RED-S Early Warning: The Devastating Impact of Energy Availability EA<30 on Bone Density, Hormones, and Immunity

1. Introduction and Cutting-Edge Research Background

In contemporary endurance sports science, the traditional myth of “train hard, eat little” is being progressively overturned by rigorous empirical data. Over the past two decades, sports medicine has undergone a paradigm shift—from the International Olympic Committee (IOC) first acknowledging the structural deficiencies of the “Female Athlete Triad” in 2005, to its official renaming as “Relative Energy Deficiency in Sport” (RED-S) in 2014. This was not merely a change in nomenclature, but an expansion from a narrow perspective limited to women, menstrual cycles, and bone health, into a comprehensive health threat encompassing both males and females, spanning endocrine, skeletal, immune, cardiovascular, and psychological domains.

The latest research data is alarming: according to a large-scale meta-analysis published in the British Journal of Sports Medicine in 2023, approximately 47% of female and 32% of male elite endurance athletes are at risk of Low Energy Availability (LEA). Even more concerning, this proportion is even higher among amateur cyclists and triathletes with high training volumes, as they often lack professional nutritional monitoring while simultaneously bearing the dual burden of high-intensity training and life stress.

From a Taiwanese local perspective, whether tackling the 55 km long climb of Eastbound Wuling, the rolling hills and attacks of Yangmingshan Fengzhongjian, or participating in the 300 km Tour of East Taiwan (Huatung) long-distance endurance race, these uniquely Taiwanese competitive scenarios place extreme demands on athletes’ energy systems. Taking Eastbound Wuling as an example, starting from Qixingtan in Hualien at 300 meters elevation to the Wuling parking lot at 3,275 meters, the total ascent exceeds 3,000 meters, with exercise durations typically ranging from 4-6 hours and estimated total energy expenditure reaching 3,500-4,500 kcal. If an athlete enters the race already in a state of low energy availability, they not only fail to produce high-quality climbing power but may also fall into a weeks-long immune window and hormonal disruption post-race.

It is worth noting that the impact of RED-S is not limited to elite athletes. The rapidly growing “Weekend Warrior” population in recent years—amateur enthusiasts who are busy with work during the week and can only perform long-distance training on weekends—are equally exposed to extremely high risk. They often attempt to “make up” training volume within limited time while neglecting the critical importance of nutritional recovery, leading to a continuous accumulation of energy deficits that ultimately triggers a cascade of physiological breakdowns.

2. Core Mechanisms of Exercise Physiology and Biomechanics

2.1 Definition and Calculation Model of Energy Availability

Energy Availability (EA) is the core indicator for RED-S diagnosis and monitoring, defined as “the energy remaining for maintaining basic physiological functions (such as cellular repair, hormone synthesis, immune function, and bone remodeling) after deducting exercise expenditure.” Its standard calculation formula is as follows:

EA (kcal/kg FFM/day) = (Energy Intake EI - Exercise Energy Expenditure EEE) / Fat-Free Mass FFM

For example, a female triathlete with a Fat-Free Mass (FFM) of 55 kg, consuming 2,200 kcal daily with a training expenditure of 800 kcal, would have an EA of: (2,200 - 800) / 55 = 25.5 kcal/kg FFM/day, which falls within the dangerous low energy availability zone (<30).

It is important to note that the method of estimating Fat-Free Mass (FFM) significantly affects the EA value. Common measurement methods include Bioelectrical Impedance Analysis (BIA), Dual-Energy X-ray Absorptiometry (DXA), and skinfold thickness estimation. Among these, DXA is considered the gold standard, but the equipment is expensive and not easily accessible; BIA is greatly influenced by hydration status, with measurement errors potentially reaching ±2-3 kg. In practical application, it is recommended to perform body composition measurements under standardized conditions (fasting in the morning, after using the restroom, wearing light clothing) every 4-6 weeks to ensure the reliability of FFM data.

