REDS and Under Fuelling
Trail Note · Health & Physiology
This Trail Note sits alongside your Her Trails program. It draws on current research in women’s sports medicine, energy availability and endurance physiology to help you recognise, prevent and recover from Relative Energy Deficiency in Sport (REDS): one of the most common yet overlooked risks for women who run trails.
REDS and Under Fuelling
What Every Woman on the Trail Should Know
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 (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” or intentionally restricting food. It is about a gap, sometimes a subtle one, between what goes in and what goes out, sustained long enough that your body starts making trade offs you never signed up for.
This Trail Note is your field guide: what REDS is, how to spot it early, and practical steps to stay fuelled, healthy and strong across every phase of your Her Trails program.
What is REDS?
REDS broadened the older concept of the “female athlete triad” and applies to athletes of all levels and genders. At its core, it describes a state of low energy availability (LEA): when the calories left over after exercise are not enough to support the body’s basic functions such as hormone production, bone remodeling, immune defence and mood regulation.
Over time, the body begins to sacrifice what it considers non essential. Reproduction slows or stops. Bone density drops. Recovery stalls. You feel flat, fragile and frustrated, despite doing “everything right” in training.
The simple equation
REDS occurs when energy in < energy out, sustained over weeks or months. The body prioritises survival over adaptation, and performance, hormones and bone health all pay the price.
Why women trail runners are especially at risk
Trail running is uniquely demanding: long hours of moderate to high intensity over variable terrain, often stacked onto already full lives of work, caregiving and daily logistics. Unlike professional athletes, recreational runners rarely have a dietitian scaling intake to match their biggest training weeks.
Low energy availability can develop unintentionally, without any deliberate restriction. A few common patterns:
- A three hour Saturday long run, followed by busy family commitments and a light dinner.
- Skipping pre run fuel on early morning sessions because “it is only an hour.”
- Under eating on rest days because you “did not earn it.”
- Layering “clean eating” plus high volume plus intermittent fasting without realising the cumulative deficit.
None of these feel extreme on their own. But over weeks and months, the gap compounds, and the body starts to speak up.
Signs and consequences of REDS
REDS symptoms span both health and performance. If you notice two or more of the following, especially irregular periods or recurrent injuries, it is worth pausing and investigating.
Warning signs to watch for
| Menstrual changes | Late, erratic or missing periods. A regular cycle is one of the most sensitive markers of balanced energy. |
| Bone injuries | Recurring stress fractures, shin splints or bone pain. Low oestrogen impairs bone remodeling and raises fracture risk. |
| Chronic fatigue | Feeling exhausted despite rest. Gains stall or reverse, and you are not bouncing back the way you used to. |
| Mood and immunity | Increased irritability, anxiety, low mood, and frequent colds or illness. |
| Metabolic shifts | Slower metabolism, feeling cold, and disrupted thyroid function. The body is conserving energy at the expense of performance. |
The performance toll is often the hardest part to recognise. Over weeks, runs feel harder at the same pace, VO₂max can drop even as mileage holds steady, and it may feel like you are losing fitness despite consistent training. This is not a mental block; it is REDS in action. The body is conserving energy instead of adapting to training stress.
Trail Fuelling Self Check
Use this quick audit regularly, weekly during big training blocks and monthly otherwise, to catch early warning signs before they become problems.
☐ Nutrition intake
Are you fuelling workouts, 30 to 60 grams of carbohydrates per hour after 60 minutes, and refuelling after. Track for one week if you are unsure.
☐ Body signals
Is your cycle regular. Cycles longer than about 35 days or skipping three or more months warrant attention. Are you often cold, tired or falling sick.
☐ Training balance
Are you building in at least one to two rest days per week. Are you avoiding random hard efforts with no clear purpose.
☐ Wellness diary
Rate your sleep, mood and hunger daily. Persistent low appetite or constant soreness can signal under fuelling before anything else shows up.
If any of these checks raise a red flag, it is time to act, not to panic. Start by making small adjustments. The steps below are your first line of defence.
Prevention: small shifts, big protection
Gradually increase food. Add 100 to 200 kilocalories per day, an extra snack or a slightly larger meal, until your cycle normalises or recovery improves. Prioritise carbohydrates and quality fats such as whole grains, fruits, nuts and avocado.
Prioritise rest. Schedule at least two recovery days per week. If you feel sluggish, swap a run for cross training, such as swimming, cycling or yoga, or take full rest. Run with intention and caution when your body is asking for a break.
Add strength training. One to two lower body sessions each week, including squats, lunges and hip work, build bone resilience and correct the muscle imbalances that come from trail running. Think of this as bulletproofing your body.
