RED-S and Under-Fuelling: When Eating Less Makes Performance Worse

Relative Energy Deficiency in Sport, now commonly written REDs, is impaired physiological and psychological function caused by prolonged or severe low energy availability. In plain language, too little energy remains for the body after training demands are subtracted.

It can affect recreational exercisers, dancers, runners, cyclists, gymnasts, weight-category athletes and team-sport players. It affects men as well as women, and it can occur at any body size.

Important: REDs is a clinical diagnosis, not a social-media checklist. Recurrent stress injuries, missed periods, fainting, chest symptoms, severe restriction, purging or rapid weight loss needs medical and eating-disorder-informed assessment.

The Body Starts Making Trade-Offs

When energy is scarce, the body protects immediate survival and reduces spending elsewhere. Possible consequences include:

• Menstrual disruption and reduced reproductive hormones.
• Low testosterone, reduced libido or fewer morning erections in men.
• Lower bone formation and greater bone-stress injury risk.
• Frequent illness, slow recovery and impaired wound healing.
• Mood change, irritability, poor concentration and sleep problems.
• Reduced training response, power, endurance and muscle gain.
• Gastrointestinal and cardiovascular effects in more severe cases.

Warning Signs Athletes Rationalise Away

• Performance falling despite more training.
• Persistent fatigue, feeling cold and poor sleep.
• Repeated tendon problems, stress reactions or fractures.
• Periods becoming irregular or stopping; this is not a normal sign of being fit.
• Low libido or hormone changes in men.
• Food rules becoming rigid, anxiety around rest days or compulsive extra exercise.
• Weight loss or failure to grow normally in an adolescent.

No single calorie-per-kilogram threshold diagnoses REDs. The IOC consensus specifically warns that individual responses vary and that a universal cut-off is not reliable.

How Under-Fuelling Happens Accidentally

Not every case starts with an eating disorder. A runner adds mileage but keeps the same meals. A busy athlete misses lunch. A vegetarian endurance athlete eats high-fibre food but not enough total energy. A hybrid athlete stacks morning cardio and evening lifting without adding carbohydrate. Appetite can also be suppressed after intense training.

What Assessment Should Include

A sports physician or knowledgeable clinician may review training, dietary intake, weight history, menstrual or sexual function, injuries, mood, medications and eating behaviours. Testing may include blood counts, iron status, thyroid and sex hormones, vitamin D, ECG or bone-density assessment depending on the picture. Normal blood tests do not automatically exclude REDs.

Recovery Is More Than “Eat a Bit More”

• Increase energy intake, often especially carbohydrate around training.
• Reduce training load temporarily when health indicators are impaired.
• Spread meals and snacks across the day; avoid long accidental gaps.
• Include carbohydrate before and after demanding sessions, plus adequate protein.
• Treat iron deficiency, bone injury and hormone consequences clinically.
• Use an eating-disorder-informed team when restriction, fear or compulsive exercise is present.

The IOC REDs Clinical Assessment Tool uses risk categories to guide participation. Return to full training is based on recovery markers, not only weight gain or one normal period.

A Simple Fuel Audit

For one week, ask:

• Did I eat within a few hours before training?
• Did I eat carbohydrate and protein afterwards?
• Did I go more than four or five waking hours without food?
• Did a second session have dedicated fuel?
• Am I avoiding entire food groups without a medical reason?
• Are fatigue, mood, libido, periods or injuries changing?

This audit cannot diagnose REDs, but it can reveal obvious energy gaps to discuss with a sports dietitian.

FAQ

Can someone with a higher body weight have REDs?

Yes. Low energy availability is about intake relative to exercise demands and body needs, not appearance or BMI.

Are missed periods normal for athletes?

No. They are common in some sports but not healthy or necessary for performance. Menstrual disruption deserves assessment.

Does REDs affect men?

Yes. Potential signs include low libido, reduced morning erections, low testosterone, poor recovery, bone stress injury and declining performance.

Can I fix REDs while keeping all my training?

Sometimes intake changes are sufficient, but moderate or high-risk cases often need temporary training reduction. Health and long-term performance take priority.

Trusted Sources

• 2023 IOC consensus statement on Relative Energy Deficiency in Sport
• IOC REDs Clinical Assessment Tool, version 2

How FitLifestyle Helps

FitLifestyle can surface rising training load, repeated fatigue and missed recovery meals, but suspected REDs needs a sports-medicine and nutrition team rather than another optimisation algorithm.