2.2 The Endocrine Collapse Cascade of Low Energy Availability

When EA remains below 30 kcal/kg FFM/day, the body initiates a series of “energy-saving mode” neuroendocrine adaptations whose complexity far exceeds general understanding:

Thyroid Axis Suppression: When the hypothalamus detects energy deficiency, it reduces the secretion of Thyrotropin-Releasing Hormone (TRH), which in turn suppresses the pituitary gland’s release of Thyroid-Stimulating Hormone (TSH). This ultimately leads to a significant decrease in triiodothyronine (T3) concentration. T3 is a key hormone regulating whole-body basal metabolic rate; a 20-30% reduction in its concentration can lower Resting Metabolic Rate (RMR) by 15-20%. This explains why many athletes with low energy availability fall into the vicious cycle of “the more they train, the more tired they get; the more tired they get, the fatter they become”—the body actively reduces metabolic efficiency to conserve energy.

Hypothalamic-Pituitary-Adrenal (HPA) Axis Dysregulation: Prolonged energy deficiency is perceived by the body as a form of “survival stress,” leading to abnormally elevated cortisol concentrations. Chronic cortisol elevation not only promotes muscle protein breakdown and inhibits collagen synthesis but also interferes with osteoblast activity in bone. Notably, this cortisol elevation is “relative”—some athletes’ absolute cortisol concentrations remain within the normal range, but their “cortisol: dehydroepiandrosterone (DHEA)” ratio has already shown significant abnormalities, indicating the body is in a state of imbalance where catabolism (breakdown) exceeds anabolism (synthesis).

Hypothalamic-Pituitary-Gonadal (HPG) Axis Suppression: In female athletes, suppression of this axis manifests as decreased luteinizing hormone (LH) pulse frequency, leading to reduced estrogen concentrations and menstrual cycle disruption (ranging from luteal phase defects to complete amenorrhea). Male athletes are equally affected, manifesting as significantly decreased testosterone concentrations. Research indicates that when EA drops below 20 kcal/kg FFM/day, male testosterone concentrations can decline by up to 40% within just 5 days. This not only affects muscle synthesis and recovery capacity but is also closely linked to mood swings, decreased libido, and irritability.

2.3 Biomechanical and Cellular Mechanisms of Bone Mineral Density Loss

Bone is a dynamic tissue continuously undergoing “bone remodeling”—a delicate balance between bone resorption carried out by osteoclasts and bone formation carried out by osteoblasts. Under normal physiological conditions, this balance is regulated by multiple factors including estrogen, testosterone, thyroid hormones, growth hormone, and mechanical loading (exercise stimulation).

However, when energy availability is insufficient, the following mechanisms lead to rapid bone mineral density loss:

Direct Effects of Estrogen Deficiency: Estrogen is a potent inhibitor of osteoclast activity. When estrogen concentrations decline, osteoclast activity increases, causing bone resorption to far outpace bone formation. Research shows that female athletes with Functional Hypothalamic Amenorrhea (FHA) can lose lumbar spine bone mineral density at a rate of 2-3% per year—5-10 times that of normal women of the same age.

Direct Effects of Energy Deficiency on Bone Cells: The latest cell biology research has found that low glucose and low free fatty acid environments directly inhibit osteoblast differentiation and activity while simultaneously promoting osteoclast formation. This means that even before hormonal concentrations show significant abnormalities, simple energy insufficiency alone is sufficient to negatively impact bone health.

The “Deafness” Effect of Mechanical Loading Signals: Under normal conditions, mechanical stress on bone (such as impact forces during running or pedaling forces during cycling) is converted into biochemical signals through mechanosensors in osteocytes, promoting bone formation. However, under low energy conditions, osteocyte sensitivity to mechanical signals significantly decreases—this is like “the ears are not deaf, but the brain refuses to receive sound.” This explains why even athletes who continuously perform high-intensity training (high mechanical loading) still experience ongoing bone mineral density loss.

