REDS and Under Fuelling
REDS and
Under Fuelling
You are putting in the hours. Long weekends on trails. Early mornings before school drop off. Strength sessions squeezed into lunch breaks. Underneath that commitment, your body might be quietly running out of fuel, and the signs are easy to miss until they are not.
Relative Energy Deficiency in Sport, or REDS, is what happens when the energy you take in cannot keep up with what your training and your life demand. It is not about being underweight, and it is not about intentionally restricting food. It is about a gap. Sometimes a subtle one. Between what you are burning and what you are bringing in.
For female trail runners, who often combine endurance work with caregiving, full work weeks, and irregular eating windows, that gap is easier to fall into than the culture tends to acknowledge. The good news is that REDS is recognisable, preventable, and recoverable when it is caught early.
What REDS actually is
REDS describes what happens when low energy availability begins to disrupt multiple body systems at once. Energy availability is the energy you have left for your body to run on after training is paid for. When that pool is too small for too long, the body starts to make protective trade offs.
Hormones shift. Bone remodelling slows. Recovery extends. Performance plateaus or regresses. Mood and immunity drop. None of these alone proves REDS, but a cluster of them together is a strong signal that energy intake is not meeting demand.
Trail and ultra training is energy expensive. Long sessions, big elevation, repeat days. Female physiology is also more sensitive to chronic low energy availability than male physiology, particularly through the hypothalamic, oestrogen and bone health pathways.
This is not a weakness. It is biology that asks for a different fuelling approach, not a smaller one.
Signs to watch for
Most women in REDS do not feel obviously unwell. They feel a slow loss of the edges. Less power on climbs. Slower recovery. Mood that dips for no reason. Recognising these patterns early matters more than any single symptom.
Missed, irregular, lighter or absent periods. One of the earliest and clearest signals of low energy availability in women.
Recurrent stress reactions, stress fractures, or vague bone pain. Low oestrogen impairs bone remodeling and raises fracture risk.
Exhausted despite rest. Gains stall or reverse. You are not bouncing back the way you used to.
Increased irritability, anxiety, low mood, and frequent colds or illness.
Slower metabolism, feeling cold often, disrupted thyroid function. The body conserves energy at the expense of performance.
Runs feel harder at the same pace. VO2max can drop even as mileage holds steady. It can feel like losing fitness despite consistent training.
REDS is rarely about one symptom. It is about a cluster. A missing period alongside low mood and a stress reaction is not three problems. It is one signal repeated in three different systems.
A trail fuelling self check
Use this quick audit weekly during big training blocks, monthly otherwise. The goal is to catch patterns before they harden into problems.
Nutrition intake. Am I fuelling within the first 30 to 60 minutes after long sessions. Am I eating regularly across the day, not just at end of day.
Carbohydrate on long runs. Am I taking in 30 to 60g of carbohydrate per hour on runs longer than 90 minutes.
Menstrual cycle. Is my cycle present and consistent. Any change in flow, length or symptoms is worth noting.
Energy and mood. Am I feeling like myself most days, or chronically flat, anxious or irritable.
Sleep and recovery. Am I sleeping well and waking restored. Are sessions feeling progressively heavier than the load justifies.
Bone and body signals. Any nagging bone aches, recurrent niggles, or unusual cold tolerance.
Recovery is a process, not a fix
REDS does not undo itself in a week. The longer the body has been in deficit, the longer it takes to feel like yours again. A general timeline looks like this. Adjust durations based on individual progress and professional guidance.
Reduce training load. Increase total intake. Re establish fuelling around every session. Work with a clinician and a sports dietitian.
Energy availability returns. Sleep, mood and recovery start to improve. Menstrual cycle may begin to normalise.
Training response improves. Strength returns. Bone and hormonal recovery continues.
Performance ceiling lifts. Long term bone density and hormonal balance restore with continued support.
It is common to worry that eating more will lead to unwanted fat gain. In reality, REDS tends to slow metabolism and encourage the body to hold onto fat when you are under fuelled.
When you replenish energy properly, you rebuild muscle, restore hormone function, and often improve body composition over time. Trust the process, and trust your body’s signals.
Frequently asked questions
What is the difference between REDS and just being underweight.
REDS is about energy balance, not body weight. Even runners with a normal weight can develop REDS if intake is too low relative to activity. It is the mismatch between energy in and energy out that triggers the cascade.
How can I tell if my problems are REDS and not something else.
REDS is often a diagnosis of exclusion. The key clue is multiple issues appearing together across menstrual, bone, recovery, mood and performance. A sports physician and dietitian can help confirm.
Do I have to stop training to recover.
Not always. Sometimes training load is reduced rather than removed. The non negotiable is restoring energy availability. Training adjusts around that.
Tamara Madden. Nutrition for endurance athletes.
Tamara Madden is a sports dietitian and specialist in fuelling for endurance sport. For women working through under fuelling, RED-S risk, or rebuilding energy availability, individual support is the safest and most effective path.
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REDS is not a weakness. It is the body asking, clearly and consistently, for more fuel than it has been given. Listening early is one of the strongest things a runner can do.
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