RED-S: Relative Energy Deficiency in Sport – What It Is, Who's at Risk, Prevention Strategies, and Seeking Help

In the world of endurance sports, where pushing limits is the norm, it's easy to overlook the fine line between optimal training and overdoing it. At Project Blue, we focus on fueling athletes for performance today and health tomorrow — stable glucose, efficient recovery, and lifelong vitality. But one silent threat can derail it all: Relative Energy Deficiency in Sport (RED-S). This syndrome, often stemming from mismatched energy intake and expenditure, affects far more than just elite competitors. It can sap strength, impair recovery, and shorten careers if ignored.

This weeks topic is inspired by a fellow Feed athlete who is passionate about preventing RED-S. We dive into RED-S: what it is, who's most vulnerable, how to prevent it, and where to turn for help. Drawing from key research, including the International Olympic Committee's (IOC) 2023 consensus statement and studies on female and male athletes, we'll equip you with actionable insights. Whether you're a cyclist grinding centuries or a runner chasing PRs, understanding RED-S is key to sustainable success.

What is RED-S?

Relative Energy Deficiency in Sport (RED-S) is a clinical syndrome caused by low energy availability (LEA), where an athlete's dietary energy intake fails to match the energy expended in training and daily life. First introduced by the IOC in 2014 as an expansion of the Female Athlete Triad (low energy availability, menstrual dysfunction (amenorrhea), and low bone mineral density), RED-S recognizes that energy deficits affect multiple body systems and both genders. The 2023 IOC consensus updates define RED-S as "impaired physiological and/or psychological functioning caused by relative energy deficiency," encompassing health issues like metabolic slowdown, hormonal imbalances, bone loss, and performance declines (Mountjoy et al., 2023).

At its core, LEA occurs when energy intake is too low relative to exercise demands, leaving insufficient fuel for essential functions. As Mountjoy et al. (2023) explain, "LEA is a mismatch between an athlete’s energy intake from their diet and the energy expended during exercise, which results in inadequate energy to support the physiological functions needed by the body to maintain optimal health and performance." This imbalance triggers a cascade: the body prioritizes survival over performance, shutting down non-essential systems.

Physiologically, RED-S disrupts:

• Hormonal Health: Low estrogen/testosterone affects reproduction, bone density, and mood.

• Bone Health: Increased fracture risk due to reduced bone mineral density (BMD).

• Metabolic Function: Slower metabolism, impaired glucose regulation, and fatigue.

• Immune System: Higher infection rates and slower healing.

• Cardiovascular Effects: Bradycardia (low heart rate) and reduced cardiac output.

• Psychological Impacts: Anxiety, depression, irritability, and disordered eating patterns.

Performance-wise, RED-S leads to decreased endurance, strength losses, and injury spikes. A 2022 review by Cabre et al. notes that "chronic LEA can lead to increased detrimental health outcomes in athletes and has been termed as the 'Relative Energy Deficiency in Sport' syndrome." RED-S isn't just "underfueling" — it's a syndrome with measurable impacts. Research from the British Journal of Sports Medicine (BJSM) shows that LEA below 30 kcal/kg fat-free mass/day triggers symptoms, with severe cases (<25 kcal/kg) accelerating damage (Loucks et al., 2011). It's not always intentional; busy schedules, travel, or miscalculated needs can contribute.

Who is Most at Risk?

RED-S can affect any athlete, but certain groups face higher risks due to physiological, cultural, or sport-specific factors. The 2023 IOC statement highlights that "REDs affects athletes of all genders, ages, abilities, and sports," but prevalence is higher in:

1. Female Athletes: Historically linked to the Female Athlete Triad, females are at elevated risk due to menstrual cycles and societal pressures for leanness. A 2024 study by Angelidi et al. reports that "women athletes are particularly susceptible, with prevalence rates up to 60% in some sports." Hormonal disruptions like amenorrhea (absent periods) are red flags, increasing osteoporosis risk. In endurance sports like running or cycling, up to 45% of females show LEA signs (Slater et al., 2016).

2. Endurance Athletes: Cyclists, runners, triathletes — anyone with high-volume training. A BJSM review found 40-70% of endurance athletes experience LEA, as prolonged sessions deplete glycogen without adequate refueling (Logue et al., 2020). Our Project Blue clients often fall here: long rides burn 500-800 kcal/hour, but underestimation leads to deficits.

3. Young Athletes: Adolescents and teens are vulnerable due to growth demands. The IOC notes that "young athletes may be at higher risk due to rapid growth phases requiring extra energy" (Mountjoy et al., 2023). In sports like gymnastics or swimming, 20-30% show RED-S markers.

