If you’re an active person or competitive athlete, chances are you’ve heard the term “RED-S” (Relative Energy Deficiency in Sport). Sometimes referred to as underfueling or low energy availability, this condition happens when athletes don’t eat enough to meet the demands of their training and daily life. 

While often unintentional, RED-S is more common than most people think, and it affects performance, recovery, and long-term health.

What is RED-S?

RED-S occurs when there’s a mismatch between how much energy an athlete expends (through training, activity, and basic body functions) and how much energy they consume (through food and fuel). 

When this energy gap persists over time, the body has to make trade-offs. Instead of putting resources toward recovery, bone health, and hormone balance, it diverts energy to keep essential systems running. 

The result? A range of health and performance consequences. 

Signs and Symptoms of RED-S 

Because RED-S affects nearly every system in the body, symptoms can look different for each athlete. Some common signs include: 

• Frequent injuries (stress fractures, recurring strains) 

• Chronic fatigue or low energy during workouts 

• Decline in performance despite consistent training 

• Hormonal changes (missed or irregular periods, low testosterone) 

• Mood changes (irritability, anxiety, or low motivation) 

• Weakened immunity (getting sick more often) 

• Digestive issues 

Many athletes chalk these symptoms up to “pushing harder in training,” when in reality, they’re signs the body isn’t getting the fuel it needs. 

How Common is RED-S? 

Research shows RED-S is highly prevalent across endurance, aesthetic, and weight-class sports. Studies suggest that: 

• Up to 50% of female endurance athletes experience low energy availability. 

• Male athletes are also at risk, but often underdiagnosed. 

• Even recreational athletes training several times per week can be affected if they consistently underfuel. 

This isn’t just an elite athlete problem, anyone increasing training volume without increasing food intake can develop RED-S. 

Why RED-S Matters 

Beyond performance setbacks, RED-S can have serious long-term health consequences if left unaddressed: 

• Decreased bone density → higher risk of fractures and osteoporosis 

• Suppressed immune function → slower recovery from illness 

• Hormonal imbalances → fertility concerns and low libido 

• Impaired heart and metabolic health 

Fueling properly isn’t just about faster race times, it’s about protecting your health for years to come. 

The good news? RED-S is preventable and reversible with the right strategies: 

• Eat enough to match training load: prioritize carbohydrates for energy and protein for recovery. 

• Don’t skip meals: regular fueling helps avoid energy dips. 

• Snack smart: especially before and after workouts. 

• Work with a dietitian: an RD can assess energy needs and create a fueling plan that supports performance and health. 

• Listen to your body: fatigue, poor recovery, or frequent injuries are cues that nutrition may need adjusting. 

  
  

RED-S is one of the most common, and overlooked issues in sport. Underfueling may feel normal in a “train harder, eat less” culture, but it comes at a cost. 

We believe that food is fuel and food is medicine. Whether you’re training for your first 10K or an Ironman, prioritizing energy balance is essential for both peak performance and long-term health. 

If you’re curious about how to fuel better or suspect RED-S might be impacting you, consider connecting with one of our Registered Dietitians for personalized guidance.