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RED-S and Underfueling: Why Healthy-Looking Athletes Can Still Be at Risk

Princeton Sports and Family Medicine, P.C. Blog RED-S and Underfueling: Why Healthy-Looking Athletes Can Still Be at Risk

RED-S and underfueling are easy to miss.

An athlete may look fit. They may train every day. They may eat what appears to be a “healthy” diet. They may be disciplined, motivated, and high-achieving. They may even be performing well for a while.

But inside, the body may not be getting enough energy to support everything being asked of it.

Training requires fuel. Growth requires fuel. Recovery requires fuel. Hormones, bone health, immune function, mood, concentration, and normal body systems require fuel too. When energy intake is too low relative to exercise demands, the body starts making tradeoffs.

That pattern is called low energy availability. When it becomes problematic and affects health or performance, it may contribute to Relative Energy Deficiency in Sport, commonly called RED-S or REDs.

For athletes in Princeton, Lawrenceville, West Windsor, Plainsboro, Hopewell, Pennington, Robbinsville, and Mercer County, the key message is simple: an athlete does not have to look underweight to be underfueled. Fatigue, recurrent injuries, poor recovery, menstrual changes, low mood, or declining performance may be early clues that the athlete’s body is not getting what it needs.

Quick Takeaways

What Is RED-S?

RED-S stands for Relative Energy Deficiency in Sport.

It refers to a state where an athlete does not have enough energy available to support both exercise and normal body function. The issue is not just calories in a general sense. It is whether enough energy remains after exercise demands are accounted for.

When energy availability is too low for too long, the body may reduce or disrupt important functions.

These may include:

RED-S is broader than the older concept of the Female Athlete Triad. It can affect male and female athletes, and it can influence many body systems.

What Is Underfueling?

Underfueling means the athlete is not eating enough to match the demands of training, recovery, growth, and daily life.

Sometimes underfueling is intentional. The athlete may be trying to lose weight, stay lean, make a weight class, improve body composition, or meet perceived sport expectations.

Sometimes it is unintentional. The athlete may simply not realize how much energy they need.

Unintentional underfueling can happen when:

Underfueling can happen even when the athlete is eating foods that look healthy.

The issue is total availability.

Why Healthy-Looking Athletes Can Still Be at Risk

One of the most dangerous myths is that underfueling only happens in athletes who look very thin.

That is not true.

Athletes can be underfueled at many body sizes. Some may have normal weight. Some may have high muscle mass. Some may look strong. Some may still be winning races or starting on varsity.

The body can compensate for a while.

An athlete may maintain performance temporarily by pushing through fatigue. But over time, signs may appear:

By the time performance clearly drops, the athlete may have been underfueled for a while.

That is why early recognition matters.

RED-S Is Not Just an Eating Disorder

RED-S and eating disorders can overlap, but they are not the same thing.

Some athletes with RED-S have disordered eating or an eating disorder. Others do not. Many are not intentionally restricting food. They may simply be underestimating their needs or following nutrition habits that cannot support their training.

Examples include:

The evaluation should be curious, not judgmental.

The goal is to understand what is happening and protect the athlete’s health.

Common Signs of RED-S and Underfueling

RED-S can show up in many ways.

Possible signs include:

No single symptom proves RED-S. But patterns matter.

An athlete with fatigue, recurrent injuries, poor recovery, and restrictive fueling habits deserves a closer look.

Fatigue Is Often the First Clue

Fatigue is one of the most common early signs.

Athletes may say:

Fatigue can come from many causes, including sleep loss, illness, iron deficiency, thyroid problems, asthma, mood concerns, training load, or stress.

But underfueling should be considered, especially if fatigue comes with poor recovery, injuries, menstrual changes, or food restriction.

The answer is not always more rest. It may be more fuel, better timing, and a smarter training plan.

Performance Decline and Plateaus

Athletes often notice performance changes before they recognize health changes.

They may train consistently but stop improving. They may lose speed, power, endurance, or the ability to repeat hard efforts. They may be able to complete workouts but need longer recovery. Race results may decline despite effort.

Performance signs may include:

Coaches may interpret this as lack of toughness or poor conditioning. Sometimes the real issue is that the athlete is not fueled enough to adapt.

Training only works when the body has the resources to recover from it.

Bone Stress Injuries Are a Major Warning Sign

Recurrent bone stress injuries should raise concern for low energy availability.

Stress reactions and stress fractures can happen for many reasons, including training errors, biomechanics, footwear, surfaces, and bone health. But underfueling can be a major contributor.

When the body does not have enough energy, bone remodeling and repair can be affected. That may increase injury risk over time.

Concerning patterns include:

A runner, dancer, gymnast, rower, or field athlete with repeated bone stress injuries should be evaluated for more than mechanics.

The question should be: why is the bone not tolerating load?

Menstrual Changes Are a Vital Sign

In female athletes, menstrual history is a key health marker.

