When a growing teenager or an active athlete fails to consume enough calories to match their energy demands, the body initiates a survival response. The high energy needed for growth, coupled with the intense fuel requirements of exercise, creates a scenario where available energy is insufficient to support all bodily functions. This deficit forces the body to conserve energy by slowing down or shutting off processes deemed non-essential for immediate survival. The consequences range from immediate performance drops to serious, long-term health issues that affect development and well-being.
The Mechanism of Low Energy Availability
The problem is defined by Energy Availability (EA), which measures the calories remaining for basic life processes after accounting for exercise. EA is calculated by subtracting the energy burned during exercise from the total caloric intake, then dividing that result by the athlete’s fat-free mass (FFM) in kilograms. This provides a normalized value, typically expressed as kilocalories per kilogram of FFM per day (kcal/kg FFM/day). If this remaining energy falls below 30 kcal/kg FFM/day, the body enters a state of Low Energy Availability (LEA). Chronic LEA leads to Relative Energy Deficiency in Sport (RED-S), affecting multiple organ systems. To preserve energy for the brain, heart, and lungs, the body downregulates energy-expensive functions such as hormone production, immune defense, and bone turnover.
Acute Effects on Athletic Performance and Recovery
The most immediate consequence of insufficient caloric intake is a significant decline in performance capacity. When the body lacks sufficient fuel, it cannot properly replenish muscle glycogen stores, the primary energy source for high-intensity exercise. This translates directly into decreased endurance and power output during training and competition.
Recovery is also impaired because LEA suppresses muscle protein synthesis, making it difficult to repair and rebuild muscle tissue after strenuous workouts. Athletes often experience chronic fatigue, prolonged muscle soreness, and an inability to adapt to training stimuli, leading to stalled strength gains. Furthermore, the body attempts to conserve energy by lowering its resting metabolic rate (RMR). This metabolic adaptation makes it increasingly difficult to lose body fat, even with continued low caloric intake. This combination of poor recovery and reduced metabolic efficiency compromises both training quality and overall energy levels.
Long-Term Hormonal and Skeletal Consequences
Chronic low energy availability poses a significant threat to skeletal health and endocrine function in developing teenagers. Adolescence is a period of peak bone mass accrual, where the body lays down the majority of its adult bone density. LEA impairs this process by disrupting calcium regulation and suppressing the hormones necessary for strong bone formation.
This hormonal disruption leads to decreased bone mineral density (BMD), increasing the risk of osteopenia or osteoporosis later in life. The primary danger is a dramatically increased risk of stress fractures, as low levels of insulin-like growth factor 1 (IGF-1) and sex steroids weaken the structural integrity of the skeleton.
The hypothalamic-pituitary axis, which controls hormone release, is highly sensitive to energy deficits. In females, a common consequence is Functional Hypothalamic Amenorrhea (FHA), marked by the loss of the regular menstrual cycle. This results in low estrogen levels, which further accelerates bone loss. Males also experience decreased production of testosterone, contributing to lower libido, impaired muscle maintenance, and reduced BMD. In teenagers, the suppression of growth hormone and thyroid function due to LEA can impede linear growth and prevent them from reaching their full genetic height potential.
Systemic Impact on Immunity and Cognitive Function
Chronic LEA compromises the body’s systemic defenses and mental acuity. The immune system, which is energy-intensive, becomes impaired when fuel is restricted. This results in a compromised ability to fight off pathogens, leading to an increased frequency and duration of common illnesses.
Low energy availability is associated with increased levels of the stress hormone cortisol, which alters immune function and inflammatory responses. This systemic stress also manifests as significant changes in mood and cognitive function. Individuals experiencing LEA often report difficulty concentrating, decreased attention span, and impaired judgment, affecting academic performance. Hormonal imbalances and nutrient deficiencies contribute to increased irritability, anxiety, and symptoms of depression.