Relative Energy Deficiency in Sport (RED-S): Causes, Warning Signs, Effects on Bone and Endocrine Health, and How to Seek Help

What Is Relative Energy Deficiency in Sport (RED-S)

Relative Energy Deficiency in Sport (RED-S) is a concept used in sports medicine in recent years to describe a phenomenon: when an athlete’s long-term caloric intake is insufficient to cover the total energy demand of “training expenditure” plus “what the body needs to maintain normal physiological functions,” the body activates a series of protective mechanisms, prioritizing the allocation of limited energy to the most basic functions needed to sustain life (heartbeat, breathing, brain function), while sacrificing functions the body deems “non-urgent,” such as the reproductive system, bone metabolism, immune system, and certain endocrine functions.

This concept was originally extended from the “Female Athlete Triad” (which traditionally describes the relationship between energy deficiency, menstrual dysfunction, and bone loss). The newer RED-S framework expands the scope beyond just women and beyond just these three aspects, encompassing a broader range of effects including decreased metabolic rate, weakened immunity, impaired protein synthesis, cardiovascular health, and mental health. However, within the endurance athlete community, the most discussed—and most easily overlooked—issue remains the cascade of effects triggered by “chronic low energy availability” in female athletes.

One important point to emphasize here is a common misunderstanding: RED-S is not the same as clinically defined eating disorders such as “anorexia nervosa” or “bulimia nervosa,” but the two can coexist and can also be mutually causal. Some athletes fall into a low energy availability state because they deliberately restrict their diet to pursue specific weight or body fat goals, while others simply “significantly increased their training volume without realizing they needed to increase their intake proportionally”—an unintentional energy deficit. Regardless of the cause, the physiological impact on the body is similar.

The Concept of “Energy Availability”: Not Simply a Weight or Body Fat Issue

To understand RED-S, one must first understand the core concept of “Energy Availability,” which is not exactly the same as the “caloric deficit” most people are familiar with. Energy availability refers to: the total calories consumed through daily diet, minus the calories expended through training, leaving the calories available for the body to allocate to maintaining basic physiological functions (breathing, heartbeat, temperature regulation, endocrine function, cellular repair, etc.).

This means that even if a person’s weight and body fat percentage appear completely normal—or even on the higher side—as long as her training expenditure is large enough and her dietary intake does not increase proportionally, she can still fall into a state of low energy availability. This is also one of the reasons RED-S is often misunderstood—many people assume it only happens to athletes who are “underweight and look very thin,” but in reality, athletes with normal physiques—or even those whose weight falls within a healthy range—can also experience physiological responses to low energy availability due to a large gap between training volume and dietary intake. This also means RED-S cannot be judged by appearance or weight numbers alone; it must be assessed through a combination of symptoms, medical history, and necessary examinations.

This concept is particularly important for endurance athletes because long road cycling rides, long-distance running training, and triathlon combined training burn far more calories than the average person’s daily activity level. Without careful attention to synchronizing dietary intake, an energy gap can easily form without one realizing it—and this gap doesn’t need to be extreme; as long as it persists over the long term, it is sufficient to trigger the body’s protective responses.

Why Endurance Athletes Are Particularly Prone to This

Endurance sports have several inherent characteristics that make athletes relatively susceptible to falling into a state of low energy availability—often “without realizing it”:

• High training expenditure, but hunger signals don’t necessarily scale up proportionally: Long-duration, moderate-to-low intensity endurance training (e.g., long rides, long runs) burns a considerable number of calories, but post-exercise hunger is not necessarily proportional to expenditure. Some people actually experience suppressed appetite after exercise, making it easy to unconsciously eat less than they burn over the long term.
• Cultural pressure toward “lightness”: The road cycling community widely holds the belief that “lighter weight means faster climbing,” and the running community has similar sayings like “being leaner makes running feel easier.” While these ideas have physiological merit (power-to-weight ratio does affect climbing performance), they are easily oversimplified and over-pursued, evolving into unhealthy dietary restriction.
• Rapid increases in training volume without dietary adjustments: Many people significantly ramp up their training volume in the months before preparing for a major event (e.g., the Wuling Challenge, the Wan Jin Shi Marathon full marathon). If dietary intake does not increase proportionally, an energy gap forms over time—even if the person has no intention of “deliberately dieting.”
• Ignoring or rationalizing bodily signals: Fatigue, menstrual irregularities, and recurrent colds or injuries are often interpreted by athletes themselves as “the normal price of training hard,” rather than as warning signs from the body.