Irregular periods, missed periods, delayed first period, or a change from previously regular cycles can be a warning sign of low energy availability.

A missing period should not be dismissed as “normal for athletes.”

Menstrual changes may reflect:

The menstrual cycle provides important information about whether the body has enough energy to support normal function.

Female athletes should be asked about menstrual history in a respectful, routine way.

RED-S in Male Athletes

RED-S can affect male athletes too.

Male athletes may be under-recognized because menstrual history is not available as a warning sign. Symptoms may be subtle.

Possible signs in male athletes include:

Sports that emphasize leanness, endurance, weight classes, aesthetics, or power-to-weight ratio may carry risk for athletes of any sex.

Male athletes also need nutrition screening when symptoms fit.

Sports Where RED-S Risk May Be Higher

RED-S can occur in any sport, but risk may be higher when low body weight, leanness, appearance, or power-to-weight ratio is emphasized.

Higher-risk contexts may include:

Risk is not limited to elite athletes.

Middle school, high school, college, recreational, and adult athletes can all be affected.

“Eating Clean” Can Still Be Underfueling

Many athletes eat what they describe as a clean diet.

They avoid processed foods. They eat salads, lean proteins, vegetables, smoothies, and snacks that seem healthy. But the total energy intake may still be too low.

A high-volume athlete may need more carbohydrate, total calories, and recovery fuel than they expect.

Common underfueling patterns include:

Food quality matters, but quantity and timing matter too.

An athlete can eat healthy foods and still not eat enough.

Carbohydrates Are Not the Enemy

Many athletes underfuel because they fear carbohydrates.

Carbohydrates are an important fuel source for high-intensity sport, endurance training, strength work, recovery, and repeated efforts.

Low carbohydrate intake may contribute to:

This does not mean every athlete needs the same carbohydrate target. Needs vary by sport, training load, body size, medical history, and goals.

But athletes should not automatically avoid carbohydrates because of social media or diet culture.

Fuel should match the work.

Protein Matters, But It Is Not Enough

Protein is important for muscle repair, recovery, satiety, and adaptation.

But many athletes focus on protein while still underfueling overall.

An athlete may eat protein at meals but still lack enough carbohydrate or total calories to support training. That can leave the body short on usable energy.

A balanced fueling plan should consider:

Protein is one piece. It cannot solve low energy availability by itself.

RED-S and Iron Deficiency

Underfueling may overlap with low iron.

An athlete who is not eating enough may not get enough iron, protein, or total nutrients. Menstruating athletes may have additional iron losses. Endurance athletes may have higher risk because of training demands, sweat, GI issues, or diet patterns.

Signs that iron should be considered include:

Iron supplements should not be started blindly. Testing and interpretation matter.

Low iron may be one clue in a larger underfueling picture.

RED-S and Mental Health

RED-S can overlap with mental health concerns.

Sometimes anxiety, depression, perfectionism, body image pressure, or disordered eating contributes to underfueling. Other times, low energy availability worsens mood, irritability, sleep, and concentration.

Warning signs may include:

These concerns should be handled with care.

The athlete needs support, not shame.

What Parents Should Watch For

Parents may notice changes before the athlete does.

Warning signs include:

Parents should approach the conversation with curiosity and concern, not accusation.

Try asking:

The goal is safety and support.

What Coaches Should Watch For

Coaches play an important role.

They may notice:

Coaches should avoid body-shaming language, public weight comments, and praise that links performance only to leanness.

A safer team culture emphasizes fueling, strength, recovery, durability, and long-term development.

If RED-S is suspected, the athlete should be referred for medical and nutrition evaluation.

How RED-S Is Evaluated

Evaluation should be individualized.

A clinician may review:

Labs may be considered depending on the athlete.

Possible evaluation may include:

Not every athlete needs every test. The story should guide the workup.

Treatment: More Than “Eat More”

Telling an athlete to “just eat more” is often not enough.

Treatment may include:

The plan should identify why underfueling is happening.

Is the athlete intentionally restricting? Unintentionally underestimating needs? Struggling with appetite? Afraid of weight gain? Missing school meals? Training too much? Dealing with GI symptoms? Experiencing anxiety?

The cause guides the treatment.

Should the Athlete Stop Training?

It depends on risk level.

Some athletes may continue modified training while fueling improves. Others may need a temporary reduction in intensity, volume, or competition. Athletes with stress fractures, significant medical abnormalities, severe underfueling, eating disorder concerns, or dangerous symptoms may need stricter limits.

Participation decisions should be individualized and medically guided.

The goal is to protect long-term health and performance.

Short-term restriction from sport may feel difficult, but it can prevent longer-term consequences.

Return to Sport After RED-S

Return to sport should be based on recovery, not just motivation.

A return plan may consider:

Returning to full training before fueling and health are restored can lead to recurrence.

The athlete should return with a plan that prevents the same pattern from repeating.