2.4 Amplification of the Immune System’s “Open Window” Effect

The transient immune suppression after strenuous exercise (the “Open Window”) is a classic concept in exercise immunology, referring to the increased risk of upper respiratory tract infections during the 3-72 hours post-exercise. However, under low energy availability conditions, this “open window” period is dramatically amplified:

• Salivary secretory Immunoglobulin A (sIgA) concentrations significantly decrease, weakening the first line of defense of the respiratory mucosa
• Neutrophil phagocytic capacity and oxidative burst activity are reduced
• Natural Killer (NK) cell cytotoxicity declines
• The balance between anti-inflammatory cytokines (such as IL-10) and pro-inflammatory cytokines (such as IL-6) is disrupted

These changes cause endurance athletes with low energy availability to face significantly increased risks of Epstein-Barr virus (EBV), cytomegalovirus (CMV), or other opportunistic pathogen infections after races or high-intensity training weeks. The clinically common presentations of “unexplained persistent fatigue,” “recurrent colds,” and “delayed recovery after training” are often associated with this comprehensive decline in immune surveillance capacity.

3. Key Parameter Measurements and Comparative Analysis

To assist athletes and coaches in performing precise self-assessments, the following two key comparative data tables are provided:

3.1 Energy Availability Status Classification and Physiological Indicator Comparison Table

3.2 Energy Demand Estimation and Nutrition Gap Analysis for Classic Taiwanese Events

4. Periodized Monitoring and Adjustment Guide

4.1 Phase 1: Baseline Assessment Period (Weeks 1-2)

In this phase, the goal is to establish a personalized “energy availability safety baseline.” Specific procedures are as follows:

• Daily Dietary Logging: Use tools such as MyFitnessPal or Cronometer to accurately record the calories and macronutrients of all foods and beverages consumed. The key point is not to “hide” any food—including energy gels, sports drinks, and even chewing gum.
• Exercise Expenditure Measurement: Use a power meter as the baseline, supplemented by heart rate monitoring. The estimation formula for cycling expenditure is: Expenditure (kcal) = Average Power (W) × Time (hr) × 3.6. For example, riding at an average power of 200W for 3 hours yields an expenditure of approximately 2,160 kcal.
• Daily Morning Measurements: Record morning body weight (at the same time daily), morning resting heart rate (HRrest), and perceived recovery level (1-10 scale). If HRrest rises more than 5 bpm above baseline, or morning body weight drops more than 1% for 3 consecutive days, these are early warning signs of energy deficiency.

4.2 Phase 2: Adjustment and Intervention Period (Weeks 3-6)

Based on the data from Phase 1, make the following adjustments:

Energy Intake Adjustment Strategies:

• If EA is between 30-45 kcal/kg FFM/day, it is recommended to increase daily intake by 300-500 kcal, prioritizing increased carbohydrates (an additional 2-3 grams per kilogram of body weight) and protein (an additional 0.3-0.5 grams per kilogram of body weight).
• If EA < 30 kcal/kg FFM/day, more aggressive intervention is required, with a daily increase of 500-800 kcal, and consideration of temporarily reducing training volume (decreasing training load by 20-30%) until EA recovers to above 35.

Training Intensity Redistribution: During this phase, it is recommended to reduce the proportion of high-intensity training (Zone 4-5) to below 15% of total training time, placing the majority of training time in Zones 1-2 (heart rate zones: 60-75% of maximum heart rate). This not only reduces energy expenditure but also promotes neuroendocrine system recovery.

4.3 Phase 3: Continuous Optimization Period (Weeks 7-12)

• Monthly Retesting: Perform a complete body composition measurement and morning blood marker testing (such as TSH, T3, cortisol, testosterone/estrogen) once per month.
• Menstrual Cycle Tracking (Female): Use an app or calendar to record menstrual cycle length and menstrual flow volume. If cycle length falls below 21 days or exceeds 35 days, heightened vigilance is required.
• Bone Mineral Density Tracking: If the previous Z-score was below -1.0, it is recommended to undergo DXA bone mineral density testing every 6-12 months to monitor trends in the lumbar spine, hip, and femoral neck.

