Updated Diagnostic Criteria for Relative Energy Deficiency (RED-S): Clinical Research on Female Athletes

Based on the Journal of Applied Physiology, Medicine & Science in Sports & Exercise, Sports Medicine, British Journal of Sports Medicine, and other international peer-reviewed journals, this article provides an in-depth analysis of the scientific evidence on “Relative Energy Deficiency in Sport (RED-S)” in the field of female exercise physiology. It also incorporates Taiwan’s local climate, race, and sports culture contexts to offer evidence-based training and health strategies.

Within the scope of female exercise physiology, “Relative Energy Deficiency in Sport (RED-S)” is a topic that carries both academic depth and practical value, yet has long been misunderstood or overlooked. Over the past few decades, knowledge accumulated in sports science has largely focused on healthy adult males, meaning that many of the unique physiological characteristics and needs of female exercise physiology have only received systematic attention and research in recent years. In fact, adolescents, females, and special populations (such as older adults, pregnant and postpartum women, and those with chronic diseases) differ fundamentally from the “standard young male athlete” in terms of physiological structure, hormonal environment, developmental stage, and health context. Directly applying adult male training principles and physiological data to these groups may yield diminished results at best, or cause health harm at worst. This is precisely why understanding “Relative Energy Deficiency in Sport (RED-S)” is so important—it allows us to move beyond the myth of a “one-size-fits-all” approach and provide scientific guidance that truly aligns with the physiology and needs of different populations. As Taiwan moves toward an aging society, gender equality awareness rises, and youth sports participation becomes increasingly common, the local application value of female exercise physiology is especially prominent. This article will guide you from the physiological mechanisms at the cellular and systemic levels, through empirical research in top international journals, the quantitative relationship between dose and effect, and differences in responses across populations, to directly applicable training practices and Taiwan’s local context. Finally, it will debunk long-standing myths, ensuring your understanding of “Relative Energy Deficiency in Sport (RED-S)” is truly built on science rather than hearsay or outdated stereotypes.

Academic Research Review

Regarding the scientific exploration of “Relative Energy Deficiency in Sport (RED-S),” the field of female exercise physiology has accumulated rigorous and rich evidence in recent years. Below are several representative studies selected for their value in methodological design, study populations, and strength of conclusions, which together form our current understanding:

1. Mountjoy et al. (2018). Published in the British Journal of Sports Medicine, this updated IOC RED-S consensus statement expanded the former “Female Athlete Triad” into a multi-system, cross-gender syndrome of energy deficiency.

2. Nattiv et al. (2007). The ACSM position stand in Medicine & Science in Sports & Exercise defined the continuum relationship among energy availability, menstrual function, and bone density.

3. Loucks et al. (2011). In the Journal of Sports Sciences, experimental evidence showed that when energy availability falls below 30 kcal per kilogram of fat-free mass, luteinizing hormone pulsatility and metabolic hormones are suppressed.

4. Melin et al. (2015). Published in the British Journal of Sports Medicine, the LEAF-Q questionnaire was developed as a screening tool for the risk of energy deficiency in female athletes.

Looking across these studies, it is clear that the scientific picture of “Relative Energy Deficiency in Sport (RED-S)” has deepened alongside advances in research methodology and a growing awareness of “population specificity.” Early research often interpreted data from adolescents, females, or special populations directly through the framework of adult males, ignoring the fundamental differences brought by developmental stage, hormonal cycles, aging processes, or disease contexts. In contrast, recent high-quality studies increasingly emphasize research methods “tailored to specific populations”—analyzing adolescents by biological maturity rather than chronological age, incorporating menstrual cycle and energy availability as control variables in female studies, and conducting stratified assessments of anabolic resistance and cardiovascular risk in older adults. This methodological evolution allows us to progress from “treating differences as noise” to “treating differences as the core of research.” It is worth noting that this field still faces several challenges: the number of studies on females and special populations remains relatively small compared to males, sample sizes are often limited; longitudinal tracking (especially long-term development in adolescents) is costly; and ethical considerations prevent certain interventions from being conducted in vulnerable populations. Therefore, when interpreting conclusions, we must both value the population-specific insights these studies reveal and remain mindful of the level of evidence and scope of applicability—a conclusion drawn from a specific age, sex, or health status group may not necessarily be extrapolated to other populations. It is this dual caution regarding population differences and evidence quality that forms the foundation for the scientific application of female exercise physiology, and it is also the consistent stance of this article.

