Low Energy Availability and RED-S: Why Underfuelling Wrecks Endurance Performance

Most endurance athletes will happily talk about intervals, watts and pace. Far fewer will admit they are quietly running on empty. Yet underfuelling is one of the most common and most damaging mistakes in our sports, and it catches out the dedicated, the disciplined and the very lean far more often than it catches out the casual. If you have been training hard, feeling flat, picking up niggles and wondering why your fitness has stalled, the answer may be on your plate rather than in your training plan.

The medical term for the problem is Relative Energy Deficiency in Sport, or RED-S. It describes what happens when you consistently take in too little energy to cover both your training and the basic running costs of your body. In this guide we will look at what low energy availability actually means, the numbers behind it, the warning signs for RED-S endurance athletes should never ignore, and the practical fuelling habits that keep you on the right side of the line.

What Low Energy Availability Actually Means

Energy availability is a simple sum: the energy you eat, minus the energy you burn in exercise, expressed per kilogram of fat-free mass. The sports medicine consensus is that around 45 kcal per kg of fat-free mass per day keeps the body running properly. Drop below roughly 30 kcal per kg and you are in low energy availability territory, where the body starts to ration energy and shut down jobs it considers non-essential.

To put that in real terms, a 70 kg athlete with about 55 kg of fat-free mass needs roughly 2,500 kcal a day before any training is counted. If that athlete burns 1,000 kcal on a long ride and eats only 2,600 kcal across the whole day, they are sitting at around 29 kcal per kg. That is a deficit many people would consider a perfectly normal diet. This is why low energy availability is so easy to slide into without ever intending to.

Importantly, it is often not deliberate. Plenty of athletes under-eat simply because training suppresses appetite, because they are busy, or because they never learned how much fuel a 4-hour session really demands. Intent does not change the physiology.

The Warning Signs Most Athletes Miss

RED-S affects far more than the menstrual cycle or bone health, which is where older ideas about the female athlete triad stopped. It touches almost every system in the body, and it affects men as well as women. The signs tend to creep up slowly:

• Persistent fatigue and flat sessions. Workouts you used to nail feel heavy, and heart rate may sit oddly high or oddly low.
• Frequent illness and slow healing. Colds that keep returning and small injuries that linger point to a suppressed immune system.
• Stress fractures and recurring bone injuries. Low energy availability reduces bone formation, so shin and foot problems become more likely.
• Hormonal disruption. Missed or irregular periods in women, and low libido or poor morning erections in men, are red flags that testosterone or oestrogen are being suppressed.
• Mood and sleep changes. Irritability, poor concentration, low mood and broken sleep are all common.
• Stalled or declining performance. Despite more training, you get slower, not faster.

If several of these sound familiar, speak to a doctor or a sports dietitian rather than trying to self-diagnose. This article is general information, not medical advice, and RED-S needs proper professional assessment.

Why Underfuelling Hurts Endurance Performance

Even before the longer-term health consequences arrive, underfuelling directly damages your training. When muscle and liver glycogen are never fully topped up, you cannot hit the intensity you planned. Sessions become a grind, quality drops, and the adaptations you are working so hard for simply do not arrive.

Recovery suffers too. Protein synthesis, tissue repair and the hormonal signals that tell your body to adapt all need energy. Train hard on a chronic deficit and you accumulate fatigue without banking fitness, which is the fastest route to overtraining. A 2023 pattern seen again and again in coaching is the athlete who responds to a plateau by training more and eating less, which makes the plateau worse.

There is also a hidden penalty during the session itself. Athletes who are chronically underfuelled often under-eat on the bike or run as well. Carbohydrate intake of 60 to 90 g per hour on long efforts protects performance and reduces the stress hormone response to exercise. Skipping it to save calories is a false economy.

How to Fuel Properly and Avoid RED-S

The good news is that prevention is largely about habits, and most of them are simple.

Eat enough every day, not just on training days. Rest days still need adequate energy because that is when adaptation happens. Cutting calories on easy days is a common trap.

Fuel the session. For anything over about 75 minutes, aim for 30 to 60 g of carbohydrate per hour, building towards 60 to 90 g per hour on long or hard efforts if your gut is trained for it. For sessions under an hour, you may need less, but a hard session still benefits from a pre-session carbohydrate top-up.

Eat soon after you finish. Taking in 1.0 to 1.2 g of carbohydrate per kg of body weight along with 20 to 30 g of protein within the first couple of hours helps restore glycogen and kick-starts repair. This matters most when you are training twice in a day.

Track the trend, not the perfection. You do not need to weigh every meal. Instead, watch for the warning signs above, and keep an eye on body weight, mood, sleep and training quality over weeks. A sudden drop in any of them is information.

Do not chase leanness at all costs. A low body weight is not the same as a fast one. Many athletes perform best at a weight that feels slightly higher than the one they imagined, because they can actually train properly.

What We Use at FTP Endurance Fuel

One of the easiest ways to stop underfuelling during training is to make eating on the move effortless. If fuel is fiddly, sticky or hard to stomach, you will skip it, and that is when the deficit builds. That is exactly why we make energy pastilles: small, pocket-friendly pieces that you can eat steadily without breaking your rhythm or fumbling with packaging.

For days when you want fast carbohydrate with real variety, our Mixed Fruit Pastilles 1kg make it easy to keep intake high for hours without flavour fatigue. The Tart Cherry Pastilles 1kg are the same carbohydrate with a sharper flavour; they are tart cherry flavoured, and real Montmorency tart cherry is in Power Fluids. Both use a glucose and fructose blend so you can build towards higher hourly intakes without upsetting your stomach.

Final Thoughts

Underfuelling is not a badge of honour, and RED-S is not something that only happens to other people. The athletes who last, stay healthy and keep improving are the ones who treat food as part of the training plan rather than a negotiable extra. Eat enough, fuel your sessions, recover properly and listen when your body starts sending signals. If you are unsure where you stand, get proper professional advice early.

Ready to make fuelling easier? Try the Taster Pack to find your favourite, or browse our bundles to stock up for the training block ahead.