4. Weight-Class or Aesthetic Sports: Boxers, wrestlers, rowers, dancers, or gymnasts where low weight is prized. Pressure to "make weight" often induces intentional LEA. A 2023 PLOS One study on netball players revealed "external pressures from coaches and social media" as key risks (O’Donnell et al., 2023).

5. Male Athletes: Once overlooked, males comprise up to 50% of cases in some studies. Symptoms include low testosterone, reduced libido, and mood issues. Angelidi et al. (2024) state, "REDs in males can impair the GH-IGF1 axis, leading to muscle loss and fatigue."

Other risks: Vegan/vegetarian diets (B12/iron shortfalls), high-stress lifestyles, or genetics affecting metabolism.

Quotes from literature underscore the scope: "RED-S is a syndrome of impaired health and performance due to low energy availability relative to energy needs in physical activity" (Langbein-Stott et al., 2025). Cabre et al. (2022) warn, "If athletes develop low energy availability, it can lead to RED-S with severe health consequences if not treated."

How to Avoid or Prevent RED-S

Prevention starts with awareness and balance. The IOC recommends "maintaining EA >45 kcal/kg FFM/day" for optimal function (Mountjoy et al., 2023). Here's how:

1. Track Energy Availability: Calculate EA = (Energy Intake - Exercise Energy Expenditure) / Fat-Free Mass. Use apps like MyFitnessPal or Project Blue's Resting Metabolic Rate testing to know exact daily calorie requirements. Aim for 45+ kcal/kg FFM; below 30 signals risk.

2. Fuel Adequately: Match intake to output. For endurance athletes, consume 50-90g carbs/hour during long sessions (Stellingwerff et al., 2019). Prioritize whole foods: proteins for repair, carbs for glycogen, fats for hormones. Thorne's Basic Nutrients 2/Day provides baseline support.

3. Periodize Nutrition: Align fueling with training. High-volume days: extra calories; rest days: maintenance. Avoid chronic restriction — even "clean eating" can undershoot if volume-focused.

4. Monitor Biomarkers: Regular bloodwork for hormones (testosterone/estrogen), BMD (DEXA scans), and ferritin.

5. Incorporate Rest and Recovery: Overtraining amplifies risks. Include off-days, sleep 7-9 hours, and manage stress. Yoga or meditation reduces cortisol, per BJSM studies.

6. Educate Teams: Coaches should promote body-positive cultures. The 2023 IOC urges "multidisciplinary teams" for early intervention (Mountjoy et al., 2023).

We need more awareness, more research, and better education for athletes, coaches, parents, and medical professionals alike. Project RED-S (https://red-s.com), set up by former elite runner Pippa Woolven, is doing a fantastic job of doing just that. Furthermore, Dr Nicky Keay has created the Personal Energy Availability Questionnaire (PEAQ) based on published research. The PEAQ is a series of questions to help you understand your potential risk of REDs.

If You're Dealing with RED-S, Who to Contact for Help

RED-S requires prompt intervention, delays worsen outcomes. Cabre et al. (2022) emphasize "early detection and multidisciplinary care" to restore energy balance.

1. Primary Care Physician or Sports Medicine Doctor: Start here for diagnosis. They can order blood tests, bone scans, and rule out other issues.

2. Registered Dietitian (RD) or Sports Nutritionist: Specialized in athlete fueling. The Academy of Nutrition and Dietetics recommends RDs certified in sports (CSSD) for personalized plans.

3. Endocrinologist: For hormonal imbalances. Angelidi et al. (2024) note "endocrine manifestations like hypoestrogenemia" need specialist care.

4. Psychologist or Mental Health Professional: Address underlying behaviors (e.g., disordered eating). CBT is effective, per IOC guidelines.

5. Project Blue or Similar Optimization Services: We offer testing (CGM, sweat, VO2) and Thorne partnerships for targeted recovery.

Seek help if symptoms persist: fatigue, injuries, menstrual changes, or performance plateaus. As Dave et al. (2022) state, "RED-S expands on the Female Athlete Triad, with impaired function across organ systems."

Conclusion

RED-S is a preventable yet pervasive threat, affecting health and performance through energy mismatches. Females, endurance athletes, and young competitors are most at risk, but awareness and balanced fueling can mitigate it. As Mountjoy et al. (2023) conclude, "REDs is a syndrome with broad implications, but early intervention restores function." At Project Blue, we help clients avoid RED-S with data-driven plans, stable energy for today's rides and tomorrow's health. Let us help you fuel your future!