Which definition describes energy availability?

November 1, 2025 •

4 min read

Athletes who train intensely but do not fuel adequately are at a high risk for compromising their health and performance. This happens when their energy intake does not sufficiently cover both exercise demands and basic bodily functions, a state known as low energy availability (LEA).

Quick Summary

Energy availability is the remaining energy for bodily processes after accounting for exercise expenditure, expressed relative to lean body mass. When this level is too low, it can lead to physiological dysfunctions and poor health outcomes, especially in athletes.

Key Points

Core Definition: Energy availability is the amount of dietary energy remaining for normal bodily functions after accounting for exercise energy expenditure.
Energy vs. Exercise: The calculation accounts specifically for the energy burned during exercise, a critical distinction from general energy balance.
Relative to Lean Mass: Energy availability is expressed relative to fat-free mass (FFM) to reflect the energy needs of metabolically active tissue.
Impact of Low EA: Insufficient energy availability can lead to hormonal imbalances, weakened immune function, decreased bone density, and impaired athletic performance.
Link to RED-S: Prolonged low energy availability is the underlying cause of Relative Energy Deficiency in Sport (RED-S), a syndrome affecting multiple body systems.
Thresholds: A threshold below 30 kcal/kg FFM/day is typically considered low energy availability, though optimal ranges are higher.
Not Just Athletes: While prevalent in athletes, anyone with a mismatch between high activity and low intake can experience low energy availability.

Defining Energy Availability

The most precise definition describes energy availability as the amount of dietary energy left for the body's physiological functions after subtracting the energy expended during exercise. It is typically expressed relative to the individual's fat-free mass (FFM), as this is the most metabolically active tissue. The standard formula is: $Energy Availability = (Energy Intake - Exercise Energy Expenditure) / Fat-Free Mass$

This calculation helps determine if an individual, particularly an athlete, is consuming enough calories to support both their training load and the body's essential metabolic processes, such as growth, immunity, and hormonal regulation. Adequate energy availability is crucial for long-term health and sustainable athletic performance.

The Crucial Distinction: Energy Availability vs. Energy Balance

While the terms 'energy availability' and 'energy balance' both relate to energy consumption and expenditure, they are not interchangeable, especially in the context of sports and fitness. Energy balance simply compares total energy intake with total energy expenditure. A person can achieve a state of energy balance (intake equals expenditure) and still experience the negative effects of low energy availability if their exercise energy expenditure is very high. This happens because a large portion of the energy consumed is used for exercise, leaving insufficient energy for the body's non-exercise physiological processes.

Comparison of Energy Balance vs. Energy Availability


Feature	Energy Balance (EB)	Energy Availability (EA)
Core Concept	Total energy in vs. total energy out.	Energy left for physiological functions after exercise.
Calculation	Dietary Energy Intake - Total Daily Energy Expenditure.	(Dietary Energy Intake - Exercise Energy Expenditure) / Fat-Free Mass.
What it reflects	Net change in body weight over time (stable weight = balance).	Adequacy of energy supply for non-exercise functions.
Relevance	Good for general population weight management.	Critical for athletes and highly active individuals.
Flaw for Athletes	An athlete can be in a state of 'balance' but still have physiological dysfunctions from undereating relative to exercise load.	Provides a more accurate picture of metabolic health for active people.

The Health Consequences of Low Energy Availability (LEA)

When energy availability drops below a certain threshold (often cited as ≤30 kcal/kg FFM/day), the body enters a state of metabolic conservation to prioritize essential survival functions. This is known as Low Energy Availability (LEA). Over time, this can lead to a multisystem impairment known as Relative Energy Deficiency in Sport, or RED-S.

The physiological effects of LEA are widespread and can include:

Reproductive Dysfunction: In females, this can lead to menstrual irregularities or complete loss of menses (amenorrhea). In males, it can result in reduced libido and lower testosterone levels.
Decreased Bone Health: Inadequate energy disrupts hormones crucial for bone formation, increasing the risk of osteopenia, osteoporosis, and stress fractures.
Impaired Immune Function: The immune system can be weakened, leading to an increased susceptibility to illness and infections.
Cardiovascular Issues: Changes in lipid profiles and cardiovascular function can occur with chronic low energy intake.
Mental and Psychological Effects: Symptoms often include irritability, depression, and poor concentration.

