What is RED-S?
If you’re training hard but not seeing progress, or worse, feeling constantly run down, it might not just be fatigue or a busy schedule. Many athletes unknowingly fall into a state of low energy availability (LEA), where they aren’t eating enough to meet the demands of both training and basic bodily functions. Over time, this can lead to a condition known as RED-S: Relative Energy Deficiency in Sport.
RED-S is a syndrome that affects multiple systems in the body, including metabolism, hormones, bone health, and mental wellbeing. It doesn’t only affect elite athletes or those with eating disorders. It can affect anyone who chronically underfuels, whether intentionally or not.
Low Energy Availability: The Root Cause
To understand RED-S, it helps to start with your body’s baseline energy needs, known as your Resting Metabolic Rate (RMR). This is the amount of energy your body needs to perform essential functions at rest, like breathing, regulating temperature, producing hormones, maintaining brain function, and supporting your immune system.
Now imagine your RMR is like the power required to keep your body’s engine idling. On top of that, you add the energy needed for training, daily activity, and recovery. If the energy coming in from food doesn’t meet all of these demands, the body has to make cuts somewhere. That’s low energy availability (LEA); when there’s not enough fuel to support both basic body functions and the stress of exercise.
When this energy shortfall persists, the body enters what you can think of as “energy-saving mode.” Like a phone in low-power mode, it starts shutting down non-essential systems to conserve energy. Hormone production, bone turnover, metabolic rate, and immune defence are all dialled down. Menstrual cycles may stop. Muscle repair and recovery slow. Mental clarity dips.
Over time, this protective mechanism becomes damaging. What begins as low energy availability progresses into Relative Energy Deficiency in Sport (RED-S); a state where health and performance are both compromised.
So, it’s not just about eating enough to train. It’s about fuelling to cover your RMR, your movement throughout the day, and the additional demands of recovery and adaptation. Failing to do that creates a silent deficit, one that can take a serious toll if left unaddressed.
You’re injured and trying to return to play, but because your body isn’t getting the fuel it needs, recovery is delayed.
Recognising the Signs of RED-S
The symptoms of RED-S vary from person to person and can easily be dismissed as unrelated or minor. But if multiple systems in the body are being affected at once, it’s worth paying attention. Some of the most common signs of RED-S in athletes include:
- Persistent fatigue or low energy, even with rest
- Frequent illness or infections due to impaired immunity
- Irregular or missed menstrual cycles in women
- Reduced libido or low testosterone in men
- Stress fractures, poor bone health, or joint pain
- Digestive issues, such as bloating or constipation
- Difficulty concentrating, low mood, or increased anxiety
- Plateaued performance despite consistent training
- Injuries that won’t heal or keep recurring
None of these signs alone confirm RED-S, but together, they tell a story. When the body is under-fuelled, it can no longer support recovery, strength gains, or peak performance.
The Impact on Health and Performance
Relative energy deficiency in sport doesn’t just stop at feeling tired or having a bad training week. The condition can affect nearly every system in the body. From an athletic perspective, it leads to a cascade of problems: decreased strength and endurance, slower reaction times, impaired coordination, and longer recovery periods.
For female athletes, menstrual irregularities are a major red flag that energy intake isn’t matching the demands placed on the body. For male athletes, signs may be more subtle — like poor sleep, low mood, or decreased motivation. Over time, low bone density becomes a serious concern, especially for young athletes who haven’t yet reached peak bone mass.
The psychological effects are equally important. RED-S is linked to increased anxiety, depression, and a strained relationship with food and body image. When you’re consistently low on energy, even small tasks can feel overwhelming.
Why This Matters for Injury and Illness
Injury recovery is significantly impacted by RED-S. If your body doesn’t have enough energy to support the healing process, injuries can become chronic or reoccur frequently. Inflammation hangs around longer, tissue repair is slowed, and immune function is compromised.
This creates a frustrating cycle for athletes: you’re injured and trying to return to play, but because your body isn’t getting the fuel it needs, recovery is delayed. You feel like you’re doing everything right, yet you’re stuck, and often, the missing piece is nutrition.
What to Do If You’re Concerned
If some of these signs sound familiar, don’t panic, but don’t ignore them either. RED-S in athletes is treatable, especially when caught early. The first step is to seek professional guidance and see your GP. Symptoms alone aren’t enough for a diagnosis, as many signs can overlap with other conditions. That’s why a full assessment, including medical screening, blood work, and a detailed review of your training and diet, is essential.
A registered sports nutritionist can help determine whether you’re in a state of low energy availability and how to reverse it. This might involve increasing overall energy intake, adjusting training loads, or addressing disordered eating behaviours, if present. A tailored nutrition plan that considers your lifestyle, training goals, and health status can support a sustainable recovery, one that doesn’t sacrifice long-term wellbeing for short-term gains.
The first step is to seek professional guidance and see your GP. Symptoms alone aren’t enough for a diagnosis, as many signs can overlap with other conditions.
Breaking the Cycle
If you’ve been stuck in a cycle of injury, fatigue, or poor results, it’s worth asking whether you’re really fuelling your body adequately. In sports, it’s easy to believe that working harder is always the answer. But sometimes, better progress comes from eating more — not less.
Letting go of rigid food rules, embracing balanced meals, and understanding the importance of carbohydrate and fat intake (not just protein) are all key parts of recovery. Food is not just fuel; it’s the foundation of your performance, recovery, and resilience.
Final Thoughts
RED-S in athletes is more common than most people realise — and often hidden in plain sight. You don’t have to be underweight, or visibly unwell, to be under-fuelled. In fact, many high-performing athletes experience symptoms without ever connecting them to low energy availability.
Awareness is the first step. By recognising the early signs, seeking appropriate support, and prioritising your body’s full energy needs, you can prevent RED-S from derailing your training, your health, and your goals.
Looking for support?
If you suspect you’re not fuelling enough, or are struggling with recovery, performance, or unexplained fatigue, get in touch. A personalised nutrition strategy can help you train smarter, recover faster, and build long-term resilience, without compromising your health along the way.
