RED-S: When an Active Body Isn't Getting Enough Fuel and the Hidden Threats

What Parents, Teens, and Active Adults Need to Know About Relative Energy Deficiency in Sport and the hidden threats to the body

There is a health condition I think far more parents, athletes, coaches, and even healthcare providers need to talk about: Relative Energy Deficiency in Sport, or RED-S (also written REDs).

I live and work in a VERY fit community, and it doesn't come without its share of health concerns. RED-S can occur in competitive athletes, recreational athletes, dancers, gymnasts, runners, skiers, cyclists, swimmers, wrestlers, weightlifters, CrossFit athletes, and highly active adults.

And one of the most important things to understand is this: Someone does not have to look underweight, have an eating disorder, or even be intentionally restricting food to develop RED-S.

RED-S develops when the body repeatedly doesn't receive enough available energy to support everything it's asked to do.

For an athlete, food doesn't just have to provide enough energy to get through a workout. The body also needs energy to maintain the brain, heart, gastrointestinal tract, immune system, hormones, reproductive system, muscles, bones, blood production, body temperature, tissue repair, and in children and teenagers, growth and puberty.

When exercise demands become high, but nutrition doesn’t increase appropriately, the body eventually has to make compromises.

That is where RED-S begins.

What Is RED-S?

RED-S stands for Relative Energy Deficiency in Sport. The International Olympic Committee describes RED-S as a syndrome of impaired physiological and/or psychological functioning caused by exposure to problematic, prolonged, and/or severe low energy availability (LEA). Low energy availability occurs when not enough dietary energy remains to support normal physiological function after accounting for exercise.

In sports medicine, energy availability is generally represented as: Energy Availability = (Energy Intake − Exercise Energy Expenditure) ÷ Fat-Free Mass

In other words, how much energy is left for your body after exercise takes its share?

Historically, sports medicine focused primarily on the Female Athlete Triad: low energy availability, menstrual dysfunction, and poor bone health. We now know the problem extends beyond those three systems. RED-S can affect both females and males, and the consequences may involve reproductive hormones, bones, muscles, cardiovascular health, immunity, metabolism, blood health, gastrointestinal function, cognition, sleep, mental health, growth, recovery, and athletic performance.

That expanded understanding is incredibly important because many people with RED-S never fit the stereotypical image of an athlete with an eating disorder.

RED-S Is NOT the Same Thing as an Eating Disorder

This distinction matters. An eating disorder can absolutely cause low energy availability and RED-S. Disordered eating behaviors, such as chronic restriction, skipping meals, fear of certain foods, bingeing, purging, laxative use, compulsive exercise, or an intense drive for thinness, are also significant risk factors. But RED-S itself is not an eating disorder.

Low energy availability can be completely unintentional. An athlete may simply underestimate how much food their body needs. A teenager might eat breakfast, lunch, dinner, and snacks and still not consume enough energy to support puberty plus two hours of sports practice. A runner may dramatically increase weekly mileage without increasing food intake. A busy adult may exercise before work, skip breakfast, eat a salad at lunch, have a healthy dinner, and genuinely believe they are eating well—while chronically consuming too little energy for their activity level.

Some athletes intentionally eliminate carbohydrates or fats because they believe doing so is healthier. Others develop food intolerances, nausea, bloating, early fullness, or gastrointestinal symptoms that make eating enough increasingly difficult. Low energy availability can therefore result from intentional restriction, unintentional under-fueling, excessive exercise, inadequate sports nutrition knowledge, gastrointestinal problems, food insecurity, dietary restrictions, psychological factors, or a combination of these.

Why I Am Particularly Concerned About RED-S in Tweens and Teenagers

Adolescence creates a unique metabolic situation. A 13-year-old athlete isn't simply a smaller adult athlete. Their body needs energy for school, concentration, movement, temperature regulation, immune function, digestion, and exercise—but it is also simultaneously building bone, muscle, reproductive capacity, brain tissue, and an adult body.

Then we add sports. Sometimes a lot of sports. Club practices. School practices. Strength training. Weekend tournaments. Ski training. Dance rehearsals. Running. Conditioning —Their energy requirements can become enormous. When food intake doesn't keep pace, the body prioritizes immediate survival over less essential processes. Growth, reproduction, bone formation, recovery, and optimal hormone production can suffer.

Research in adolescent athletes increasingly emphasizes RED-S as a concern for both girls and boys, particularly because adolescence represents an important period for bone mineral accrual and physical development. This is one reason I don't consider chronic fatigue, recurrent injuries, disappearing menstrual cycles, or stalled growth to be simply "part of being a serious athlete." They deserve investigation.