Core Mechanisms

At the heart of Relative Energy Deficiency in Sport (RED-S) is “energy intake that fails to meet the remaining amount after exercise expenditure plus basic physiological needs”—that is, energy availability (EA) that is too low. When EA remains below the critical threshold of approximately 30 kcal per kilogram of fat-free mass over the long term, the body activates an energy-saving mode, prioritizing the reduction of “non-essential” physiological functions to maintain survival. Endocrinologically, the pulsatility of hypothalamic gonadotropin-releasing hormone (GnRH) is suppressed, leading to declines in luteinizing hormone and follicle-stimulating hormone, which in turn results in low estrogen levels, oligomenorrhea, or amenorrhea. The low estrogen state directly impairs bone formation, accelerates bone loss, and increases the risk of decreased bone mineral density and stress fractures. The damage of RED-S extends far beyond reproduction and bone: it also affects reduced metabolic rate, hypothyroidism, immune suppression, impaired protein synthesis, altered cardiovascular function, gastrointestinal issues, and worsened mental health (depression, disordered eating tendencies). A key significance of the 2018 IOC consensus update is its clear statement that this is a multi-system syndrome affecting both men and women, not merely a “triad” limited to females, and its emphasis on the importance of early screening and multidisciplinary intervention. The key to recovery is not treating individual symptoms but restoring adequate energy availability.

To truly understand “Relative Energy Deficiency in Sport (RED-S),” one must return to the physiological context unique to female exercise physiology: how the developing body, fluctuating hormones, aging systems, or disease effects alter exercise responses at the cellular, tissue, and systemic levels. The table below summarizes the key points of action for this topic across different physiological levels, helping you build a complete mechanistic picture:

| Physiological Level | Key Mechanisms | Implications for Training and Health |

|—|—|—|

| Endocrine/Hormonal | Population differences in sex hormones, growth and metabolic hormones | Affects adaptation direction, energy regulation, and reproductive/bone health |

| Bone and Muscle | Bone mass accumulation/loss, muscle fiber composition, and protein synthesis | Determines bone density, strength development, and injury risk |

| Cardiovascular and Metabolic | Population-specific characteristics of cardiac remodeling, oxygen uptake, and substrate utilization | Affects endurance performance, recovery, and long-term health |

| Neural and Psychological | Neuromuscular control, motivation, and psychosocial needs | Determines skill development, injury prevention, and sustained participation |

Special emphasis should be placed on the two dimensions of “developmental stage” and “individual differences.” The same intervention, applied under different maturity levels, ages, sexes, hormonal states, or health conditions, may produce vastly different or even opposite effects—this is precisely where female exercise physiology is most susceptible to being misled by oversimplified recommendations. For adolescents, for example, the effects, risks, and optimal timing of a training stimulus differ before and after PHV; for females, energy availability and menstrual function are key regulators behind many physiological responses; for older adults, anabolic resistance and the rate of decline give “stimulus intensity” a different meaning than in younger individuals. “Relative Energy Deficiency in Sport (RED-S)” deserves in-depth exploration precisely because it can specifically affect certain critical links in female exercise physiology. The more thoroughly you understand the mechanisms, the better you can determine “for whom, at what stage, what to do, and how much,” rather than blindly applying unsuitable general rules. This ability to adjust according to population and individual context is precisely the dividing line between those who understand female exercise physiology and those who train blindly.

Dose-Response Relationship

In female exercise physiology, “dose determines effect” is a core principle, but this dose often needs to be recalibrated for population-specific characteristics. Stimuli that are too low fail to reach the adaptation threshold and produce no benefit; loads that are too high may exceed the compensatory capacity of vulnerable populations, causing injury, developmental disruption, or health damage. The table below summarizes the dose-response relationships for Relative Energy Deficiency in Sport (RED-S) and serves as the most important quantitative reference when designing training and health plans for specific populations:

Dose / Condition	Physiological State	Effects and Key Points
EA ≥45 kcal/kg FFM	Healthy energy status	Supports normal endocrine function, bone health, and training adaptation
EA 30–45 kcal/kg FFM	Suboptimal/at-risk zone	Some physiological functions begin to be affected; monitoring required
EA <30 kcal/kg FFM	Clinical energy deficiency	LH pulse suppression, decreased estrogen, bone loss
Chronic long-term low EA	Full RED-S presentation	Amenorrhea, stress fractures, metabolic and psychological deterioration

