What Is RED-S? Symptoms, Causes, and Recovery Explained
What is RED-S? Relative Energy Deficiency in Sport happens when you're not eating enough to support both your training and your body's basic functions. It's not just "overtraining" or "burnout", it's a systemic problem that affects hormones, metabolism, bone density, mood, immunity, and performance. And it doesn't just happen to elite athletes or women.
RED-S occurs when energy intake is too low relative to exercise expenditure, leaving insufficient fuel for the body to maintain normal physiological function. The result is a cascade of symptoms that many athletes dismiss as "just part of hard training", but chronic fatigue, feeling cold all the time, irritability, lost periods, frequent injuries, and stalled performance aren't badges of honor. They're warning signs.
How RED-S differs from the Female Athlete Triad: The older "triad" model focused on three connected problems in female athletes: disordered eating, amenorrhea (lost periods), and low bone density. RED-S expanded this framework in 2014 to recognize that under-fueling affects all body systems—and all genders. Men can and do develop RED-S, though it often goes undiagnosed because the warning signs (like hormonal disruption) are less visible than a missed period.
Early warning signs of RED-S: declining performance despite consistent training, frequent illness or injuries that won't heal, loss of menstrual period (in women) or low libido (in men), feeling cold when others are comfortable, mood changes including irritability, depression, or anxiety, obsessive thoughts about food or body composition, and stress fractures or unexplained bone injuries. Many athletes experience these symptoms for months or years before connecting them to under-fueling.
What the research says: A 2024 paper asked "Does RED-S Exist?", not to dismiss the condition, but to push for clearer diagnostic criteria and better research. The science is still evolving, but the clinical reality is clear: athletes who chronically under-fuel experience measurable hormonal, metabolic, and bone density changes that resolve when energy availability improves.
Why athletes fall through the cracks: Diet culture celebrates restriction. Endurance sports often reward lighter bodies in the short term. Coaches and even medical providers may miss the signs, especially in male athletes or those who don't "look" underweight. And many athletes genuinely don't realize they're under-eating, high training loads make it easy to fall into energy deficit without intentional restriction.
What recovery actually requires: Eating enough is non-negotiable—and "enough" is usually more than athletes expect. Recovery also means reducing training load, sometimes significantly, to allow the body to repair. This isn't a quick fix; hormonal and metabolic recovery can take months to years depending on severity. Working with professionals who understand RED-S—sports dietitians, endocrinologists familiar with athletes, therapists if disordered eating is involved—makes a meaningful difference.
Being tired, cold, and cranky isn't "just part of training." Losing your period isn't a sign you're finally lean enough. Getting injured every season isn't bad luck. These are symptoms, and they're telling you something. Whether you're chasing PRs or just trying to feel good in your body again, the answer is almost always the same: eat more, rest more, and stop treating your basic needs as obstacles to performance.
What is RED-S (Relative Energy Deficiency in Sport)? How do you know if you have it? And why is it so hard to diagnose? In this episode, Zoë and Kylee break down the science behind RED-S—what causes it, who's at risk, how it shows up differently in men and women, and what the latest research says about treatment and recovery. Whether you're an elite athlete or a weekend warrior, if you're not eating enough for your activity level, RED-S is on the table.
What Is RED-S? A Simple Definition
KYLEE: Welcome to Your Diet Sucks, the podcast that won't say your symptoms are normal because you do sports.
ZOË: So Kylee, we are in your zone today. This is your specialty—RED-S, Relative Energy Deficiency in Sport.
KYLEE: RED-S is simply put an energy mismatch. A mismatch between energy intake and energy expenditure.
ZOË: Not eating enough...
KYLEE: ...and doing too much. But it's also important to remind people that it has downstream effects on different systems in the body. Technically, a mismatch could be low energy availability or LEA. When we take it a step further, RED-S includes the different systems of the body that it's affecting and the symptoms that people are experiencing.
ZOË: So low energy availability or LEA is the cause. RED-S—the downstream impacts—is the effect, or how it shows up in the human body.
KYLEE: I really want to point out that this isn't just like being tired from your training.
The History: From "Female Athlete Triad" to RED-S
ZOË: So quick history. In the late 1980s and early 1990s, researchers started noticing a troubling pattern in female athletes—a cluster of three interrelated health issues that eventually became known as the Female Athlete Triad: disordered eating, menstrual dysfunction, and low bone mineral density.
