Energy Availability and the Prevention of Relative Energy Deficiency in Sport (RED‑S)

Energy availability (EA) is the amount of dietary energy remaining for the body’s physiological functions after the energy cost of exercise has been subtracted. It is expressed relative to lean body mass (LBM) and is calculated as:

\[

\text{EA (kcal·kg}^{-1}\text{ LBM·day}^{-1}) = \frac{\text{Energy intake (EI)} - \text{Exercise energy expenditure (EEE)}}{\text{Lean body mass (kg)}}

\]

When EA falls below the threshold required to support normal metabolic processes, athletes may develop a condition known as Relative Energy Deficiency in Sport (RED‑S). Unlike acute energy deficits that resolve quickly, RED‑S reflects a chronic mismatch between energy intake and the combined demands of training, recovery, growth, and basic physiological maintenance. Understanding and managing EA is therefore a cornerstone of long‑term athlete health and performance.

Understanding Energy Availability

1. Components of Energy Balance

• Energy Intake (EI): All calories consumed from foods, beverages, and any supplemental sources.
• Exercise Energy Expenditure (EEE): The caloric cost of planned training sessions, competitions, and any additional physical activity (e.g., warm‑ups, cool‑downs, sport‑specific drills).
• Resting Metabolic Rate (RMR) and Non‑Exercise Activity Thermogenesis (NEAT): The energy required for basal physiological functions (cardiac output, respiration, thermoregulation) and everyday movements (fidgeting, walking to class, etc.).

EA isolates the energy left for RMR, NEAT, growth, immune function, and other essential processes after accounting for EEE. This distinction is critical because an athlete can appear to be in “energy balance” on a daily scale (EI ≈ total daily energy expenditure) yet still suffer from low EA if a large proportion of intake is consumed by training.

2. Thresholds of Concern

• Optimal EA: ≥ 45 kcal·kg⁻¹·LBM·day⁻¹ – supports normal endocrine function, bone remodeling, and immune competence.
• Suboptimal EA: 30–44 kcal·kg⁻¹·LBM·day⁻¹ – may begin to impair some physiological systems, especially in highly trained individuals.
• Low EA: < 30 kcal·kg⁻¹·LBM·day⁻¹ – associated with the onset of RED‑S and its cascade of health consequences.

These thresholds are derived from controlled laboratory studies and field observations, but individual variability exists. Factors such as training status, age, sex, and genetic predisposition can shift the point at which symptoms appear.

The Spectrum of Relative Energy Deficiency in Sport (RED‑S)

RED‑S is not a binary condition; it exists on a continuum ranging from subtle metabolic disturbances to severe clinical syndromes. The International Olympic Committee (IOC) conceptualizes RED‑S as comprising three interrelated domains:

1. Metabolic: Low EA, impaired substrate utilization, altered hormone profiles (e.g., reduced leptin, insulin, thyroid hormones).
2. Menstrual (Female‑Specific): Disruption of the hypothalamic‑pituitary‑gonadal axis, leading to menstrual irregularities or amenorrhea.
3. Bone Health: Decreased bone formation, increased resorption, and heightened risk of stress fractures and osteoporosis.

Athletes may experience isolated effects (e.g., metabolic dysfunction without menstrual changes) or a full‑blown syndrome affecting all three domains. Recognizing the spectrum helps clinicians and coaches intervene before irreversible damage occurs.

Physiological Consequences of Low Energy Availability

These alterations are interdependent; for example, chronic cortisol elevation can exacerbate bone loss, while reduced leptin further suppresses reproductive function. The cumulative effect is a decrement in both health and performance capacity.

Assessing Energy Availability in Athletes

1. Direct Calculation

• Step 1: Record 3–7 days of detailed food intake (including timing and portion sizes).
• Step 2: Quantify EEE using heart‑rate monitors, power meters, or validated metabolic equations.
• Step 3: Measure LBM via dual‑energy X‑ray absorptiometry (DXA) or bioelectrical impedance analysis (BIA).
• Step 4: Apply the EA formula.

While precise, this method is time‑intensive and prone to reporting bias. It is best suited for research settings or high‑performance teams with dedicated sport‑science staff.

2. Proxy Markers

• Resting Metabolic Rate (RMR) Suppression: A > 10 % reduction from predicted RMR suggests low EA.
• Hormonal Panels: Low triiodothyronine (T3), estradiol (in females), and testosterone (in males) can indicate metabolic stress.
• Bone Turnover Markers: Elevated CTX or reduced P1NP point toward compromised bone health.
• Questionnaires: The Low Energy Availability in Females Questionnaire (LEAF‑Q) and its male counterpart (LEAM‑Q) provide rapid screening but should be followed by objective measures.

3. Clinical Evaluation

A comprehensive assessment includes menstrual history (for females), injury record, psychological screening for disordered eating, and a physical exam focusing on signs of undernutrition (e.g., low body fat, hair loss, nail brittleness).

