Low Energy Availability (LEA) is a condition where the body does not have enough energy remaining after exercise to support all its basic physiological functions. This is a state of insufficient fuel for processes such as breathing, temperature regulation, and maintaining a robust immune system. When energy intake cannot cover both the energy burned during physical activity and the energy needed for survival, the body enters a forced “survival mode.” This results in the suppression of systems deemed non-essential to immediate survival, leading to various health and performance impairments.
Defining the Energy Availability Equation
Low Energy Availability is a technical concept defined by a specific calculation that quantifies the energy left over for the body’s internal machinery. The equation is: Energy Availability equals (Energy Intake minus Exercise Energy Expenditure) divided by Fat-Free Mass (FFM). The result is expressed in kilocalories per kilogram of fat-free mass per day (kcal/kg FFM/day).
Energy Intake is the total calories consumed from food and beverages, while Exercise Energy Expenditure is the calories burned during intentional physical activity. Fat-Free Mass (FFM) is the body’s lean tissue, including muscle, bone, and organs, and is considered the most metabolically active tissue that requires constant fueling. Normal energy availability is generally considered to be around 45 kcal/kg FFM/day.
A state of low energy availability is clinically defined by a threshold, typically considered to be less than 30 kcal/kg FFM/day for women. This threshold has shown demonstrable negative impacts on hormonal markers and bone metabolism. For men, the threshold is less clear but appears to be lower, possibly around 20–25 kcal/kg FFM/day.
Common Causes of Low Energy Availability
The underlying cause of LEA is an imbalance between the amount of energy consumed and the amount of energy expended. This imbalance can happen accidentally or intentionally, stemming from a failure to match fuel intake with activity levels.
One frequent cause is intentional severe caloric restriction, where an individual deliberately limits food intake, often to lose weight or achieve a lean physique. This can be compounded by a lack of nutritional knowledge, leading to meal-skipping or poor timing of meals around high-intensity exercise sessions.
Another pathway to LEA is a sudden, drastic increase in training volume or intensity without a corresponding increase in food intake. For example, an athlete who doubles their weekly running mileage without consuming more calories creates a significant energy deficit. Certain sports with aesthetic demands or weight classes can also increase the pressure to restrict calories, contributing to the development of LEA.
Systemic Effects on the Body
When the body senses a prolonged energy deficit, it initiates a series of internal system shutdowns to conserve energy for survival. This shift away from optimal function is known as the body entering a state of energy crisis.
One of the most noticeable effects is endocrine disruption, involving the suppression of reproductive and thyroid hormones. The body decreases the production of hormones like estrogen and testosterone, signaling that the reproductive system has been suppressed. Thyroid hormone production may also decrease, which acts to slow the body’s overall metabolism to further conserve energy.
LEA severely impairs bone health by disrupting the balance of bone remodeling. The lack of energy, combined with hormonal changes, prevents the body from properly rebuilding bone tissue, leading to a long-term reduction in bone mineral density. This raises the risk of developing stress fractures and chronic osteoporosis.
To stretch its limited energy supply, the body actively decreases its Resting Metabolic Rate (RMR), the energy required to sustain life at rest. This metabolic slowdown is an adaptation to low fuel, making it harder to maintain body temperature and contributing to feelings of being cold. The immune system is also suppressed, as energy is diverted away from immune cell production and function, leading to a higher susceptibility to frequent or prolonged illness.
Recognizing the Signs of Chronic LEA
Chronic LEA manifests through several practical, observable symptoms. One of the most common indicators is persistent fatigue or a feeling of low energy, despite getting adequate rest or sleep.
Recurrent injuries, particularly stress fractures or muscle strains that do not seem to heal completely, are a physical sign of impaired bone and tissue repair. In women, a major red flag is the loss of the menstrual cycle, known as amenorrhea, or having irregular periods, which signals significant reproductive hormone suppression.
Individuals with chronic LEA may frequently feel cold, even in warm environments, due to the body’s compromised ability to regulate its temperature. Other signs include frequent bouts of illness, such as colds or infections, which point to a suppressed immune system. Changes in mood, such as increased irritability, difficulty concentrating, or a general feeling of being down, can also be symptoms of this ongoing energy crisis.