ZOË: The name "triad" was officially coined at a 1992 ACSM conference. At the time, it was framed as primarily a women's issue, largely because menstrual irregularity was such a visible red flag. If your period goes missing, something is clearly off. But this framing had a significant downside—it implied that men couldn't experience the same underlying problem.
ZOË: By the 2000s, cracks in this framework started to become obvious. Male athletes, especially in weight-sensitive sports like distance running, cycling, and rowing, were showing up with the same problems—stress fractures, low testosterone levels, chronic fatigue, and stalled performance.
ZOË: Since the 2018 RED-S consensus, there have been more than 170 original research publications advancing the field. The big takeaway from the history lesson is that it took literally decades for the sports world to admit that this isn't just a "women's bones problem"—it's a whole body, all athlete problem.
Who Gets RED-S? Intentional vs. Unintentional Under-Fueling
KYLEE: When we look at it, we can look at an intentional state of RED-S—like an under-fueling situation—or an unintentional situation. Even if you are unintentionally under-fueling, you can still have RED-S. Your body doesn't care what your intention is.
ZOË: A study on Western States athletes found that women were more likely to experience low energy availability, and more likely to report being in that state on purpose. Men were slightly less likely to have low energy availability, but more likely to accidentally have it.
Unintentional Under-Fueling
KYLEE: Athletes feel like, "I have a handle on this. I kind of know what I'm doing." But appetite can change with training intensity, volume, and heat. Hunger and fullness cues get altered. And athletes who are good at listening to their bodies might under-fuel because they're like, "Well, I'm not hungry."
KYLEE: Schedule is another factor. People have work, travel, family, kids, pets—their nutrition gets put on the back burner. The fly-by-the-seat-of-my-pants approach while training 15 hours a week for a hundred-miler is not going to work. Eventually, you'll hit a breaking point.
ZOË: A 2025 study of student athletes found only 45% correctly answered nutrition questions. And pretty much all of them believed they needed significantly fewer carbohydrates and more protein than recommended.
Intentional Under-Fueling
KYLEE: Then we have intentional under-fueling—weight loss and restriction. People can fall into this trap whether they're intentionally cutting calories to lose weight or doing things like fasted training, restrictive diets like keto or low-carb, or intermittent fasting that shortens the window to get nutrition in.
ZOË: The reason restrictive diets "work" isn't because there's some magical thing about cutting out legumes or whatever. It's simply because you're limiting your energy intake.
Signs and Symptoms of RED-S
KYLEE: This is where it gets wild. The threshold for under-fueling can be really different in different people. It's not apples to apples because we're all genetically and physiologically different.
Physical Signs
- Reproductive: Missed or irregular periods in women; reduced testosterone in men; low sex drive in both
- Bone health: Frequent stress fractures, shin splints, bone pain; long-term risk of osteopenia and osteoporosis
- Immune function: Frequent illnesses, always having a low-grade cold
- GI symptoms: Gas, bloating, food intolerances, nausea, diarrhea, constipation—one of the biggest things I see
- Other: Feeling cold all the time, thinning hair, brittle nails, thyroid dysfunction
KYLEE: I liken it to dimming the lights on systems so they don't have enough energy to function. Your body just slowly dims the lights.
Mental Signs
- Irritability, feeling "hangry"
- Increased anxiety
- Brain fog, difficulty concentrating
- Preoccupation with food—if you're thinking about food more than 75% of your day, that's a problem
- Denial that you need fuel for your training level
- Feeling the need to exercise more to "make up" for eating
- Sneaking in extra movement (even walks)
- Low motivation and mood changes
Performance Signs
- Stalled performance despite consistent training
- Impaired recovery
- Decreased strength and endurance
- Reduced coordination and focus
- Chronic injury cycles
Why RED-S Is So Hard to Diagnose
ZOË: Is this a diagnosis a regular doctor will give? There is no single specific ICD medical code for Relative Energy Deficiency in Sport. Clinicians have to use a combination of existing codes to reflect the various symptoms. So it's a syndrome, not a specific diagnosis. And there's no single test for RED-S.
KYLEE: That's what makes it so slippery and complicated. If physicians haven't been trained in this, they're not going to automatically assume it's RED-S. They might not even know what RED-S is. They're going to go to diagnoses they're more familiar with—Crohn's disease, pre-diabetes, something else.
The "Does RED-S Exist?" Paper Explained
ZOË: In 2024, a paper came out with the provocative title "Does RED-S Exist?" It's been a big point of discussion.
