Energy Balance and Performance in Athletes
Appropriate energy intake (or fuelling) is crucial for the health and performance of all athletes. When an athlete's energy intake does not align with energy expenditure, both short and long-term performance can suffer and negative health effects may arise.
Understanding Energy Balance
Energy balance refers to the equilibrium between total energy intake from food and total energy expenditure. Energy output includes:
The energy needed to fuel exercise,
Incidental daily movement and
Basal metabolic rate (BMR)—the energy required to maintain vital functions while at rest.
When energy intake matches expenditure (at rest and during exercise), you are in neutral energy balance. Adjusting either food intake or exercise levels shifts this balance. A negative energy balance occurs when energy intake is less than expenditure, typically leading to weight loss.
Energy Availability (EA)
For athletes, it's important to consider Energy Availability (EA), which separates energy output into:
Energy for exercise
Energy for health and maintenance
In an ideal state, energy intake supports both components. However, endurance athletes often create large negative energy balances due to high training demands and increased muscle mass, which raises BMR. If this negative energy balance persists for a week or more, initial weight loss may occur. However, over time, the body adapts by lowering Basal Metabolic Rate (BMR) to conserve energy, reducing available energy for key functions such as bone health, immune function and lean muscle mass preservation.
Low Energy Availability (LEA) and its Effects
When energy intake is sufficient for exercise but insufficient for recovery and physiological functioning, the body enters a state of low energy availability (LEA). LEA is highly stressful and triggers a survival response, where the body conserves energy by slowing or shutting down non-essential functions.
Over time, chronic LEA leads to metabolic and physiological adaptations:
BMR decreases, making the body more energy-efficient but compromising overall health.
Muscle mass decreases, training performance declines and weight loss may plateau or even reverse.
Athletes may struggle to lose weight despite under-fuelling due to a sluggish metabolism.
Psychological and cognitive effects such as increased stress, mood disturbances and impaired decision-making may occur.
Gut health and nutrient absorption can also be affected, potentially impairing digestion and recovery.
When an athlete corrects their energy intake, short-term weight gain is common as the body restores normal metabolic function. However, this weight gain is often due to restored glycogen stores and water retention rather than fat gain, making it a normal and necessary part of recovery.
Achieving Energy Balance
Restoring normal metabolic and physiological processes requires fuelling in line with actual metabolic demands. Because BMR is suppressed in LEA, athletes may experience an initial period of energy excess before metabolic function normalizes.
Although individual optimal EA levels vary, research suggests the following general thresholds for maintaining physiological functions:
Women: Around 45 kcal/kg fat-free mass/day for sedentary, eumenorrheic individuals, but some may require more (Loucks, 2014)
Men: Around 40 kcal/kg fat-free mass/day for exercising individuals, though variability exists (Koehler et al., 2016)
(Energy availability = (Energy intake (kJ) − Energy expenditure during exercise (kJ)) / fat-free mass (kg))
Anecdotal evidence suggests that the longer an athlete remains in LEA, the longer it takes to restore normal physiological function. This process can impact body composition, which may have psychological effects within athletic populations.
The Risks of Chronic LEA
In 2014, the International Olympic Committee (IOC) introduced the term Relative Energy Deficiency in Sport (RED-S) to describe a state where low food intake fails to meet both training and basic physiological demands.
Chronic LEA in athletes is associated with numerous health risks, including:
Nutrient deficiencies due to inadequate food intake
Chronic fatigue and increased susceptibility to illness and infection
Psychological effects, ranging from heightened stress to depression
Reduced testosterone, lower bone density and increased fracture risk in male athletes, even if weight loss isn’t apparent
Menstrual dysfunction, lower bone density and increased fracture risk in female athletes, even if weight loss isn’t apparent
Optimizing Performance Through Proper Fuelling
Balancing training adaptations, maintaining health and achieving body composition goals for peak performance is a complex challenge. A preoccupation with leanness can contribute to chronic under-fuelling and LEA. Developing a deeper understanding of personal nutritional needs and adjusting energy intake to support fitness and weight goals—without compromising health—allows athletes to maximize their performance potential.
Bevan McKinnon / February 2025