5. Race Nutrition, Environmental Adaptation, and Practical Strategies

5.1 Carbohydrate Loading Strategy for the 48 Hours Before Race Day

For Taiwan’s classic long-distance climbing events (such as Eastbound Wuling), carbohydrate loading in the 48 hours before the race is crucial. The specific strategy is as follows:

• 48-24 hours before race: Consume 8-10 grams of carbohydrates per kilogram of body weight daily. For a 65 kg athlete, this means 520-650 grams of carbohydrates daily, equivalent to 2,080-2,600 kcal. Prioritize high glycemic index (GI) sources such as white rice, white bread, and sports drinks to maximize muscle glycogen supercompensation.
• 24-4 hours before race: Maintain 7-8 grams of carbohydrates per kilogram of body weight, and begin reducing dietary fiber intake to lower the risk of gastrointestinal discomfort.
• 4-1 hours before race: Consume 1-2 grams of carbohydrates per kilogram of body weight, preferably in liquid form (such as sports drinks), to ensure stable blood glucose before the start.

5.2 Scientific Quantification of In-Race Nutrition

Taking Eastbound Wuling as an example (estimated 4-6 hours to finish), the recommended nutrition strategy is:

• Carbohydrates: 60-90 grams per hour (with a 1:0.8 glucose:fructose ratio for optimal absorption efficiency). Using a 25-gram energy gel as an example, 3-4 gels are needed per hour.
• Electrolytes: 300-600 mg of sodium per hour to maintain blood sodium concentration and nerve conduction function.
• Fluids: 500-750 ml per hour, adjusted based on temperature and sweat rate. It is recommended to adopt a “drink when thirsty” strategy to avoid hyponatremia caused by overhydration.

5.3 Special Responses to Taiwan’s Climate and Environment

Taiwan’s summer high-temperature and high-humidity environment (temperature >30°C, relative humidity >80%) significantly raises the perceived temperature, increasing heat stress and energy expenditure. Research shows that during submaximal exercise in hot environments, carbohydrate oxidation rates increase by 15-20%. Therefore, for summer events, it is recommended to:

• Increase hourly carbohydrate intake to 80-100 grams
• Supplement additional electrolyte tablets (500 mg sodium per hour)
• Undergo 5-7 days of heat acclimatization training before the race (60-90 minutes of low-intensity exercise in a hot environment daily)

6. Common Operational Misconceptions and Scientific Myth-Busting

Myth 1: “Cycling doesn’t hurt the knees, so bone loss doesn’t matter”

This is one of the most dangerous myths. Although cycling is a low-impact sport with less mechanical stress on the joints, bone mineral density loss affects the overall health of the entire skeleton. Recent research shows that professional cyclists have significantly lower lumbar spine and hip bone mineral density compared to age-matched general populations, and their stress fracture risk is 2-3 times that of runners. Furthermore, bone loss is irreversible—once the bone mineral density Z-score falls below -2.0, even subsequent restoration of energy availability yields very limited bone density recovery.

Myth 2: “Thin equals fast; the lighter you are, the stronger you climb”

This claim seems physically reasonable (power-to-weight ratio = power/weight), but it ignores physiological compensatory mechanisms. Indeed, on Wuling’s long climbs, power-to-weight ratio is the key factor determining speed. However, when weight loss results from muscle loss (rather than fat reduction), maximum power output decreases simultaneously, and the power-to-weight ratio may actually worsen. More importantly, the hormonal disruption caused by chronic low energy availability significantly impairs training adaptation capacity, ultimately trapping the athlete in a predicament of “can’t train, can’t recover, performance plateau.”

Myth 3: “I take supplements, so that should be enough, right?”

This is a serious misunderstanding. Supplements marketed as “promoting recovery,” such as BCAAs, glutamine, and zinc-magnesium tablets, cannot replace adequate total caloric and macronutrient intake. BCAAs can indeed stimulate muscle protein synthesis, but if total caloric intake is insufficient, the body will still prioritize using amino acids for energy metabolism rather than muscle repair. The core principle is always: first meet total caloric needs, then consider micronutrients and supplements.

Myth 4: “Missing my period means training is effective; it’s a ‘normal phenomenon for athletes’”

This is an extremely dangerous outdated notion. The menstrual cycle is a sensitive indicator of female health status; amenorrhea is by no means a symbol of being “well-trained” but rather a serious warning sign that the body is issuing an “energy crisis.” Functional Hypothalamic Amenorrhea (FHA) not only causes bone loss but is also closely associated with cardiovascular endothelial dysfunction, cognitive decline, and mood disorders. If menstrual cycle disruption persists for more than 3 months, immediate professional sports medicine assistance should be sought.