From the table above, it is clear that dose-response relationships in female exercise physiology often present as threshold-type or inverted U-shaped curves: before reaching an effective dose, benefits increase with dose; but beyond a certain critical point, not only are there no additional benefits, but risks and costs rise sharply—this is especially critical for vulnerable populations (developing adolescents, women prone to energy imbalance, and older adults with diminished compensatory capacity). This means that “finding the optimal dose for the specific population and individual” matters far more than “blindly pursuing more and stronger.” In practical application, it is recommended to continuously monitor responses using objective indicators (such as performance, recovery, health markers, and subjective feelings) and calibrate according to population characteristics and individual data. Remember: the group average reported in research is a starting point, not an endpoint; each person’s maturity, hormonal status, health background, and genetics will cause the optimal dose to shift individually. Only by calibrating with your own data and professional assessment can group science be safely translated into an individual prescription.

Differences Across Populations

The impact of Relative Energy Deficiency in Sport (RED-S) is not equal for everyone. Age and maturity, sex, training status, hormonal state, health conditions, and genetic background all significantly modulate the magnitude of individual responses. Ignoring these differences and applying a one-size-fits-all recommendation is one of the most common and dangerous mistakes in the application of female exercise physiology.

Population Aspect	Response Characteristics	Practical Recommendations
Beginners vs. advanced athletes	Advanced athletes have mature adaptations and better tolerance but less marginal room for improvement	Beginners should progress conservatively, building a foundation before increasing load
Males vs. females	Hormonal, body composition, skeletal, and metabolic characteristics differ	Females require individualized assessment of energy, iron status, and bone health
Young vs. older adults	Older adults recover more slowly, have anabolic resistance, and accelerated degeneration	Older adults need sufficient stimulus intensity but longer recovery and screening
Developmental stage	Maturity influences adaptation direction, risk, and timing	Arrange training based on biological maturity rather than chronological age

Regarding population-specific considerations for this topic: RED-S is not limited to females; male athletes also experience decreased testosterone, bone mineral density loss, and performance decline. Adolescent athletes face more severe consequences of energy deficiency because they simultaneously need to meet growth requirements.

When interpreting individual differences, one must also be wary of a statistical trap: research reports mostly describe “group average responses,” but beneath the average often lies enormous individual variability. In the same intervention, some may be strong responders while others barely respond. This is why even when a study shows “average effectiveness,” you still need to confirm applicability through professional assessment and your own response. Taking common athletic populations in Taiwan as an example—whether it be adolescents burdened with heavy academic workloads, women balancing family and training, or middle-aged and older adults pursuing healthy aging—correctly understanding the physiological characteristics of your own population group is the only way to avoid the ineffective or even harmful consequences of “copying someone else’s training plan.” After understanding population differences, you will realize: truly professional female exercise physiology recommendations are always individualized prescriptions that vary “by person and by stage,” not one-size-fits-all slogans.

Practical Training Application

Theory must ultimately translate into actual training and health practices. Below is a practical framework for converting Relative Energy Deficiency in Sport (RED-S) into concrete applications:

• Population matching: All training and health recommendations must first ask, “Is this suitable for this population?”—adolescents prioritize development and protection, women prioritize energy and bone health, older adults prioritize safety and functional maintenance; the starting points differ for each.

• Progression and monitoring: Progress gradually from an appropriate starting point, continuously monitoring responses with objective indicators (performance, recovery, health markers) and subjective feelings, dynamically adjusting based on individual conditions.

• Health over performance: For vulnerable populations, long-term health (development, bone, endocrine, cardiovascular) always takes precedence over short-term performance; never sacrifice health for temporary numbers.

• Holistic context: Training is only one piece of the puzzle; sleep, nutrition (especially energy availability), recovery, psychological and social support are equally critical—no single intervention can compensate for overall imbalance.

• Professional collaboration: When dealing with growing adolescents, female-specific health issues, or older adults and chronic disease populations, seeking collaborative assessment from coaches, medical professionals, nutritionists, and psychologists in a timely manner is the safeguard for safety and effectiveness.

As an example of practical planning: when designing a program, one should first clarify the subject’s population characteristics and health context, then set reasonable goals, doses, and monitoring indicators accordingly. The most common mistake many people make is directly applying practices seen on social media or in adult elite athletes to adolescents, women, or older adults, ignoring the underlying physiological differences—this is precisely what female exercise physiology strives to avoid. Daily training and life serve as the best laboratory for observing population and individual responses and building personalized data.

