Understanding Relative Energy Deficiency in Sport: Why Fueling is the Foundation of Athletic Performance

When it comes to exercise, there is one non-negotiable factor that ranks alongside training intensity, recovery protocols, and periodization: nutritional fueling. Sustaining physical performance requires a precise balance of caloric intake and nutrient density, which not only powers the body through immediate athletic demands but also facilitates long-term physiological homeostasis. For many athletes, however, the act of eating is relegated to an afterthought, a practice that can lead to a precarious physiological state known as Relative Energy Deficiency in Sport (RED-S). Recent high-profile disclosures from elite athletes have brought this condition—previously termed the "female athlete triad"—into the public consciousness, highlighting the urgent need for a more nuanced understanding of energy availability.

The Scope and Definition of RED-S

At its core, RED-S occurs when an individual’s energy intake is insufficient to cover the energy expended through exercise while simultaneously supporting basic metabolic functions. Dr. Elizabeth Matzkin, an orthopedic surgeon and chief of women’s sports medicine at Brigham and Women’s Hospital, explains that the body requires a consistent energy surplus to maintain vital systems, including bone remodeling, hormonal regulation, endocrine health, cardiovascular efficiency, and immune system resilience.

When the body is chronically underfueled, it begins a process of physiological rationing. It downregulates non-essential functions to preserve energy for basic survival. This transition is not merely about weight loss or aesthetic changes; it is a systemic breakdown. The condition is termed "relative" because it is entirely dependent on the individual’s specific training load and metabolic needs. A sedentary individual requires significantly fewer calories than a professional marathoner; therefore, RED-S can affect anyone, from a recreational gym-goer attempting to lose weight to an elite athlete training for Olympic-level competition.

Chronology of a Crisis: The Case of Emma Navarro

The professional tennis circuit recently witnessed a candid acknowledgment of this struggle. Top-ranked tennis player Emma Navarro publicly detailed her multi-year battle with the condition via social media. Navarro’s account provides a clinical look at how the symptoms of RED-S manifest over time. According to her statement, she had been operating in a state of chronic underfueling for nearly three years.

The timeline of her symptoms illustrates the insidious nature of the condition:

• Initial Phase: Persistent hunger and subtle declines in recovery capacity.
• Secondary Phase: Onset of cognitive issues, including "brain fog," and extreme, unexplained mood swings.
• Tertiary Phase: Physiological collapse, marked by a weakened immune system, frequent headaches, and dizzy spells that made previously manageable training loads feel impossible to complete.

Navarro’s experience underscores that RED-S is not an acute injury that happens overnight, but rather a slow-moving erosion of health that can be difficult for the athlete to recognize until it has significantly impacted their professional performance and personal well-being.

Data and Clinical Prevalence

Research suggests that the incidence of RED-S is alarmingly high within professional sports populations. A 2026 study published in the Journal of Science and Medicine in Sport found that approximately 70 percent of high-level female athletes were at risk of developing the condition. This statistic is particularly concerning given that the clinical consequences of RED-S often do not present until significant internal damage has occurred.

Historically, the medical community categorized this issue as the "female athlete triad," which focused on the intersection of three conditions: disordered eating, menstrual dysfunction, and low bone mineral density. The shift to the term "RED-S" in 2014 was intended to broaden the clinical scope, acknowledging that the condition affects men as well and impacts far more physiological systems than just those associated with the original triad. Despite this, women remain at a disproportionately higher risk, influenced by biological factors, the specific demands of sports that emphasize aesthetic leanness (such as gymnastics, figure skating, and distance running), and pervasive societal pressures regarding body image.

Clinical Symptoms and Long-Term Implications

The symptoms of RED-S are diverse and often mimic other medical conditions, which complicates diagnosis. Dr. Amy West, a sports medicine physician at the Northwell Orthopedic Institute, notes that clinicians must look for a constellation of indicators rather than a single symptom.

Short-term indicators include:

• Unexplained, persistent fatigue.
• Impaired concentration and cognitive decline.
• Increased frequency of illness due to a suppressed immune system.
• Menstrual irregularities or amenorrhea (the loss of a period).
• Decreased muscle strength and endurance.

The long-term risks, however, are where the medical community expresses the most concern. Adolescence and early adulthood represent the critical window for bone accrual. If an athlete spends these years in a state of low energy availability, the resulting reduction in bone mineral density can lead to early-onset osteoporosis and an increased risk of stress fractures that never fully heal. Furthermore, chronic hormonal imbalances caused by prolonged underfueling can have lasting impacts on fertility and cardiovascular health.

Dietary Analysis and Preventative Strategies

Amy Stephens, a registered dietitian who specializes in the treatment of eating disorders, emphasizes that preventing RED-S does not necessarily require rigorous calorie counting or the use of complex tracking applications. Instead, she advocates for an intuitive, structured approach to nutrition.

Stephens suggests a three-pillar strategy for maintaining adequate energy availability:

1. Consistent Meal Structure: Aiming for three balanced meals per day, with each plate consisting of approximately one-third protein, one-third carbohydrates, and one-third vegetables. Breakfast is identified as the most commonly skipped meal, serving as a primary culprit in daily energy deficits.
2. Training-Dependent Intake: Athletes must scale their nutritional intake to match their training cycle. A standard rule of thumb suggested by experts is to add approximately 500 to 600 calories for every hour of intense physical activity.
3. Proactive Monitoring: Regular blood workup, including checks on thyroid function, ferritin levels, Vitamin D, and white blood cell counts, can provide an objective baseline for metabolic health.

When to Seek Professional Intervention

The threshold for seeking medical advice should be low. Dr. Matzkin advises that athletes should not wait for a catastrophic injury, such as a stress fracture, to consult a physician. If an athlete experiences persistent fatigue, sleep disturbances, or menstrual irregularities, these should be treated as diagnostic red flags.

Treatment protocols for diagnosed RED-S are multidisciplinary. They often involve a team consisting of a primary care physician, a sports dietitian, and, in many cases, a sports psychologist to address the psychological underpinnings of restrictive eating habits. The primary goal is the gradual restoration of energy balance, which may involve reducing training intensity while simultaneously increasing caloric intake.

Broader Implications for the Sports World

The conversation surrounding RED-S is part of a larger shift in professional and amateur sports toward prioritizing athlete health over "win-at-all-costs" performance models. By acknowledging that a body in a state of deficiency cannot perform at its peak, the sports world is beginning to treat nutrition as a fundamental pillar of training rather than an optional add-on.

The implications for the next generation of athletes are significant. As organizations, coaches, and athletes become more educated on the mechanics of energy availability, the hope is to destigmatize the act of fueling and normalize the idea that an athlete’s worth—and their career longevity—is tied to their health, not their ability to maintain an unrealistic or unsustainable weight.

In conclusion, the path to athletic excellence is paved with fuel. Whether competing on a grand stage or training for a personal milestone, the human body acts as a biological engine. Without the necessary input, the engine will eventually fail. The growing awareness of RED-S serves as a necessary reminder that while grit and determination are essential components of success, they are no substitute for the physiological fuel required to sustain the pursuit of greatness. The expert consensus remains clear: athletes almost always require more nutrition than they initially believe, and the key to longevity in sport is recognizing the body’s warning signs long before they manifest as lasting damage.