The Parent RED-S Checklist: Red Flags You Shouldn't Ignore

No single symptom automatically means that your child has RED-S. What concerns me is when several of these begin appearing together or develop alongside increasing training volume, dietary restriction, or weight loss.

Physical and Medical Red Flags

• Unexplained weight loss or significant weight fluctuations

• Failure to gain expected weight during adolescence

• Slowed growth or delayed puberty

• Frequently feeling cold

• Persistent fatigue or low energy, dizziness or lightheadedness

• Recurrent illness, frequent colds or infections, and/or poor wound or injury healing

• Hair thinning or hair loss

• Difficulty sleeping

• Declining strength and/or Loss of muscle mass

• Recurrent muscle or tendon injuries or Multiple sports injuries

• Bone pain, Stress reactions, or stress fractures, or declining bone density

• Iron deficiency or anemia

• New or worsening gastrointestinal problems, Constipation, bloating, abdominal discomfort, nausea, or early fullness

• Declining athletic performance despite increased training

Hormonal and Reproductive Red Flags

For girls and women:

• Delayed first menstrual period

• Increasingly irregular periods

• Cycles becoming farther apart

• Missing periods

• Three or more consecutive missed periods

• Loss of a previously regular menstrual cycle

• Fertility concerns

• Symptoms suggesting low estrogen

For boys and men:

• Reduced libido

• Reduced morning erections

• Possible declining testosterone

• Poor muscle development or recovery

• Fatigue despite adequate sleep

• Declining strength or performance

Behavioral and Psychological Red Flags

Pay attention to:

• Increasing anxiety around food

• Suddenly eliminating major food groups

• Fear of carbohydrates or dietary fat

• Labeling foods as "clean," "bad," or "unhealthy" with increasing rigidity

• Skipping meals or choosing to eat in their room consistently during meal times

• Exercising despite illness or injury

• Significant distress when a workout is missed

• Adding exercise beyond what the coach prescribes

• Obsessive tracking of calories or activity

• Frequent weighing or body checking

• Increasing concern about body fat or weight

• Irritability

• Depression or low mood

• Difficulty concentrating

• Perfectionistic behavior

• Social withdrawal

• Eating differently from the rest of the family

• Avoiding restaurants, team meals, or social events involving food

Athletes often underreport disordered eating behaviors, which is another reason observable changes, such as sudden weight loss, hair loss, menstrual changes, recurrent injuries, or stress fractures, can be particularly important clues.

"But She Still Gets a Period."

This is another misconception worth discussing. Having menstrual bleeding does not necessarily mean reproductive hormones are functioning optimally. A female athlete can bleed but experience anovulatory cycles, meaning ovulation did not occur. Menstrual cycles may also gradually lengthen or become increasingly irregular before disappearing altogether. As low energy availability worsens, signaling through the hypothalamic-pituitary-ovarian axis can become suppressed. Gonadotropin-releasing hormone (GnRH), LH, and FSH signaling may decline, contributing to reduced ovarian estrogen and progesterone production and potentially progressing to functional hypothalamic amenorrhea. A disappearing period should therefore not be celebrated as a convenient side effect of athletic training. Losing your menstrual cycle because you exercise heavily is not a sign of superior fitness. It is a physiologic signal that deserves attention.

RED-S Happens in Boys and Men, Too

We used to talk primarily about female athletes because menstrual changes gave clinicians an obvious sign that something was wrong. Males don't have that built-in monthly indicator. That doesn't mean they are protected. Research has associated low energy availability in males with changes involving testosterone, leptin, insulin, and thyroid hormone T3, as well as metabolic adaptation. Male athletes with low energy availability have also reported more immunologic, metabolic, psychological, cardiovascular, and gastrointestinal dysfunction, along with poorer endurance, training response, judgment, coordination, and muscle strength. This means a teenage boy who becomes progressively leaner while experiencing recurrent injuries, fatigue, irritability, declining performance, poor recovery, or stalled development deserves the same attention.

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What Happens When the Body Doesn't Have Enough Energy?

The human body is remarkably adaptive, and when energy is insufficient, it begins conserving resources to protect the functions most essential for survival. This adaptation can help during a brief period of inadequate intake, but when the energy deficit becomes significant or persists over time, the body must prioritize where it spends its limited energy.

As a result, hormone production and resting metabolism can change, reproductive function may be suppressed, bone formation and remodeling can become impaired, and muscle repair and recovery may become less efficient. Immune defenses can also suffer, gastrointestinal symptoms may develop or worsen, and changes in mood, concentration, cognition, and sleep can begin to appear. In other words, RED-S is not simply about being tired or not eating enough, it can become a whole-body physiologic response to chronic under-fueling.

