Running on Empty. Iron, Ferritin and the Fatigue That Does Not Budge.
Trail Notes | Female Physiology
When tired is more than training load
Running on Empty
Iron, Ferritin and the Fatigue That Does Not Budge
You can train consistently, sleep reasonably well, fuel with care, and still feel like you are dragging yourself through every session.
If that pattern has been sitting with you for weeks or months, iron and ferritin deserve a closer look.
Fatigue is one of the most common complaints we hear from female trail runners. It is also one of the easiest to dismiss. Training stress, life load, poor sleep, under-fuelling, overreaching, hormonal changes and low iron can all produce symptoms that look similar.
Without the right screening, it is easy to spend months adjusting training variables while the underlying issue remains unaddressed. This Trail Note is about iron and ferritin specifically: what they are, what low levels can feel like in a trail runner, what to ask your doctor to test for, and how to think about training when your numbers are low.
Low ferritin does not always show up as anaemia.
Your haemoglobin may sit in the normal range while your iron stores are still too low to support the training load you are carrying.
Trail Note · 01
Fatigue in female trail runners is not simple
Tiredness in training is expected. Persistent fatigue that does not respond to rest, sits beyond what your training load explains, or creeps in despite doing the basics well, is a signal worth investigating rather than pushing through.
Iron deficiency is one of the more common nutritional issues seen in female endurance athletes. It can be easy to miss because the early signs often look like normal training fatigue, poor sleep, life stress or under-recovery.
Trail Note · 02
Iron and ferritin are not the same thing
Iron is the mineral. Ferritin is the protein that stores it. When you get a standard blood test, your haemoglobin may be normal while ferritin is low.
This matters because iron-deficiency anaemia is usually a later stage of the problem. Before haemoglobin drops, ferritin can become depleted enough to affect energy, recovery, training tolerance and how hard the same effort feels.
A full blood count is useful, but it is not enough on its own if iron stores are part of the question. You need serum ferritin specifically, interpreted by a clinician in the context of your symptoms, training load, menstrual history and overall health.
Normal haemoglobin does not always mean your iron stores are where they need to be for endurance training.
Trail Note · 03
Why female trail runners can be at higher risk
Iron depletion in female endurance athletes can have several overlapping drivers. Menstrual blood loss is a major one, especially for women with heavy periods. Training can also increase iron demand as the body adapts to endurance load.
Running impact may contribute through foot-strike haemolysis, where red blood cells are damaged through repetitive impact. Hard training can also temporarily influence iron regulation and absorption through inflammation-related pathways.
Dietary patterns matter too. Low overall energy intake, plant-based eating without adequate absorption strategies, heavy training without enough food, or regular under-fuelling around sessions can all reduce the iron available to replenish what is being lost.
Trail Note · 04
The symptoms can look like everything else
One of the most frustrating things about low ferritin is that it can feel like many other training and life issues. The symptoms can overlap with overtraining, under-fuelling, burnout, thyroid dysfunction, low vitamin D and general life fatigue. Without blood testing, there is no reliable way to know what is driving the feeling.
Possible signs of low ferritin
Persistent tiredness that does not improve with rest or extra sleep.
Heart rate higher than expected during controlled sessions.
Perceived effort significantly higher than pace or terrain would usually suggest.
Flatness in sessions despite adequate sleep and fuelling.
Breathlessness on climbs that previously felt manageable.
Difficulty recovering between training days.
Brain fog, low mood, poor concentration or irritability.
Increased susceptibility to illness or slow recovery from minor infections.
Several of these symptoms together, especially without a clear training explanation, are a strong reason to seek blood work before making more changes to your training plan.
Trail Note · 05
What to ask your doctor to screen for
A standard full blood count will show haemoglobin and red blood cell indices, but ferritin is not always included automatically. Ask for it by name.
Ask your GP about
Serum ferritin.
Full blood count including haemoglobin, MCV and MCH.
Iron studies, including transferrin saturation.
Thyroid function if fatigue is significant.
Vitamin D and B12 if clinically relevant.
