Under-fuelling (RED-S/LEA) in athletes: the signs and the fix
Relative Energy Deficiency in Sport (REDs) is the set of health and performance problems caused by low energy availability: eating too little for the training you do. Signs include lost or irregular periods, bone stress injuries, frequent illness, low mood and stalled performance. Diagnosis and treatment belong with a doctor or sports dietitian.
This article is information, not a diagnosis. If any of the signs below sound familiar, especially missed periods, a stress fracture or symptoms of disordered eating, see a doctor or a registered sports dietitian. REDs is diagnosed clinically, and the right fix depends on the cause.
What is RED-S, according to the IOC?
REDs is a syndrome of health and performance effects in athletes of either sex who are exposed to low energy availability. That is the definition in the 2023 IOC consensus statement on REDs (Mountjoy et al., British Journal of Sports Medicine, 2023).
The consensus describes low energy availability as “inadequate energy intake in relation to exercise energy expenditure”. It separates problematic exposure from short-term exposure the body can adapt to. It also highlights a growing role for low carbohydrate availability, the link with mental health, and new data on male athletes.
The 2023 statement introduced a clinical assessment tool, REDs CAT2, for clinicians to detect and diagnose REDs. It is not a self-test.
What are the signs of under-fuelling?
The signs fall into two groups: health and performance. The ACSM, AND and Dietitians of Canada joint position on nutrition and athletic performance (Thomas, Erdman and Burke, 2016) lists possible effects across both.
Health signs worth raising with a clinician:
- Missed, irregular or absent periods.
- Low libido or other hormonal changes, in men too.
- Bone stress injuries or low bone density.
- Getting ill often, or slow recovery from illness.
- Gut problems.
- Changes a clinician may find in blood tests or heart rhythm.
- Low mood, irritability or anxiety around food.
Performance signs:
- Lower endurance and muscle strength.
- Poorer training response.
- More injuries.
- Weaker concentration and coordination.
- Sessions that feel harder at the same effort.
None of these proves under-fuelling on its own. Several have other causes, which is exactly why they need a professional to sort out.
How is energy availability calculated?
The formula is energy intake minus exercise energy expenditure, divided by fat-free mass. It gives the energy left for the rest of the body after training.
The joint position gives this formula and two reference points from earlier research. It associates about 45 kcal per kg of fat-free mass per day with energy balance and optimal health. It associates sustained intakes below about 30 kcal/kg FFM with impaired body functions.
It then adds a caveat. Impairments appear “across the continuum of reductions” in energy availability, not at a single threshold. Treat 30 as a research reference point, not a diagnostic line.
What does that look like in numbers?
Take a 58 kg runner at 22% body fat, which gives 45.2 kg of fat-free mass. She trains enough to burn about 800 kcal a day.
| Daily intake | Calculation | Energy availability |
|---|---|---|
| 2,100 kcal | (2,100 - 800) ÷ 45.2 | about 29 kcal/kg FFM |
| 2,600 kcal | (2,600 - 800) ÷ 45.2 | about 40 kcal/kg FFM |
On 2,100 kcal, she sits in the range the research associates with impaired function. Her weight may still hold steady, because the body can lower its resting energy use to match a low intake. The joint position notes that stable weight is not proof of adequate fuelling.
An extra 500 kcal a day moves her to about 40. That is the level at which, in one six-month trial cited by the joint position, female athletes with menstrual dysfunction regained their periods. The average recovery time was 2.6 months.
Take this example as illustration only. Measuring intake, exercise expenditure and fat-free mass outside a lab carries real error, so a home calculation can’t rule REDs in or out.
What is the fix for low energy availability?
In general, the fix is to eat more, especially carbohydrate around training, and sometimes to train less for a while. The details should be set with a clinician. The IOC consensus sets out prevention and treatment principles for clinicians and sports organisations.
In practice, care usually involves:
- More energy, often starting with regular meals and fuel around sessions.
- A look at whether a weight-loss goal is driving the deficit.
- Checks on bone health and hormones.
- Support for disordered eating where it is present.
If you are under 18, or if food feels like a source of anxiety or control, involve a professional early. Self-managing in that situation is not advisable.
Can a nutrition app help?
An app can help with the everyday part: seeing whether intake rises to meet training. A target that ignores a 3-hour ride makes under-fuelling easy. A target sized to the day makes it harder, as covered in eat for the training you did. That support is not screening or treatment.
How Sweatmeal handles this
Sweatmeal sizes your daily calorie and carb target to your training load. Training days keep a carb floor, and the target never drops below your estimated resting metabolic rate, whatever the goal. Sweatmeal is not a medical device and cannot detect or diagnose REDs. If you notice the signs above, see a doctor or sports dietitian. The calculations are on our methodology page.