The 2023 IOC RED-S model recognizes potential effects across energy metabolism, reproductive function, bone health, immunity, glycogen synthesis, cardiovascular and blood health, gastrointestinal function, neurocognitive and mental health, sleep, skeletal muscle, and growth and development. Ironically, an athlete who initially restricts food or increases training to become leaner, faster, or more competitive may eventually undermine the very systems required for athletic performance and begin to perform worse.

RED-S and Bone Health

Bone health deserves special attention. Childhood and adolescence are critical periods for building the skeleton that will support you for the rest of your life. Low energy availability can alter reproductive and metabolic hormones and interfere with normal bone formation and remodeling.

The result may be:

• Reduced bone mineral accrual

• Lower bone mineral density

• Bone stress reactions

• Stress fractures

• Recurrent fractures

• Increased musculoskeletal injury risk

This is particularly concerning because some consequences may not immediately disappear once an athlete starts eating more. The review literature notes that bone recovery can take months to years after intervention. In adolescent and young adult athletes, accumulating RED-S/Triad risk factors have also been associated with higher-risk bone stress injuries. For a growing teenager, we are not merely trying to prevent today's stress fracture. We are trying to protect the skeleton they will have at 30, 50, and 70.

RED-S, the Gut, and the Microbiome

This area deserves considerably more attention. Gastrointestinal dysfunction is recognized within the RED-S spectrum, and studies of athletes with low energy availability have reported increased gastrointestinal symptoms. Clinically, an under-fueled athlete may report symptoms such as bloating, abdominal discomfort, constipation, nausea, altered bowel movements, early fullness, or difficulty tolerating larger meals.

Unfortunately, this can create a vicious cycle: The athlete isn't eating enough → gastrointestinal symptoms develop or worsen → eating becomes uncomfortable → the athlete eats even less → the energy deficit becomes worse.

But another piece of the puzzle may be the intestinal microbiome.

Your intestinal bacteria require food, too. Dietary carbohydrates, fibers, resistant starches, polyphenols, fats, and proteins influence which microorganisms live in the gastrointestinal tract and what metabolites those organisms produce.

One particularly important group of microbial metabolites is short-chain fatty acids (SCFAs), including butyrate, acetate, and propionate. These compounds participate in intestinal barrier health, immune regulation, metabolic signaling, and other physiological processes. If an athlete chronically restricts total food intake or begins removing carbohydrates, whole grains, fruits, legumes, fats, and other foods to become leaner, the intestinal nutritional environment changes as well.

Emerging research is investigating whether prolonged low energy availability may alter microbial diversity, reduce beneficial SCFA-producing organisms or their metabolites, affect intestinal barrier function, and contribute to inflammatory or metabolic consequences. This is a very exciting area of research, but an important scientific distinction needs to be made:

We do not yet have strong evidence proving that RED-S causes a specific pattern of gut dysbiosis in athletes. A 2026 review examining low energy availability, the microbiome, and bone health found biologically plausible connections based on athlete data and evidence from other states of undernutrition, including altered microbial diversity, depletion of some SCFA-producing organisms, reduced SCFA availability, impaired intestinal barrier function, and low-grade inflammation. However, the authors specifically concluded that a causal gut-mediated pathway in athletes has not yet been established. We cannot diagnose RED-S based on a stool microbiome test, but I also would not ignore the gut.

When I evaluate an under-fueled athlete with chronic bloating, constipation, diarrhea, nausea, food intolerance, or early satiety, gastrointestinal health becomes an important part of understanding why that athlete is struggling to adequately nourish their body.

Under-Fueling Affects Anxiety and Depression

The relationship can go in both directions. Psychological factors may contribute to RED-S, but RED-S may also contribute to psychological symptoms.

Someone with anxiety, perfectionism, body-image concerns, or an eating disorder may intentionally restrict food or compulsively exercise, leading to low energy availability, but prolonged inadequate nutrition also affects the biological systems necessary for normal brain function.

The brain needs glucose, amino acids, essential fatty acids, vitamins, minerals, and adequate total energy. Chronic energy deficiency also influences endocrine and metabolic signaling. The primary review used for this article reported that athletes with low energy availability had a 2.4-times greater likelihood of psychological disorders, with reported problems including irritability, depression, impaired judgment, decreased coordination, and decreased concentration. The updated IOC consensus similarly identifies mood disturbances, depressive symptoms, reduced well-being, sleep disturbance, anxiety, cognitive dietary restraint, perfectionistic tendencies, exercise dependence, and eating disorders among psychological concerns associated with problematic low energy availability and RED-S.

