Diet culture would have us believe that 'eating clean' or following strict ways of eating is to be celebrated. For athletes this can also mean trying to adhere to a certain body size either for aesthetics or for perceived performance gains. This can often result in intentional under-eating to try to meet these ideals.

What is under-fuelling?

Under-fuelling happens when your body doesn't get the energy (calories) it needs to function properly. This isn't just about notable weight loss or strict diets. It can happen to those who are eating three meals a day and feeling like they are doing well with their dietary intake. Many underestimate just how energy-demanding running can be.

What is RED-S?

RED-S (Relative Energy Deficiency in Sport) is defined as insufficient dietary energy to support the combined energy demands of exercise and normal body functioning. Often I see runners mismatch their energy intake and training – accidentally under-fuelling as they haven't increased their food intake despite their training increasing. Trying to train heavily but still eating what you normally do is often not enough. Some may feel uncomfortable eating more than others who are not training and so hold back.

RED-S affects everything from bones, hormones, immune system, performance and mood. It can happen to anyone, not just elite athletes. Signs may include poor recovery, getting ill more often, feeling colder, periods changing or stopping, feeling more tired and having less stamina. Short term our bodies will adapt, but in the long term the compensatory changes can seriously compromise health and performance.

The IOC framework recognises RED-S as a syndrome resulting from chronic low energy availability. The body always must prioritise energy for exercise, but normal bodily functions are compromised when there's not enough total energy. Imagine the 'low power mode' on your phone battery – it's still operating but not at full capacity. Our bodies can turn on this lower power mode when energy is scarce.

Symptoms you should be aware of (potential red flags for under-fuelling)

  • Constant fatigue – Feeling tired all the time, even after sleeping well; your body might be running on empty. To compensate the body starts to slow things down.
  • Not making progress despite training hard – Under-fuelling will negatively impact the muscle's ability to repair and you start to stall in progress. Also having less energy in the tank makes workouts feel harder than they should if you were properly fuelled.
  • Feeling moody and irritable – Our brains rely on glucose (in fact they use around 130g glucose per day). Under-fuel and your brain gets grumpy! You end up more irritable and will struggle to concentrate. Everyday tasks can feel harder than normal. This can impact your mental health too.
  • Losing your menstrual cycle (amenorrhea) – For women, a lost or irregular period is a major red flag. Hormonal disruption from under-fuelling is a classical symptom of RED-S.
  • Cold all the time – Feeling like you're freezing even though it's not cold out. That's a sign your metabolism has slowed and is prioritising essential organs over thermoregulation.
  • Hair thinning or hair loss – Your body will prioritise vital organs over non-essentials like hair growth when it's not getting enough nutrients.
  • Constantly thinking about food? Undereating can increase cravings and fixate your brain on finding energy as the body's way of trying to protect itself.
  • Digestive issues – Slowed digestion, constipation or bloating can result when your gut isn't getting the fuel it needs to function smoothly.

What next?

Recognising the signs of under-fuelling is the first step toward returning to a normal, healthy body. Our bodies need fuel to work – a bit like a car; you wouldn't expect it to run without petrol. If you recognise these symptoms, it may be time to do something about it. By taking the time to support and nourish your body you are giving yourself the best base from which to chase your goals!

If any of this resonates and you are concerned about your health, a visit to your GP can be a good starting point.