Prevention: Building a Fueling Culture

Prevention starts with culture.

Teams and families can reduce risk by emphasizing:

Athletes should understand that food is not the enemy of performance. Food is part of performance.

RED-S and Underfueling Near Princeton, NJ

Athletes and families in Princeton, Lawrenceville, West Windsor, Plainsboro, Hopewell, Pennington, Robbinsville, and Mercer County often seek care when fatigue, injury, or performance decline does not make sense.

They may be asking:

Those questions deserve a whole-athlete evaluation.

How PSFM Can Help

At Princeton Sports and Family Medicine, RED-S and underfueling concerns are evaluated by looking at the full athlete: training load, nutrition, recovery, sleep, growth, menstrual history, injury pattern, labs, performance, and mental health context.

Sports Medicine Services can help evaluate fatigue, performance decline, recurrent injury, bone stress injury, underfueling concerns, RED-S risk, and return-to-sport decisions.

Primary Care Services can help evaluate medical contributors such as anemia, thyroid concerns, illness, menstrual changes, mood symptoms, medications, and lab abnormalities.

Physical Therapy can help when RED-S concerns overlap with pain, injury, strength deficits, deconditioning, or return-to-training progression.

For condition-specific education, PSFM provides pages on Relative Energy Deficiency in Sport, Fatigue, Iron Deficiency / Anemia, Nutrition Basics, Sports Performance Testing, and Sports Physical.

VO₂max & Lactate Testing can help guide training zones once medical contributors to fatigue and under-recovery are appropriately addressed.

For runners and field athletes, the Run Stride & Performance Evaluation can help connect symptoms to mechanics, impact loading, and training patterns after the athlete is medically ready.

PSFM Wellness can support healthy training habits, recovery, and long-term wellness. Fuse Sports Performance can help athletes build strength, durability, and performance capacity when fueling and medical status support progression.

The goal is not simply to get the athlete back quickly. The goal is to restore health, improve resilience, and support long-term performance.

Related Pages

FAQs

What is RED-S?

RED-S, or REDs, stands for Relative Energy Deficiency in Sport. It occurs when an athlete does not have enough available energy to support training and normal body function.

Is RED-S only a problem for female athletes?

No. RED-S can affect athletes of any sex. Female athletes may have menstrual changes as an important warning sign, but male athletes can also experience fatigue, low hormones, injury, and performance decline.

Can an athlete have RED-S without looking underweight?

Yes. Athletes can be underfueled at many body sizes. Looking fit, lean, muscular, or healthy does not rule out low energy availability.

What are signs of underfueling in athletes?

Signs may include fatigue, poor recovery, recurrent injuries, frequent illness, mood changes, sleep disruption, performance decline, feeling cold, menstrual changes, and bone stress injuries.

Are missed periods normal for athletes?

No. Missed or irregular periods should not be dismissed as normal training effects. Menstrual changes can be a warning sign of low energy availability or other medical issues.

Can underfueling cause stress fractures?

Underfueling can contribute to impaired bone health and poor recovery, which may increase risk for bone stress injuries. Recurrent stress fractures should prompt evaluation for RED-S risk factors.

How is RED-S treated?

Treatment is individualized and may include medical evaluation, nutrition support, training-load modification, mental health support, lab monitoring, injury treatment, and return-to-sport planning.

Where can athletes be evaluated for RED-S near Princeton, NJ?

Princeton Sports and Family Medicine evaluates RED-S, underfueling, fatigue, recurrent injuries, bone stress injuries, and performance decline in Lawrenceville for athletes in Princeton and surrounding Mercer County communities.

The Bottom Line

RED-S and underfueling can affect athletes who look healthy, fit, and disciplined.

The warning signs may be subtle at first: fatigue, poor recovery, recurrent injuries, illness, mood changes, menstrual changes, or performance decline. The athlete may not need more toughness. They may need more fuel, better recovery, medical evaluation, and a smarter training plan.

If an athlete is struggling with fatigue, repeated injuries, underfueling concerns, menstrual changes, or unexplained performance decline, request an appointment with Princeton Sports and Family Medicine.

This content is educational only and is not a substitute for individualized medical advice. If an athlete has chest pain, fainting, unexplained shortness of breath, severe fatigue, rapidly worsening symptoms, unintended weight loss, new weakness, confusion, fever, severe dehydration, abnormal bleeding, black or bloody stools, severe abdominal pain, major trauma, inability to bear weight, significant swelling, or other concerning symptoms, seek medical evaluation.

Author
Peter Wenger, MD Peter C. Wenger, MD, is an orthopedic and non-operative sports injury specialist at Princeton Sports and Family Medicine, P.C., in Lawrenceville, New Jersey. He is board certified in both family medicine and sports medicine. Dr. Wenger brings a unique approach to sports medicine care with his comprehensive understanding of family medicine, sports medicine, and surgery. As a multisport athlete himself, he understands a patient’s desire to safely return to their sport.

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