What is Energy Availability?
Energy availability (EA) is a powerful concept in nutritional science, defined as the amount of dietary energy left over for the body to perform all necessary physiological functions after accounting for the energy expended during exercise. Unlike simple 'energy balance,' which compares total calories in versus total calories out, EA specifically prioritizes the energy needed for basic cellular and organ function, known as resting metabolic rate (RMR). This is calculated by dividing energy intake minus exercise energy expenditure by fat-free mass (FFM). If this calculation results in a low number, the body is forced into a 'power saving' mode, downregulating essential systems to conserve fuel.
The Three Levels of Energy Availability
Energy availability can be viewed as a continuum, with three distinct levels that have profound implications for health and performance. These levels apply to everyone, though they are most studied and relevant for athletes with high training loads.
1. Optimal Energy Availability
This is the ideal state for maintaining peak physiological function and health. It occurs when energy intake is sufficient to cover both exercise demands and the energy required for all basic bodily processes.
- Thresholds: For women, this is often defined as >45 kcal/kg FFM/day, while for men, it's typically >40-45 kcal/kg FFM/day.
- Characteristics: In this state, hormones (like reproductive hormones and thyroid hormones) function normally, bone mineral density is maintained, and the immune system is robust.
- Outcomes: This level supports consistent high-quality training, effective recovery, and long-term athletic development without compromising health.
2. Subclinical Low Energy Availability (LEA)
Falling into this range means there is a mild to moderate energy deficit that isn't severe enough to cause overt clinical symptoms, but can still trigger subtle metabolic and hormonal changes.
- Thresholds: This range is typically between 30 and 45 kcal/kg FFM/day for women and 30 and 40 kcal/kg FFM/day for men.
- Characteristics: The body begins to adapt by reducing its RMR. An athlete might experience early signs of fatigue, slower recovery, or slight hormonal disruption, such as a subtle reduction in testosterone or thyroid function.
- Outcomes: Performance can be maintained for short periods, and an athlete may even perceive a temporary performance boost due to lighter body weight. However, this is a precarious state that can lead to more severe issues if continued.
3. Clinically Low Energy Availability (LEA)
This is a state of severe energy deficiency where the body lacks sufficient energy to support essential physiological functions. Prolonged periods in this state can lead to the clinical condition known as Relative Energy Deficiency in Sport (RED-S).
- Thresholds: Defined as an energy availability of <30 kcal/kg FFM/day for both men and women.
- Characteristics: The body is forced to drastically conserve energy, leading to a cascade of negative health consequences. Symptoms include menstrual dysfunction (amenorrhea) in women, reduced bone mineral density, chronic fatigue, increased injury risk (especially stress fractures), impaired immune function, and psychological issues like depression and irritability.
- Outcomes: This is a serious health risk that requires intervention. It results in a significant and sustained decline in athletic performance and can have long-lasting effects on health.
Comparison of Energy Availability Levels
| Feature | Optimal Energy Availability | Subclinical Low Energy Availability | Clinically Low Energy Availability |
|---|---|---|---|
| Energy Level | Sufficient to support all physiological functions and exercise demands. | Minor to moderate deficit where the body starts to conserve energy. | Severe deficit leading to the shutdown of non-essential bodily functions. |
| Performance | Sustained peak performance, effective recovery, reduced injury risk. | Potential for short-term performance gains, but increased risk of fatigue and slow recovery. | Significant and sustained drop in performance, high risk of overtraining. |
| Physiological Effect | Normal hormonal function, strong immune system, healthy bone density. | Reduced resting metabolic rate, mild hormonal changes, early signs of adaptation. | Hormonal disruption (e.g., amenorrhea), reduced bone mineral density, weakened immunity. |
| Health Consequences | Excellent overall health. | Subtle health risks that can worsen over time. | High risk of illness, stress fractures, cardiovascular issues, and psychological distress. |
How Can LEA Occur?
Low energy availability isn't always intentional. It can stem from a variety of factors, often without the individual's full awareness.
- Unintentional Under-fueling: Athletes, particularly those with very high training volumes in sports like endurance running or cycling, may simply fail to consume enough calories to match their energy expenditure.
- Poor Nutritional Planning: Lack of knowledge about proper sports nutrition can lead to inadequate fueling strategies, such as restricting specific food groups or eating meals that are not calorie-dense enough.
- Rapid Weight Loss Attempts: Intense pressure to reduce body weight for performance in weight-sensitive sports (e.g., gymnastics, wrestling) can lead to deliberate and rapid calorie restriction.
- Disordered Eating Patterns: Though not all LEA is caused by it, underlying disordered eating behaviors or eating disorders are a significant cause of chronic low energy intake.
- High Training Volume: An acute increase in training load without a corresponding increase in caloric intake can quickly plunge an athlete into a state of LEA.
Key Strategies for Maintaining Optimal Energy Availability
Maintaining sufficient EA requires a proactive and informed approach to nutrition. Implementing these strategies can help prevent the health and performance pitfalls of LEA:
- Plan Nutrition Around Training: Focus on consuming adequate carbohydrates and protein before and after workouts to fuel performance and kickstart the recovery process.
- Eat Frequently: Aim for regular meals and healthy snacks every 3-4 hours to keep energy levels stable throughout the day and avoid large energy deficits.
- Work with a Professional: Consulting with a sports dietitian or nutritionist can help create a customized fueling plan that matches individual energy needs, training volume, and lifestyle.
- Track Progress: Pay attention to hunger cues, mood, sleep quality, and overall energy levels. A decline in these areas can be an early indicator of developing LEA.
- Prioritize Recovery: Adequate rest, including sufficient sleep, is essential for metabolic and hormonal balance. Poor sleep can exacerbate the effects of poor energy availability.
Conclusion
Understanding what the levels of energy availability are is fundamental for anyone serious about their health, particularly athletes. The body's energy budget is finite, and when exercise demands take up a large portion of it, the energy left for basic life-sustaining functions—known as energy availability—is critical. While optimal energy availability supports peak health and performance, both subclinical and clinical LEA can undermine long-term wellness. By proactively managing nutritional intake and being aware of the warning signs, individuals can ensure they are properly fueled to thrive in all aspects of their lives. Ignoring the signs of low energy availability can trigger a metabolic crisis with serious and lasting health consequences.
Future Research and Considerations
While significant progress has been made, further research is needed to refine energy availability thresholds, especially for male athletes, and to develop more reliable assessment tools for real-world application. Developing clearer, sport-specific guidelines and better educating both athletes and the general public on the importance of EA will be crucial for improving health outcomes. A critical review of measuring energy availability in athletes highlights the need for ongoing research in this area to minimize potential inaccuracies.
Disclaimer: The information provided is for educational purposes only and is not a substitute for professional medical advice. Always consult with a healthcare provider or a sports dietitian before making significant changes to your diet or exercise regimen.