It is recommended to integrate training logs with health monitoring, recording key indicators (such as growth and injuries in adolescents, menstrual and iron status in women, strength and recovery in older adults) alongside training content and physical responses. After weeks to months of accumulation, the value of this personalized database will far exceed any general guideline. Furthermore, do not overlook the often-underestimated component of “recovery and long-term development”—for vulnerable populations, excessively pursuing short-term progress at the expense of recovery and health often leads to injury, burnout, or health problems, thereby interrupting the engine of long-term progress. Treat population matching and health priority as core training principles, and both your results and safety will be markedly different.

Local Application in Taiwan

Taiwan’s unique climate, terrain, social structure, and sports culture add distinct local color to the application of Relative Energy Deficiency in Sport (RED-S). Climatologically, summers are hot and humid while winters are damp and cold, posing additional challenges for different populations (especially adolescents and older adults with differing thermoregulatory capacities); topographically, the extreme elevation gain from sea level to Wuling at 3,275 meters provides a rich training environment; socially, Taiwan is entering an aged society, academic pressure is heavy, and gender equality awareness is rising—all of which profoundly affect the sporting circumstances of various populations.

Taking local scenarios as examples: adolescent athletes often face dual pressures from academics and training, lacking systematic recovery and long-term development planning; female sports enthusiasts face insufficient attention to energy availability, iron status, bone health, and female-specific health issues; older adults need friendly, safe exercise environments and communities that accommodate different abilities. Making good use of Taiwan’s dense convenience store supply points, diverse cycling and running routes, and the growing sports community—while designing population-appropriate activities (such as multi-sport development for adolescents, female-friendly equipment and environments, and group rides for older adults)—is how the science of female exercise physiology can truly be implemented for every sports enthusiast in Taiwan, promoting public health and sports participation for all.

Common Myth-Busting

Myth: “Being lighter makes you run faster, and the lower your body fat, the better.” Short-term weight loss may improve power-to-weight ratio, but long-term low energy availability can trigger RED-S, which instead leads to fractures, amenorrhea, immune suppression, and long-term performance decline; a healthy energy supply is the foundation of sustainable performance.

This type of myth spreads widely often because it “sounds reasonable,” is easy to pass along by word of mouth, or stems from inappropriately applying concepts about adult males to other populations. Yet the value of science lies precisely in testing intuition with rigorous evidence: many seemingly obvious ideas do not hold up under rigorous research on specific populations. The field of female exercise physiology in particular is rife with outdated stereotypes and oversimplified claims that compress complex population differences, developmental stages, and individual variation into a single slogan. The next time you hear a categorical training recommendation aimed at adolescents, women, or special populations, it is worth asking: “What is the level of evidence for this claim? Was it studied in this population? Or is it a conclusion from another group being applied directly here?” Cultivating this evidence-based, population-specific critical thinking is more valuable than memorizing any single conclusion, and it is also a key step toward making female exercise physiology more scientific and preventing injury.

Conclusion

“Relative Energy Deficiency in Sport (RED-S)” is a topic in female exercise physiology that carries both theoretical depth and practical value. As can be seen from the international journal evidence reviewed in this article, adolescents, women, and special populations have unique characteristics and needs in exercise physiology—they are by no means “scaled-down” or “special-case” versions of adult males. Understanding and respecting these differences is precisely the starting point of scientific, individualized training. The key lies in grasping the mechanisms, calibrating the dosage, adjusting according to population and individual, and always placing long-term health above short-term performance. For sports enthusiasts in Taiwan, while mastering scientific principles, it is equally important to integrate local climate, environment, and social context, translating general rules into prescriptions suited to one’s own population and individual needs. May every adolescent, woman, and older adult who trains hard at every stage of life and growth safely, healthily, and sustainably enjoy the joy and benefits of sport through the wisdom of female exercise physiology. The value of sport has never been divided by age, gender, or circumstance—let science become a force from which everyone can benefit.

Related Reading

• Relative Energy Deficiency (RED-S): Causes, Warning Signs, Effects on Bone and Endocrine Health, and How to Seek Help
• RED-S Relative Energy Deficiency: Energy Supply Warning Signs for Female Athletes
• Energy Deficiency in Female Runners (RED-S): Recognizing the Hazards and Prevention
• RED-S in Female Runners: Relative Energy Deficiency Is Not as Simple as Eating Less