This is another reason we need to stop separating nutrition, mental health, hormones, gastrointestinal health, and athletic performance into completely different boxes. They communicate constantly. The microbiome may eventually prove to be another piece of this relationship through the gut-brain axis, but research specifically demonstrating that RED-S-induced microbiome changes cause anxiety or depression is not yet strong enough to make that claim.

What we can say is that inadequate energy and nutrient availability, RED-S, gastrointestinal dysfunction, and psychological symptoms can coexist and potentially reinforce one another.

What About Carbohydrates?

This deserves its own section because carbohydrates have become unnecessarily controversial. Current RED-S research suggests that low carbohydrate availability may add to the physiological consequences of low energy availability. Athletes have significant carbohydrate requirements because carbohydrates help replenish glycogen used during exercise. A teenager training hard while simultaneously following a low-carbohydrate diet may therefore be creating two problems: 1) not enough total energy and not enough readily available fuel for training. 2) Carbohydrates also provide many of the fibers and resistant starches used by intestinal microorganisms.

Fruit is not the enemy. Potatoes are not the enemy. Rice is not the enemy. Oatmeal is not the enemy.

For an athlete, appropriate carbohydrate intake can be essential for fueling, recovery, hormone health, gastrointestinal health, and performance.

RED-S Can Look Like "Overtraining"

RED-S can easily be mistaken for overtraining because many of the symptoms overlap. An athlete may become increasingly tired, notice that performance has plateaued or declined, sleep poorly, become more irritable, experience persistent muscle soreness, recover more slowly between workouts, or seem to get sick more frequently. From the outside, it may seem like the athlete is simply training too hard and needs more rest.

Sometimes overtraining is truly part of the problem, but in other cases, the athlete may not be exercising too much; they may be eating too little for the amount of exercise they do. Inadequate total energy and carbohydrate availability can contribute to many of the same symptoms traditionally associated with excessive training. This is why an exhausted athlete should not automatically be encouraged to simply "push through" fatigue or train harder. Instead, we need to consider whether the body is receiving enough nutrition to support the demands being placed upon it. Are we adequately fueling what we are asking this body to do?

Athletic Performance Eventually Suffers

This is sometimes what finally gets an athlete's attention. Early in energy restriction, an athlete may lose weight and temporarily feel faster or see an improvement in power-to-weight ratio. That can reinforce the idea that eating less is working, but it is generally not sustainable.

With prolonged low energy availability, athletes may experience:

• Reduced endurance

• Reduced strength

• Poorer training response

• Decreased coordination

• Impaired concentration and judgment

• Slower recovery

• More illness

• More injuries

• Loss of lean body mass

• Reduced glycogen availability

• Declining performance

The irony is important: An athlete may start eating less to perform better, only to eventually lose the physiological capacity needed to perform well.

"My Child Eats All the Time. There's No Way They Are Under-Fueled."

An athlete can appear to eat plenty of healthy food and still be under-fueled. A large salad with grilled chicken, Greek yogurt with berries, vegetables, and lean proteins are all nutritious choices, but nutrient-dense food isn't necessarily energy-dense food. An athlete can eat an exceptionally "clean" diet while still consuming too few total calories, particularly when meals emphasize vegetables and lean proteins but contain inadequate amounts of carbohydrates, starches, dietary fats, or other energy-dense foods. This can be especially problematic for teenagers, who often have limited opportunities to eat throughout the day due to early school mornings, short lunch periods, after-school activities, practices that overlap with dinner, and homework late into the evening. In these situations, the problem may have nothing to do with intentionally restricting food; there may simply not be enough eating opportunities, or enough energy in those meals and snacks to meet the demands of growth, daily life, and athletic training.

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What Should a Medical Evaluation for RED-S Look Like?

RED-S is a clinical diagnosis and should be evaluated by a healthcare professional familiar with the condition. There is no single blood test that says:

"Positive for RED-S." Evaluation starts with the whole picture.

A clinician should consider:

• Growth and weight trajectory

• Changes in body composition

• Dietary intake and meal timing

• Training volume

• Recent increases in training

• Injury history

• Stress fracture history

• Menstrual history

• Pubertal development

• Libido and reproductive symptoms

• GI symptoms

• Sleep

• Mood and anxiety

• Food restriction

• Body-image concerns

• Exercise compulsion

• Recurrent illnesses

• Athletic performance and recovery

The International Olympic Committee has developed the REDs Clinical Assessment Tool Version 2 (IOC REDs CAT2) to help clinicians identify indicators of RED-S and assess severity and sport-participation risk.