7. Expert FAQ

Q1: How can I accurately measure my Fat-Free Mass (FFM)? Are consumer body composition scales accurate?

Home body composition scales (based on BIA principles) can serve as trend-tracking tools under standardized conditions (fasting in the morning, after using the restroom, with dry soles and hands), but their absolute value error can reach ±3-4%. For more precise data, it is recommended to undergo DXA testing at a medical facility or professional fitness center every 3-6 months (costing approximately NT$1,500-3,000). DXA not only provides whole-body and regional FFM data but also simultaneously measures bone mineral density—one test tracks two of RED-S’s core indicators, offering excellent value for money.

Q2: I’m male; am I also at risk of RED-S? Should I be concerned?

Absolutely! Although past research has focused primarily on women, recent data has clearly confirmed that male endurance athletes are equally exposed to significant risk. The manifestations of RED-S in males are more subtle, primarily presenting as decreased testosterone concentrations, reduced libido, mood swings, and delayed training recovery. Notably, male athletes are often even more reluctant to admit they are “not eating enough,” leading to delayed diagnosis and intervention. If you find yourself experiencing persistent fatigue, reduced morning erections, or training performance stagnation exceeding 6 weeks, blood hormone testing is recommended.

Q3: If I’ve already experienced bone mineral density decline, is there still a chance to reverse it?

Although bone remodeling capacity declines with age, it is not entirely irreversible. The key lies in “early intervention” and a “multi-pronged approach.” First, EA must be restored to above 45 kcal/kg FFM/day and maintained for at least 6-12 months. Second, incorporate weight-bearing training (such as resistance training and jumping exercises) 2-3 times per week to provide mechanical stimulation to the hip and spine. Finally, ensure adequate intake of calcium (1,200-1,500 mg daily) and vitamin D (2,000-4,000 IU daily). Research shows that after restoring energy availability, bone mineral density can recover by 2-5% within 12-24 months. However, if the Z-score has already fallen below -2.5, pharmacological intervention options should be discussed with a sports medicine physician.

Q4: How can I convince my coach to let me “eat more” without being perceived as lacking discipline?

This is a communication strategy issue. It is recommended to communicate with “data” rather than “feelings.” The specific approach is: keep a 7-14 day diet and training log, calculate your EA value, and share the IOC’s latest 2023 RED-S consensus statement with your coach (downloadable from the IOC official website). The emphasis should not be “I want to eat more” but rather “current energy intake is insufficient to support training adaptation and recovery.” A coach who values athletes’ long-term development should accept training adjustments based on scientific evidence.

Q5: In the week before a race, how should I adjust my diet to avoid falling into a low energy state?

The goal for the week before a race is “supercompensation,” not “cutting calories to make weight.” It is recommended to increase daily caloric intake to 110-120% of the calories needed to maintain body weight, with carbohydrates increased to 65-70% of total calories. Simultaneously, reduce training volume by 40-60% while maintaining 1-2 sessions of intensity stimulation (such as 4×4 minutes of Zone 4 intervals) to preserve neuromuscular readiness. On the day before the race, consume 8-10 grams of carbohydrates per kilogram of body weight and avoid trying any new foods. The key mindset is: the recovery benefits of “eating well and sleeping well” before a race are far more important and reliable than the power-to-weight ratio improvement from “losing half a kilogram more” before the event.

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References and Further Reading:

1. Mountjoy M, et al. IOC consensus statement on relative energy deficiency in sport (RED-S): 2023 update. Br J Sports Med. 2023.
2. Logue DM, et al. Low energy availability in athletes: A review of prevalence, physiology, and clinical implications. Sports Med. 2020.
3. Ackerman KE, et al. Bone mineral density in female athletes with RED-S. J Clin Endocrinol Metab. 2019.
4. Burke LM, et al. Carbohydrates for training and competition. J Sports Sci. 2022.

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This article was compiled and written by the CTYeh Sports Science Platform. The content is provided solely for sports science education and training reference and does not constitute any medical advice. If you suspect you have RED-S-related health issues, please be sure to consult a professional sports medicine physician and registered dietitian.