Timing matters. For the most useful baseline, talk to your doctor about testing when you are well and after at least 48 hours of lighter activity, as recent hard training or illness can influence some markers.
Trail Note · 06
Understanding what the numbers mean
Laboratory reference ranges are designed to flag population-level clinical concern. They do not always tell the full story for an endurance athlete with symptoms and high training load.
A result may be reported as normal while still being low for the demands of your training. This is why context matters: symptoms, cycle history, training load, diet, recovery, bleeding patterns and previous results all help interpret the number.
Within range is not always the same as well-resourced for performance. Ask your clinician to interpret your results in context.
Trail Note · 07
Dietary iron: sources and absorption
There are two main forms of dietary iron. Haem iron, found in animal foods, is generally absorbed more readily. Non-haem iron, found in plant foods such as lentils, chickpeas, tofu, dark leafy greens and fortified cereals, is absorbed less efficiently but can still make an important contribution.
Vitamin C can help support non-haem iron absorption. Tea, coffee and some high-phytate foods can reduce absorption when consumed at the same time as iron-rich meals. A simple strategy is to pair plant iron with vitamin C and give tea or coffee some space from iron-rich meals.
Her Trails coaching cue
Focus on absorption as much as intake. Pair plant iron with vitamin C and give coffee or tea space from iron-rich meals where practical.
Trail Note · 08
Supplementation: get the numbers first
Iron supplementation can be effective when ferritin is depleted, but it should not be started blindly. Excess iron carries risks and does not offer a performance benefit if you do not need it.
Get blood work first, then make a plan with your doctor or sports dietitian. The right approach depends on your ferritin level, haemoglobin, symptoms, diet, menstrual blood loss, gut tolerance and training load.
Trail Note · 09
Training with low ferritin
Training decisions while ferritin is low depend on how low your levels are, whether haemoglobin is affected, and how pronounced your symptoms are. With mildly low ferritin and manageable symptoms, modified training may be appropriate while treatment takes effect. With more significant depletion or heavy symptoms, a genuine reduction in load may be needed.
If you are training through a period of low ferritin, effort should guide your sessions more than pace targets. Your usual outputs may not reflect your current physiology.
Use effort as your anchor. Give your body the conditions to recover, and your training capacity can return.
Trail Note · 10
The coach’s role: flagging, not diagnosing
As coaches at Her Trails, we do not diagnose iron deficiency. What we can do is notice when the pattern of your fatigue does not fit the training explanation, and direct you toward appropriate investigation.
Her Trails coaching cue
If your training is not making sense and rest is not solving it, do not keep adjusting variables. Get the blood work done. The answer you need might be in a number, not a training block.
Trail Note · 11
Iron is information, not weakness
Feeling flat is not a reflection of your commitment or character as an athlete. It may be a physiological reality that a blood test can identify and a clear plan can address.
Getting your ferritin checked when persistent unexplained fatigue arrives is not overreacting. It is part of training intelligently as a female endurance athlete.
Your body is not failing you. It may be running on insufficient resource.
Get the blood work. Know the numbers. Then train and recover with that information in your hands.
Trail Note · 12
The invitation
If you have been sitting with unexplained fatigue and blaming your training plan, your sleep, your stress, your life stage or your body being difficult, consider the possibility that there is a physiological answer worth looking for.
Ask your GP to test serum ferritin specifically. Understand your results in the context of your body, your symptoms and your training load. If the numbers are low, work with your doctor and coach to make a plan that includes restoring iron and protecting training in the interim.
Running on empty is not a badge of honour. Running well-resourced, well-supported and with a clear picture of what your body needs is.
Know your numbers, then train from there
Written by the Her Trails coaching team
Trail Notes are evidence-informed coaching journals written for women who train, race and run on trails.
General education only, not medical or nutrition advice. Persistent fatigue, breathlessness, menstrual changes, low mood, suspected iron deficiency or abnormal blood results should be discussed with a qualified health professional. Do not start iron supplementation without blood testing and medical guidance.