Summary of Common Causes

RED-S is usually not caused by a single factor but rather by multiple factors stacking together. Common situations include:

• Deliberate caloric restriction to pursue a specific weight or physique: Including aiming for a visually “leaner” appearance, targeting a specific power-to-weight ratio, or being influenced by external comments (from coaches, teammates, social media) about body shape.
• Unintentional inadequate energy intake: Training volume increases but eating habits do not adjust accordingly, especially in busy lifestyles where meals are rushed or insufficient.
• Gastrointestinal or dietary habit limitations: For example, long-term adoption of extreme restrictive diets (unnecessary excessive low-carb, over-restriction of certain food groups), or existing gastrointestinal issues causing poor absorption.
• Coexisting eating disorder tendencies: This falls into a category requiring professional psychological and medical intervention—it cannot be addressed simply by adjusting a meal plan.
• Anxiety-driven overtraining for performance: Training volume keeps climbing without corresponding recovery and nutritional support, creating a long-term deficit of “output exceeding input.”

What Warning Signs May Appear in the Body

RED-S affects a wide range of systems. Below is a summary of common warning signs organized by system. However, please note that the appearance of any individual symptom does not necessarily indicate RED-S—it could be caused by other factors, and professional evaluation is needed for confirmation. This article serves only as a reminder checklist of “things worth paying attention to and getting checked by a doctor,” not as a self-diagnosis tool.

Menstrual and Reproductive System

• Menstrual cycles become irregular, or the cycle length noticeably lengthens.
• No menstrual period for three or more consecutive months (assuming pregnancy has been ruled out)—medically termed secondary amenorrhea, which is a warning sign requiring prompt medical evaluation.
• Delayed menarche (applicable to adolescent athletes).

Skeletal System

• Recurrent stress fractures, especially fractures occurring without obvious trauma or falls.
• Abnormal bone density test results (this can only be confirmed through a bone density scan at a medical facility—it is not something one can judge on their own).

Energy and Metabolism

• Chronic abnormal fatigue that does not resolve even after rest.
• Training performance plateaus or even declines, despite continuously increasing training volume.
• Abnormal basal metabolic indicators (e.g., abnormally low resting heart rate, low body temperature, heightened sensitivity to cold).
• Recurrent colds and slower wound healing, indicating the immune system may be affected.

Psychological and Emotional

• Low mood, increased anxiety, excessive anxiety about food or weight.
• Social withdrawal, beginning to avoid food-related social situations (group meals, celebrations).
• Compulsive obsession with training, unable to allow oneself to rest even when the body signals fatigue.

Digestive and Other Physical Signs

• Increased gastrointestinal issues, such as constipation and bloating.
• Thinning hair or increased hair loss.
• Cold hands and feet, dry skin.

A Table Summarizing Warning Signs and Severity Concepts

This table lists areas worth paying attention to and pursuing further examination—it is not a basis for diagnosis. Actual diagnosis and severity grading must be determined by medical professionals based on a complete medical history, physical examination, and necessary tests.

Why Bone Health and Endocrine Effects Deserve Special Attention

Among all the potential impacts of RED-S, bone loss and endocrine disruption deserve particular attention from endurance athletes, for the following reasons:

The effects of bone loss can be long-term and partially irreversible. The younger years (especially from your twenties to early thirties) are actually a critical period for accumulating bone density over a lifetime. If you remain in a state of low energy availability and low estrogen for an extended period during this time, bone accumulation can be suppressed, and you may even experience bone loss earlier than what typically occurs after menopause. This is why “recurrent stress fractures” in young female endurance athletes should not be treated merely as “too much training, not enough rest”—underneath, there may be issues of low energy availability and bone health that need to be assessed together.

The effects on the endocrine system are not limited to menstruation. Long-term energy deficiency can affect the normal function of the hypothalamic-pituitary-gonadal axis (the hormonal axis that regulates the menstrual cycle), and may also affect thyroid function and the secretion pattern of cortisol (the stress hormone), further impacting metabolic rate and overall recovery capacity, creating a vicious cycle of “the more you train, the more tired you get; the more tired you get, the worse you train.”

Common Myth: Cessation of Menstruation Does Not Mean “Training Hard and Being Healthy”

In some outdated views within the sports community, there has been a saying that “cessation of menstruation means your body fat is low enough and you’ve trained hard enough”—this is a myth that must be clearly corrected. Secondary amenorrhea (no menstruation for three consecutive months or more, after excluding physiological causes such as pregnancy) is a warning sign from the body, not a symbol of “effective training.” It reflects a protective response in which the body is in a state of severe energy deficiency and the hormonal axis is forced to shut down some of its functions. Long-term neglect will only accumulate more health risks, not lead to better athletic performance. In fact, many athletes caught in RED-S see their actual competition performance stagnate or decline, which is why the sports medicine community increasingly emphasizes that “adequate nutrition is the foundation of performance, not the enemy of performance.”

