Low Energy Availability in Athletes: Fatigue, Menstrual Cycle and RED-S Risk

Energy availability is the energy the body has left after exercise to support essential functions, including hormone production, bone health, immunity, recovery, metabolism, brain function and everyday activities. When it becomes problematically low, health, well-being and performance can suffer. [1][3]

Fatigue, declining performance or slower recovery alone cannot establish a diagnosis of RED-S. These signs need to be assessed in context, taking into account training load, diet, sleep, injuries, menstrual history, stress, illness and medication. [6]

The energy you consume each day does more than fuel running, cycling, swimming or heavy lifting. After accounting for the energy used during exercise, enough must remain to keep the body functioning. If food intake consistently falls short of overall needs and training demands, the energy available for these functions can become inadequate.

This shortfall may be intentional, resulting from food restriction or efforts to reach a target weight. It can also be unintentional: training increases, appetite is low, busy schedules leave little time to eat, or travel, stress and digestive problems get in the way. Some athletes may believe that “eating light” is always the more professional approach. RED-S can develop without an eating disorder, although disordered eating behaviors increase the risk. [1][2]

Scientific measures of energy availability describe a physiological concept; they are not a home test. Calculating an individual's energy availability and interpreting the result require professional assessment. [3]

Persistent fatigue is a warning sign, not a diagnosis. Too little sleep, infection, excessive training, a nutrient deficiency, illness, psychological stress or an inadequate diet can cause similar symptoms. The IOC consensus describes how problematic low energy availability may affect recovery, concentration, the body's response to training and performance. [1][3][6]

Struggling with previously manageable training sessions, an unusual drop in power output, repeated injuries, new irritability or longer recovery times should all be taken seriously. The aim is not to jump to a diagnosis of RED-S, but to avoid dismissing a sustained decline as normal.

In some people who menstruate, problematic low energy availability can disrupt reproductive function, leading to longer gaps between periods, irregular periods or periods stopping altogether. These changes should not be accepted as the normal price of intense training. They can also have other causes, including pregnancy, an endocrine disorder, significant stress or illness. An assessment helps ensure that changes are not automatically attributed to sport or diet. [1]

An apparently unchanged menstrual cycle does not rule out a problem. RED-S is not just about menstruation. In men, it can affect reproductive function, bone health, mood, metabolism and performance. Research in men is growing, but the thresholds involved and how the condition develops remain less clearly defined. [1][5]

Bone is constantly being renewed. This process requires energy, nutrients, appropriate mechanical loading and adequate recovery. Problematic low energy availability can impair bone remodeling and may be associated with a higher risk of bone injuries in some athletes. [1][6]

A stress fracture, recurring bone pain or a series of injuries cannot be addressed simply by adding calcium, vitamin D or collagen. A suitably qualified team needs to assess training load, diet, medical history, hormones, recovery and any accompanying signs or symptoms.

RED-S can involve changes in immune function, recovery and other physiological processes. A single cold or a week of fatigue is not enough to suggest RED-S on its own. What matters is the overall pattern: repeated infections, slow wound healing, digestive problems, feeling cold, reduced libido, mood changes, disrupted sleep, injuries or a prolonged performance plateau.

Low energy availability may occur alongside inadequate carbohydrate availability, the exclusion of food groups or disordered eating behaviors. Good care investigates the underlying causes rather than treating the problem as a matter of willpower. [1][2]

The risk is well recognized in endurance sports, weight-class sports, aesthetic sports and environments that place a strong emphasis on body composition. It can also arise in other sports when training load increases or food intake becomes restricted. Competing at a particular level offers no protection: dedicated recreational athletes, young athletes, elite competitors and people returning to intensive exercise may all be at risk. [1][2]

Particular care is needed for adolescents, people who are pregnant or breastfeeding, athletes with a history of an eating disorder, and people who are ill or receiving medical treatment.

Talk to a doctor, sports dietitian or another suitably qualified professional. Describe your fatigue, performance, sleep, injuries, digestion, mood, infections and diet, as well as your cycle if you menstruate. The REDs CAT2 is designed for clinical assessment by a physician-led health and performance team. It is neither a self-diagnosis questionnaire nor an online calculator. [2]

Treatment aims to restore adequate energy availability and address the causes and consequences identified during assessment. This may involve changes to diet, training and rest, alongside monitoring of the body systems affected. [1][4]

Problematic low energy availability can contribute to fatigue and affect performance, recovery, menstrual function in some people, bone health, immunity and other body functions. It can affect people of any sex and cannot be identified from a single symptom. The safest approach is professional assessment and a gradual return to an energy intake that supports both health and training.