Get professional support. If problems persist, especially amenorrhoea lasting more than six months or multiple stress fractures, see a sports dietitian or sports medicine doctor. Early intervention prevents long term harm.
Managing and recovering from REDS
If REDS is suspected or confirmed, recovery is not about stopping everything. It is about restructuring. Three pillars support this process.
1 Nutrition rehabilitation
Work with a dietitian to restore energy balance. Start by adding extra snacks and modestly larger portions, especially around workouts. Focus on carbohydrate rich foods such as whole grain pasta, rice bowls, oatmeal and energy bars, since trail running heavily taxes glycogen. Do not fear temporary weight change. You are rebuilding lean mass and restoring the metabolic environment your body needs to adapt.
2 Adjust training
Cut back endurance volume for a period. Replace one long run per week with rest or cross training. Keep some intensity, such as short hill sprints or fartlek sessions, to maintain fitness without endless mileage. Increase strength training to two or three sessions per week, using weight bearing and plyometric exercises to rebuild muscle and bone.
3 Education and monitoring
Share what you are learning with your coach, partner or training group. Track key markers, including your menstrual diary, weight trends, sleep quality and mood. Schedule regular check ins with a health professional to make sure you are moving back toward balance, not just patching symptoms.
Example recovery timeline, 12 weeks
| Phase | Weeks | Focus |
|---|---|---|
| Stabilise | 1 to 4 | Increase nutrition and rest. Reduce weekly mileage by about 30 per cent. Add two days of strength training. Prioritise sleep. |
| Rebuild | 5 to 8 | Gradually reintroduce longer runs, increasing by around 10 per cent per week. Maintain high quality fuelling and regular sleep and recovery habits. |
| Maintain | 9 to 12 | Return to a balanced training plan. Monitor your cycle and performance. Continue ongoing nutrition support if available. |
The timeline is illustrative. Adjust durations based on individual progress and professional guidance.
A note on eating more
It is common to worry that eating more will lead to unwanted fat gain. In reality, REDS tends to slow your metabolism and encourage your 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 their intake is too low relative to their activity. It is the mismatch between calories and exercise that triggers the cascade of effects.
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: missed periods, fatigue, stress fractures and a training plateau despite adequate sleep. A sports medicine specialist can also test for other causes such as thyroid issues or vitamin deficiencies.
Do I need to stop running if I think I have REDS.
You do not usually need to stop completely, but you will need to restructure. Lower your mileage and intensity until energy balance is restored. Focus on low impact cross training and strength work, and return to full training gradually under professional guidance.
Is REDS reversible.
Yes. With proper nutrition and adjusted training, most athletes regain normal cycles, rebuild bone health and return to full performance. The earlier you catch it, the faster the recovery.
Can men get REDS.
Yes. REDS affects male athletes as well, often as loss of libido, low testosterone and similar performance declines. Women’s menstrual cycles simply provide an earlier and more visible warning signal.
What are the best foods for REDS recovery.
Focus on whole foods rich in complex carbohydrates, including oats, quinoa, sweet potatoes and rice, and quality protein, including legumes, dairy, lean meats and eggs. Calcium and Vitamin D can support bone health, but getting enough total energy is the most important step. A sports dietitian can tailor specific choices to your needs.
When should I see a professional.
If your menstrual cycle stops for more than three months, if you have had multiple stress fractures, or if fatigue is unrelenting despite rest and better fuelling, consult a sports medicine doctor or registered dietitian. Early intervention protects bones, hormones and long term health.
Resources and next steps
✓ Download: Trail Fuelling Self Check and Recovery Checklist, PDF. A concise worksheet to track your fuelling, workouts, sleep and cycle, with action items to close any nutrient gaps.
✓ Sign up: Fuel Strong Newsletter: weekly tips on women’s sports nutrition, recipes and healthy fuelling habits delivered to your inbox.
✓ Join: Her Trails Coaching Waitlist: personalised coaching for women who want to train hard and stay healthy. Limited spots available.
✓ Read more: Shaping Your Ultra Weekends at Every Age: the companion Trail Note covering age specific training patterns, hormonal guidance and the REDS fuelling callout for every life stage.
Under fuelling is one of the most fixable problems in women’s endurance sport, but only once you see it. The trail will always be there. Your job is to arrive at the start line with a body that has been supported, not just pushed. Fuel the engine, listen to the signals, and trust that a well fed athlete is a fast, resilient and long lasting one.
Further reading: American Academy of Family Physicians, Relative Energy Deficiency in Sport and low energy availability in athletes. Freetrail, REDS overview for trail and ultra runners.
