make sure your nutrition is supporting your training, recovery, and long-term health?Join The Flight Collective Membership for expert guidance, personalized strategies, and a supportive community of endurance athletes who get it. Together, we’ll help you fuel smarter, train stronger, and feel better—without restriction.
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