Depending upon the individual, medical evaluation may also include laboratory testing, electrocardiography, assessment of hormones and nutritional status, and/or DXA imaging to evaluate bone mineral density and body composition. Importantly, normal basic laboratory results do not automatically rule out RED-S. The diagnosis requires context.

When Should Parents Seek an Evaluation?

I would strongly consider medical evaluation when an active child or teenager develops a combination of:

• weight loss + fatigue

• missed periods + heavy exercise

• stress fracture + restrictive eating

• recurrent injuries + poor recovery

• stalled growth + high training volume

• new anxiety around food + increased exercise

• declining performance + weight loss

• GI symptoms + inability to adequately fuel

• recurrent illness + chronic fatigue

or simply "Something has changed, and my child doesn't seem healthy anymore." You know your child. A teenager who used to have energy, sleep well, enjoy meals, recover from practices, and love their sport but is now exhausted, injured, anxious about food, irritable, cold, losing weight, or struggling through workouts deserves more than being told that this is what serious training looks like.

Treating RED-S Is About Restoring Energy Availability

The primary goal of RED-S treatment is straightforward: restore adequate energy availability.

That sounds simple, but in practice, it can be complicated. Treatment may require increasing overall food intake, improving meal and snack timing, increasing carbohydrate availability, correcting nutrient deficiencies, reducing or temporarily modifying training, treating gastrointestinal problems, addressing an eating disorder or disordered eating, and supporting anxiety, depression, or compulsive exercise behaviors.

Depending upon severity, an athlete may temporarily need restrictions on training or competition. The primary review emphasizes a multidisciplinary approach involving physicians, registered dietitians, athletic trainers/physiotherapists, coaches, and other healthcare providers as appropriate. When an eating disorder is present, treatment of that disorder becomes essential rather than simply giving the athlete a higher-calorie meal plan.

And when gastrointestinal symptoms are preventing adequate intake, those symptoms need to be addressed rather than telling the athlete to "just eat more."

The underlying question is always: Why isn't enough energy reaching this body? Then we work from there.

The Goal Isn't Just to Get an Athlete Back on the Field

One of the most important shifts in our understanding of RED-S is the focus on long-term athlete health, not simply how quickly someone can return to competition. This is especially important for children and teenagers, whose bodies need adequate energy not only for sports, but also for growth, bone development, hormone production, brain function, digestion, sleep, recovery, and emotional health.

Sport can be incredibly beneficial, building strength, confidence, resilience, cardiovascular fitness, coordination, friendships, and lifelong movement habits. The goal isn't less sport; it is adequately fueled sport. A healthy athlete has enough energy to train and perform while still supporting everything the body needs outside of athletics. If you recognize signs of RED-S in yourself or your child—particularly with high training demands, restrictive eating, menstrual changes, recurrent injuries, GI symptoms, fatigue, or declining performance—consider discussing these concerns with a healthcare provider familiar with RED-S and sports nutrition.

Here to empower you with the knowledge of health,

Dr. Meg Holpuch

References & Further Reading

Cabre HE, Moore SR, Smith-Ryan AE, Hackney AC. Relative Energy Deficiency in Sport (RED-S): Scientific, Clinical, and Practical Implications for the Female Athlete. PMID: 36479178. This review discusses low energy availability, reproductive and metabolic hormone changes, bone health, gastrointestinal and cardiovascular effects, psychological consequences, screening, and multidisciplinary treatment.

Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine. 2023;57:1073-1098.

Gould RJ, Ridout AJ, Newton JL. Relative Energy Deficiency in Sport (RED-S) in Adolescents – A Practical Review. International Journal of Sports Medicine. 2023;44(4):236-246.

Ackerman KE, Holtzman B, Cooper KM, et al. Low energy availability surrogates correlate with health and performance consequences of Relative Energy Deficiency in Sport. British Journal of Sports Medicine.

Wagner et al. Low energy availability, the gut microbiome, and bone health in athletes: a mechanistic narrative review based on athlete evidence and clinical analogues. 2026. This emerging literature explores possible relationships between energy deficiency, intestinal microbial ecology, short-chain fatty acids, intestinal permeability, inflammation, and skeletal health while emphasizing that direct causation in athletes remains unproven.

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Disclaimer: The information provided on this blog is for educational and informational purposes only and is not intended as medical advice, diagnosis, or treatment. The content shared here is not meant to replace or supersede the guidance or recommendations of your personal healthcare provider. Always consult your physician or qualified healthcare professional before making any changes to your diet, exercise routine, supplement regimen, or overall health plan. Your health and well-being are unique, and decisions regarding your care should always be made in consultation with your trusted healthcare provider.