What You Can Do If You Suspect You Have Similar Issues

If you’ve read this far and find that you have multiple warning signs mentioned above, here are practical action steps:

1. Do not attempt extreme “catch-up” measures on your own: For example, drastically increasing calorie intake in a short period or making dramatic changes to your eating patterns. Such approaches are also stress on the body; it is recommended to adjust gradually under professional guidance.
2. Seek appropriate medical and nutritional professional help: This includes obstetrics/gynecology (to assess menstrual and reproductive health), sports medicine or family medicine (for overall assessment and referrals), and dietitians with a sports nutrition background (to help design eating plans that meet training needs).
3. Honestly record your diet and training: For one to two consecutive weeks, record what you actually ate and how much you trained. This record can help professionals more quickly understand the gap between your energy intake and expenditure.
4. If necessary, reduce training volume in the short term: If medical professionals recommend a recovery period, accept temporary adjustments to training volume, and prioritize “being able to train consistently in the long term” over “not interrupting training in the short term.”
5. Examine your beliefs about weight and body image: If there is intense anxiety about weight or body shape behind this, psychological professional help may be needed; simply adjusting the eating plan may not solve the root problem.

Medical Warning Signs Checklist (Please Pay Close Attention—Seek Medical Care Promptly If Any of the Following Occurs)

The following situations are warning signs that require priority attention and prompt medical assistance. The content of this article is for educational purposes only and cannot replace professional medical diagnosis and treatment. If you have any of the following conditions, please seek medical care—do not self-diagnose or delay:

• No menstruation for three consecutive months or more (pregnancy has been ruled out).
• Stress fractures, especially if they recur or occur without obvious trauma.
• Obsessive anxious thoughts about food, weight, or body shape that already affect daily life and social activities.
• Behaviors related to eating disorders, such as binge eating, self-induced vomiting, or excessive use of laxatives or diuretics.
• Long-term unusual fatigue combined with symptoms such as palpitations, dizziness, or chest pain.
• Persistent low mood, loss of interest in training and life, or thoughts of self-harm—in such cases, please seek psychiatric or psychosomatic help immediately, or call the Ministry of Health and Welfare’s安心專線 (1925) for immediate support.

The Role Coaches and Teammates Can Play

The risk of RED-S is often amplified in a training culture that emphasizes “pursuing progress,” and the attitudes of coaches and training partners are very important:

• Avoid using weight or body shape as criteria for evaluating athletes: Publicly praising someone for “getting thinner” or commenting on body type may unintentionally reinforce unhealthy eating behaviors.
• Watch for warning signs where training volume and performance are not proportional: If an athlete’s training volume keeps increasing but performance plateaus, or they repeatedly get injured or sick, take the initiative to show concern rather than adding more training.
• Build a team culture where diet and body issues can be safely discussed, so that athletes are willing to seek help proactively when something feels wrong, rather than hiding it.
• When coaches do not have medical or nutritional expertise, they should make appropriate referrals rather than giving specific dietary or weight management advice on their own.

The Unique Context of Taiwan’s Sports Community: Race Season Planning and Dietary Concepts

Endurance sports events in Taiwan have a distinct seasonality. For example, many road races are concentrated in autumn and winter (when the weather is cooler and more suitable for long-distance running), and key road cycling challenges (such as the Wuling series events) are also often scheduled in specific seasons. This concentration of race season can easily lead athletes to do intensive volume training in the months before races without realizing that their diet also needs to be adjusted accordingly. Several common situations in Taiwan’s sports community are worth noting:

• The “pre-race weight loss” trend on social media: Many amateur athletes deliberately attempt short-term weight loss before races, hoping to compete at a lighter weight. If the weight loss method involves severely restricting calories and coincides with increased training volume, it can easily compound into a significant energy deficit—especially when preparing for long climbing challenges like Wuling, the risk deserves extra attention.
• Comparison mentality in group training: The group training culture of cycling teams and running clubs can sometimes unconsciously create an implicit comparison atmosphere of “who is thinner and climbs faster.” This atmosphere can easily cause some members to develop unnecessary body image anxiety.
• Suppressed appetite in hot weather: Taiwan’s summer heat and humidity are high, and it is common to experience decreased appetite after long outdoor training sessions. Combined with the already high training expenditure, the risk of insufficient calorie intake over time can be amplified. It is necessary to proactively replenish rather than waiting for hunger to appear before eating.
• The compounding risk of vegetarian or special dietary patterns: Taiwan has a relatively high proportion of vegetarians. If someone is both an endurance athlete and follows a vegetarian or stricter dietary restriction (such as a vegan diet), they need to plan more carefully for the intake sources of nutrients such as protein, iron, and calcium, to avoid unintentional insufficiency due to limited food choices. It is recommended to consult a professional with a sports nutrition background to help plan menus.