Practical Strategies to Optimize Energy Availability

1. Align Energy Intake with Training Load

• Periodized Nutrition: Increase caloric intake on high‑intensity or high‑volume days, and allow modest reductions on lighter days.
• Macro Distribution: Prioritize carbohydrate for glycogen replenishment on heavy training days, while ensuring adequate protein (1.6–2.2 g·kg⁻¹·day⁻¹) for tissue repair. Fat intake should meet at least 20 % of total calories to support hormone synthesis.

2. Timing of Fueling

• Pre‑Exercise: Provide 1–2 g·kg⁻¹ of carbohydrate 2–3 hours before training to spare glycogen and reduce perceived effort.
• During Exercise: For sessions > 90 minutes, ingest 30–60 g of carbohydrate per hour to maintain EA.
• Post‑Exercise: Replenish energy stores within the first 2 hours with a balanced meal containing carbohydrate, protein, and healthy fats.

3. Nutrient‑Dense Food Choices

• Emphasize whole grains, legumes, nuts, seeds, dairy or fortified alternatives, and lean animal proteins (or high‑quality plant proteins) to maximize micronutrient intake without excessive volume.
• Incorporate energy‑dense options (e.g., nut butters, dried fruit, avocado) for athletes who struggle to meet caloric targets due to limited appetite.

4. Manage Training Load

• Use objective metrics (session RPE, heart‑rate variability, GPS load) to adjust training volume when EA is borderline.
• Incorporate “recovery weeks” or active‑recovery sessions to allow physiological systems to reset.

5. Education and Behavioral Strategies

• Teach athletes to recognize early signs of low EA (persistent fatigue, mood changes, menstrual irregularities).
• Encourage regular self‑monitoring of body weight, training logs, and dietary intake.
• Provide counseling on intuitive eating and body image to mitigate the risk of disordered eating.

Nutrition Planning for Different Training Phases

Adjustments should be individualized; athletes with higher LBM or those competing in weight‑class sports may require tighter caloric control while still preserving EA.

Special Considerations for Female Athletes

• Menstrual Cycle Monitoring: Track cycle length, flow characteristics, and any amenorrhea. A sudden change often precedes measurable hormonal alterations.
• Estrogen’s Role in Bone Health: Low estrogen accelerates bone resorption; therefore, maintaining EA above the suboptimal threshold is critical for preserving bone mineral density.
• Energy Availability vs. Body Composition Goals: Female athletes may feel pressure to reduce body fat for aesthetic or performance reasons. Education should emphasize that excessive restriction jeopardizes health and can ultimately impair performance.
• Pregnancy and Post‑Partum: Energy needs increase substantially; RED‑S risk persists if intake does not keep pace with training and fetal demands.

Monitoring and Adjusting: A Dynamic Approach

1. Weekly Check‑Ins
• Review training logs, dietary records, and subjective wellness scores.
• Flag any trends (e.g., decreasing RMR, rising fatigue scores).

2. Monthly Objective Testing
• Re‑measure body composition, RMR, and hormonal panels.
• Update EA calculations based on the latest data.

3. In‑Season Rapid Response
• If an athlete reports persistent low mood, decreased performance, or menstrual changes, implement an immediate nutrition and training load audit.
• Provide short‑term caloric increase (e.g., + 300–500 kcal/day) and reduce training volume by 10–20 % until markers normalize.

4. Long‑Term Tracking
• Maintain a digital health dossier that integrates nutrition, training, medical, and psychological data.
• Use trend analysis to predict periods of heightened RED‑S risk (e.g., pre‑competition weight cuts).

Integrating Coaching, Medical, and Nutrition Support

• Interdisciplinary Communication: Regular meetings (bi‑weekly) among coaches, sport‑medicine physicians, dietitians, and psychologists ensure consistent messaging and rapid identification of RED‑S signs.
• Shared Decision‑Making: Athletes should be active participants in setting EA targets, fostering ownership and adherence.
• Education Modules: Provide sport‑specific workshops on EA, emphasizing practical meal planning, reading nutrition labels, and using portable tracking tools (e.g., smartphone apps).
• Policy Development: Teams can adopt guidelines that prohibit extreme weight‑cutting practices and require baseline EA assessments for all athletes.

Future Directions and Research Gaps

• Individualized EA Thresholds: Current cut‑offs are based on population averages; genetic and epigenetic markers may help refine personalized thresholds.
• Non‑Invasive Biomarkers: Development of wearable sensors that estimate EEE and metabolic stress in real time could simplify EA monitoring.
• Male RED‑S Phenotype: While much of the early literature focused on females, emerging data suggest that low EA also disrupts testosterone, bone health, and mood in male athletes; more longitudinal studies are needed.
• Interaction with Sleep and Circadian Rhythms: Preliminary evidence links low EA to altered sleep architecture, which may further impair recovery; integrated research could inform holistic interventions.
• Psychological Interventions: Understanding how cognitive‑behavioral strategies can prevent the progression from low EA to disordered eating remains an underexplored area.

By systematically evaluating energy availability, recognizing the early signs of RED‑S, and implementing evidence‑based nutritional and training adjustments, athletes and support staff can safeguard health while optimizing performance. The principle is simple yet powerful: the body must be fueled not only for the next workout, but for every physiological process that keeps the athlete thriving day after day.