KYLEE: At first glance, they made a catchy title on purpose. But I worry that if people don't read the whole thing, they'll think the conclusion is that RED-S isn't real.
ZOË: The paper isn't dismissing RED-S. The authors are challenging how the model is often applied. They point out measurement limitations—LEA is basically impossible to measure accurately in the field. Studies are often short-term and observational because you can't do a randomized controlled trial where you tell people to develop RED-S. There are ethical concerns.
ZOË: They also flag an over-reliance on generic symptoms. Things like fatigue and poor mood are non-specific—they could stem from LEA or from having a new puppy.
KYLEE: The paper talks about making sure medical providers consider differential diagnoses. But I find that more often than not, medical professionals are making differential diagnoses rather than considering RED-S.
ZOË: Despite the title, it doesn't disprove RED-S. But the current evidence framework might be too calorie-centric relative to the quality and quantity of available research. The authors are calling for a shift toward a more holistic, evidence-based, athlete-centered approach.
How to Recover from RED-S
KYLEE: If you're questioning whether you have it, I don't see a downside in focusing on trying to get a handle on your fueling. The challenging part is that if you're actually in a RED-S state, your body is in a stressed state, and your needs might be elevated over what we'd typically calculate.
Recovery Steps
- Increase energy intake: This is non-negotiable. You have to eat more to match your training.
- Reduce training volume and intensity: Give your body room to recover.
- Work with professionals: A sports dietitian, a therapist if there's disordered eating involved, and an MD who understands RED-S.
- Be patient: Recovery timelines vary widely. Going "all in" speeds recovery; half-measures draw it out.
ZOË: Are there instances where you recommend athletes stop all exercise?
KYLEE: That's tricky because people's mental health can tank if you take exercise out completely. I think gentle movement forms are fine—yoga, hiking—in concert with investigating your relationship with these movements.
How to Monitor Without Becoming Obsessive
KYLEE: I'm not saying you have to obsessively track in Cronometer, but some kind of journal or record of what you're eating is important. The reality is, we forget. We're our own worst enemy at remembering what we ate.
ZOË: People are terrible at self-reporting what they eat. Studies show people under-report what they're actually eating and over-report healthy foods. So much of it gets filtered through our cultural understanding of food.
KYLEE: If you log or journal and you don't have somebody help you, you might not be honest with yourself. You also have to know how much you need. If you don't understand that piece, you're shooting in the dark.
The Bottom Line
KYLEE: From my opinion, there's no harm in learning how to fuel yourself and understanding what your body's signals are telling you. Educate yourself on what you actually need fuel-wise. Put that into practice in your training. There won't be harm in understanding that.
ZOË: No one's ever gonna be like, "Oh shit, I wish I didn't know about my body."
References:
Mountjoy, M., Ackerman, K. E., Bailey, D. M., Burke, L. M., Constantini, N., Hackney, A. C., … Verhagen, E. (2023). International Olympic Committee’s (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine, 57(17), 1073–1097. https://doi.org/10.1136/bjsports-2023-106994
Stellingwerff, T., Mountjoy, M., McCluskey, W. T. P., Ackerman, K. E., Verhagen, E., Heikura, I. A., … et al. (2023). Review of the scientific rationale, development and validation of the International Olympic Committee Relative Energy Deficiency in Sport Clinical Assessment Tool: V.2 (IOC REDs CAT2). British Journal of Sports Medicine. https://doi.org/10.1136/bjsports-2023-106914
Jeukendrup, A. E., Areta, J. L., Van Genechten, L., Langan-Evans, C., Pedlar, C. R., Rodas, G., … Walsh, N. P. (2024). Does relative energy deficiency in sport (REDs) syndrome exist? Sports Medicine, 54, 2793–2816. https://doi.org/10.1007/s40279-024-02108-y
Areta, J. L., Jeukendrup, A. E., Van Genechten, L., Langan-Evans, C., Pedlar, C. R., Rodas, G., … Walsh, N. P. (2025). Limited empirical support for relative energy deficiency in sport (REDs) syndrome. Sports Medicine, 55(8), 2051–2055. https://doi.org/10.1007/s40279-025-02220-7
Ackerman, K. E., Palacios, C., Lambert, M. I., & others. (2023). Methodology for studying Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine, 57(17), 1136–1144. https://bjsm.bmj.com/content/bjsports/57/17/1136.full.pdf