Recovering from Low Energy Availability: How Long It Takes and What to Expect

If you have been professionally assessed and confirmed to have low energy availability or RED-S-related conditions, the recovery process is usually not achieved overnight. Here are several key points to be mentally prepared for in practice:

• The recovery period may take several months or longer: Especially if menstrual dysfunction or abnormal bone density has already occurred, the recovery of the endocrine axis and the replenishment of bone mass take time—they cannot be immediately reversed by simply increasing calorie intake in the short term.
• Training volume may need to be reduced in stages: This is often the most psychologically difficult part for athletes accustomed to high training volumes, but continuing to train under an energy deficit will only delay recovery and may even cause the condition to worsen.
• Weight may change, and this is part of the recovery process: As energy intake is replenished, changes in weight are a normal phenomenon of the body restoring normal metabolism and physiological function. There is no need to feel frustrated or to want to restrict food again because of this.
• The timing of menstrual recovery varies from person to person: Some people see their menstruation return within a few months after energy status improves, while others need longer. This should be continuously monitored and assessed by an obstetrician/gynecologist, rather than self-judging “how long it should take to recover.”
• Rebuilding a healthy relationship with food and your body is one of the core goals of recovery: This often requires collaborative efforts from dietitians, psychological professionals, and the sports medicine team. Trying to figure out dietary adjustments on your own usually yields limited results.

For You Training for a Race: Long-Term Consistency Matters More Than Short-Term Weight Numbers

Preparing for the Wuling Challenge, the Wan Jin Shi Marathon, or any race that matters to you, the drive to improve is a natural motivation, but the foundation of your health must come before training performance. Adequate energy is not an obstacle to training; it is a necessary condition that allows your body to withstand training load, adapt, and become stronger. Keeping yourself in a long-term energy deficit may not show obvious costs in the short term, but damage to your bones, endocrine system, and immune system is often cumulative. Once problems arise (such as fractures or amenorrhea), the recovery period is often longer than expected and may leave long-term effects on future family planning or bone health.

If you are preparing for an important race, rather than obsessing over the number on the scale, it is more worthwhile to invest your effort in: ensuring adequate sleep, keeping your food intake in line with training expenditure, listening to the fatigue and discomfort signals your body sends, and regularly allowing yourself to rest and recover. These seemingly “conservative” practices are, in the long run, the key to allowing you to continue participating in the sport you love and to go further.

Key Action Points

• RED-S is a chain reaction that occurs when long-term energy intake cannot support training expenditure and basic bodily functions, causing the body to sacrifice “non-essential” functions such as reproduction, bone health, and immunity.
• Common warning signs include menstrual irregularities or amenorrhea, recurrent stress fractures, chronic abnormal fatigue, frequent colds, and excessive anxiety about food and body weight. Individual symptoms do not confirm a diagnosis, but they warrant a medical evaluation.
• “Amenorrhea means you are training hard enough” is an outdated and dangerous myth. Secondary amenorrhea is a warning sign that requires medical attention, not a symbol of training effectiveness.
• If you suspect you have a related condition, seek help from an OB-GYN, a sports medicine specialist, or a dietitian with a background in sports nutrition, and avoid attempting extreme dietary changes on your own.
• If you experience consecutive missed periods, recurrent fractures, eating-disorder-related behaviors, or concurrent emotional distress or thoughts of self-harm, please seek medical attention as soon as possible. If necessary, you can call the 1925安心專線 (Comfort Line) for assistance.
• Coaches and training partners should avoid evaluating athletes based on weight or body shape and should foster a team culture where body-related issues can be discussed safely.
• Adequate energy is the foundation of training adaptation and long-term athletic performance. Long-term, consistent health management matters more than short-term weight numbers.

Related Reading

• Relative Energy Deficiency in Sport (RED-S): The Most Dangerous Dietary Trap for Endurance Athletes
• RED-S: The Energy Supply Warning Sign for Female Athletes
• Energy Deficiency in Female Runners (RED-S): Identifying the Risks and Prevention
• RED-S in Female Runners: Relative Energy Deficiency Is Not as Simple as Eating Less