Who is at risk for low energy availability?

While often associated with endurance sports like running, cycling, and swimming, LEA can affect any athlete, male or female, with a high training load relative to their energy intake. Athletes in weight-sensitive or aesthetic sports (e.g., gymnastics, wrestling, ballet) are also at a higher risk due to a tendency to restrict calories in pursuit of a lean physique. Unintentional LEA can also occur simply due to a lack of awareness or poor fueling strategies during periods of intense training. The core issue is the gap between energy consumed and energy used for exercise, leaving a deficit for all other biological processes.

How to Measure and Manage Energy Availability

Accurately measuring energy availability can be complex and is often difficult for free-living athletes. It requires precise tracking of both dietary energy intake and exercise energy expenditure, along with an assessment of fat-free mass.

Common methods for measurement include:

Dietary Records: Recording food intake over several days to estimate calorie consumption.
Exercise Tracking: Using a heart rate monitor, GPS device, or other technology to estimate calories burned during exercise.
Body Composition Analysis: Techniques like Dual-Energy X-ray Absorptiometry (DXA) or bioelectrical impedance analysis (BIA) to determine fat-free mass.

For athletes and coaches, the goal is to maintain a healthy energy availability level, typically above 45 kcal/kg FFM/day for females and 40 kcal/kg FFM/day for males. If low energy availability is suspected, management strategies often involve increasing energy intake, especially around training sessions, and potentially reducing exercise load. Professional guidance from a sports dietitian or physician is recommended to develop a safe and effective plan.

Conclusion

Ultimately, the definition of energy availability is a crucial concept for understanding the metabolic health of active individuals. It goes beyond the simple energy balance equation by focusing on the energy remaining for vital physiological processes after exercise demands are met. A clear understanding of this definition and its calculation is the first step toward preventing the damaging effects of low energy availability and supporting an athlete's long-term health and performance. Recognizing the signs and symptoms of LEA is essential for coaches and athletes to address the issue proactively. Adopting needs-based nutritional strategies that ensure sufficient fueling for both training and normal bodily functions is fundamental to avoiding health problems and reaching full athletic potential. For more information on Relative Energy Deficiency in Sport (RED-S), consult the resources available from authoritative sports science organizations, such as the International Olympic Committee's consensus statement.

International Olympic Committee (IOC) consensus statement on relative energy deficiency in sport (RED-S)

Frequently Asked Questions

Energy balance compares total energy consumed with total energy burned over a day. Energy availability, however, focuses specifically on the energy remaining for vital bodily functions after exercise expenditure is deducted, making it a more specific metric for active individuals.

Energy availability is calculated using the formula: $EA = (Energy Intake - Exercise Energy Expenditure) / Fat-Free Mass$. The result is measured in kilocalories per kilogram of fat-free mass per day (kcal/kg FFM/day).

When an athlete has low energy availability (LEA), their body enters a 'power-saving mode,' downregulating non-essential functions like reproductive health and immune function. This can lead to health issues and reduced performance, a condition known as RED-S.

Optimal energy availability is often cited as 45 kcal/kg FFM/day for females and 40 kcal/kg FFM/day for males. Low energy availability is typically defined as less than 30 kcal/kg FFM/day.

Yes, absolutely. The body can maintain a stable overall weight while still being in a state of low energy availability. It achieves this by suppressing key metabolic functions, masking the underlying energy deficit. This is common in athletes who under-fuel relative to their high training loads.

Fat-free mass (FFM), which includes bone, muscle, and water, is the body's most metabolically active tissue. Calculating energy availability relative to FFM provides a more accurate measure of energy sufficiency for vital physiological processes than using total body weight.

Athletes in endurance sports and weight-sensitive or aesthetic sports, such as distance running, gymnastics, and ballet, are at a higher risk. However, any athlete with a mismatch between high training volume and insufficient caloric intake can be affected.