In sport culture, “fitness” is often treated as synonymous with health. Faster splits, lower resting heart rates, leaner bodies, and higher training volumes are seen as proof that the body is adapting well. But for many, these outward markers of underfueling in athletes can hide a bigger issue.
Relative Energy Deficiency in Sport (REDs) is often overlooked because the signs are missed as red flags. They’re often looked at as common in athletes. And while they may be common, they’re not normal. REDs often develops quietly, while performance appears stable, training continues uninterrupted, and the athlete is praised for discipline and resilience. By the time symptoms become impossible to ignore, multiple physiological systems may already be affected.
This is where the distinction between appearing fit and being healthy becomes critical.
When Performance Masks Underfueling in Athletes
One of the most challenging aspects of REDs, for athletes and clinicians alike, is that performance metrics often hold up longer than physiology does.
Athletes may:
- Continue training at high volumes
- Maintain race results or fitness benchmarks
- Appear lean, strong, and highly conditioned
Meanwhile, behind the scenes, the body isn’t adapting for performance, it’s adapting for survival.
When energy intake consistently fails to meet energy demand (kind of like trying to pull money out of a bank account that doesn’t have the funds), the body prioritizes systems essential for immediate survival. Processes that support long-term health, recovery, and adaptation are quietly downregulated. This allows athletes to keep training in the short term, but at a growing physiological cost.
This is why REDs is so frequently missed in high-performing athletes. The absence of immediate performance decline is often misinterpreted as evidence that fueling must be “fine,” yet underfueling in athletes is the real culprit.
Early REDs Signals That Get Normalized as “Just Training”
Many athletes who eventually meet criteria for REDs can trace symptoms back months or even years before diagnosis. These early signals are often subtle and culturally normalized within sport environments.
Common early signs of underfueling in athletes include:
- Persistent fatigue that feels disproportionate to training
- Needing more caffeine to get through workouts
- Increased soreness or slower recovery
- Feeling cold more often, especially in the hands and feet
- Disrupted sleep or difficulty staying asleep
- Plateauing performance despite increased training load
Because these symptoms develop gradually, they are often framed as:
- “Normal training fatigue”
- “Just part of getting older”
- “A sign I need to train harder”
- “What it takes to be competitive”
This normalization is one of the reasons REDs can progress without being diagnosed. Athletes are taught to override internal cues in favor of external metrics, and those metrics may lag far behind physiological strain.
A Systems Problem, Not a Single Symptom
REDs is not a condition of willpower, motivation, or discipline. It is a systems-level physiological response to low energy availability.
The human body does not experience energy deficiency in isolation. When one system is stressed, others feel the effects and suffer with it. This concept, often referred to as inner organ cross-talk, helps explain why REDs can affect such a wide range of functions, including:
- Endocrine signaling
- Metabolic regulation
- Cardiovascular function
- Thermoregulation
- Bone remodeling
- Reproductive health
Rather than failing outright, the body adapts by reallocating resources. Hormones shift, metabolic pathways adjust, and nervous system activity changes. These adaptations are protective in the short term, but maladaptive when sustained.
Understanding REDs requires stepping back from individual symptoms and looking at how systems interact under chronic energy stress.
Why the Body Downregulates to Survive
From a physiological standpoint, REDs represents a smart yet costly response to perceived scarcity.
When energy is limited, the body:
- Conserves fuel by slowing metabolic rate
- Reduces hormone production not essential for survival
- Alters autonomic nervous system activity
- Shifts substrate utilization to preserve energy
None of these changes are accidental. They are protective mechanisms designed to keep the body alive during periods of shortage.
The problem arises when athletes continue to train as though the body is fully resourced while it is quietly operating in conservation mode.
Over time, these survival adaptations begin to show up in places athletes and clinicians don’t always expect.
Why the Heart Is Often Where REDs Gets Missed
Cardiovascular changes are among the most misunderstood adaptations in athletes. A low resting heart rate is often celebrated as a marker of fitness, and “normal” or even favorable lab values are assumed to reflect good health. But in the context of chronic under-fueling in athletes, these same findings can tell a very different story.
A suppressed heart rate is not always a sign of efficiency. Bloodwork can shift in ways that resemble overnutrition—even when intake is inadequate. These changes are not random; they reflect the body’s attempt to conserve energy and preserve survival when resources are limited. When systems are evaluated in isolation, these nuances are easy to miss.
As Heart Health Month approaches in February, we’ll take a closer look at how REDs shows up in the cardiovascular system, examining heart rate, heart rate variability, and cholesterol through a REDs-informed lens. Understanding these patterns requires moving beyond surface-level metrics and learning to interpret the body as an integrated system.
This systems-based framework is the foundation of how I train clinicians in my REDs Informed Provider Certification Program®. Under-fueling in athletes rarely presents as a single symptom or a sudden performance collapse. It reveals itself through subtle, interconnected shifts across metabolic, endocrine, and cardiovascular systems, often long before traditional red flags appear.
For clinicians and coaches who want to identify REDs earlier and provide more informed care, join the program today to become a certified REDs Informed Provider.