Quick answer: Ferritin is the blood marker for your stored iron, and it can fall low enough to wreck your running long before a standard anemia test looks abnormal. Female runners lose iron through menstrual bleeding, sweat, and hard training, and heavy perimenopausal periods make it worse. If easy runs feel disproportionately hard, ask your doctor for ferritin plus a full blood count before you self-prescribe iron.
There is a specific kind of tiredness that runners learn to distrust. Not the honest fatigue of a hard week, but the sort where easy pace feels like tempo, your heart rate is oddly high for the effort, hills you have run for years leave you gasping, and a rest day changes nothing. You assume you are unfit, or stressed, or getting older.
Often you are just low on iron.
Iron deficiency is one of the most common and most missed causes of a female runner losing her legs, and it gets more likely, not less, as you move through your forties. This guide covers what ferritin actually measures, why running and perimenopausal bleeding drain it, what to ask your doctor for, and how to fix it without making yourself miserable. This is general information, not medical advice. Iron problems should be diagnosed and treated with a clinician.
What is ferritin, and why do runners care?
Ferritin is the protein that stores iron, and the blood test for it estimates how much iron you have banked. Iron’s day job is carrying oxygen to working muscle, so when stores fall, endurance is the first thing to go. Your heart works harder to deliver the same oxygen, which is why the earliest sign is usually a higher heart rate at your normal easy pace, not an obvious collapse in performance.
The important part: ferritin falls before hemoglobin does. Your body protects circulating red blood cells by raiding the stores first. So there is a long window, often months, where your stores are running on empty, you feel progressively worse, and a standard anemia check still looks perfectly normal.
That window has a name: iron deficiency without anemia. It is the version that catches runners out, because it is invisible to the test most people are given.
Why do female runners lose so much iron?
Because several drains stack on top of each other, and only one of them is obvious.
- Menstrual blood loss. The single biggest factor for most women who are still cycling, and the reason iron deficiency is so much more common in women than men.
- Hard training. Sweat carries small amounts of iron, endurance exercise causes some gastrointestinal losses, and repeated foot strike damages a small number of red blood cells. Individually trivial, cumulatively not.
- The hepcidin problem. Hard exercise triggers a rise in hepcidin, a hormone that temporarily blocks iron absorption for several hours afterwards. Training hard both spends iron and briefly makes it harder to absorb more, which is a genuinely unhelpful combination.
- Eating less meat. Iron from animal sources is absorbed several times more efficiently than iron from plants. Women who eat little or no red meat start from a lower baseline, and vegetarian and vegan runners need to be particularly deliberate about it.
Does perimenopause make iron deficiency worse?
Yes, and this is the part almost nobody joins up. Perimenopausal cycles often become heavier, longer, closer together, or wildly unpredictable, and heavy menstrual bleeding is one of the largest iron drains there is. The result is that a woman can be losing considerably more iron at 44 than she did at 34 while eating exactly the same way and training exactly the same amount.
So when running starts feeling harder in your forties, iron belongs on the list alongside the hormonal explanations, not underneath them. If your periods have got heavier and your training has got harder, treat those two facts as connected until a blood test tells you otherwise.
Worth flagging clearly: ACOG treats genuinely heavy or abnormal uterine bleeding as something to be assessed rather than endured, and the NHS says the same. “Heavy periods are just perimenopause” is half true and a poor reason to skip a conversation. Soaking through protection hourly, flooding, clots larger than a coin, bleeding for more than a week, or bleeding between periods all deserve a proper look. See irregular periods in perimenopause for what is normal in this transition and what is not.
What does low iron feel like?
Two clusters, and it is usually the combination that gives it away.
In your running:
- Fatigue that does not match your training load
- Heart rate 5 to 10 beats higher than usual at your normal easy pace
- Breathlessness on efforts that never used to trouble you
- Legs that feel heavy from the first mile rather than the last
- Recovery that takes days instead of hours
- A slow, unexplained slide in performance across weeks
Away from running:
- Cold hands and feet
- Hair shedding more than usual
- Restless legs at night
- Poor concentration and low mood
- Frequent illness
- Occasionally, a strange craving to chew ice
Read that list and you will notice the problem: it overlaps almost perfectly with perimenopause fatigue, with overtraining, and with plain underfueling. That is precisely the argument for testing rather than guessing. Four different causes with the same symptoms need four different fixes, and iron is the cheapest one to rule in or out.
Which blood tests should I ask for?
Ask for these by name. “Can you check my bloods” often returns a full blood count alone, which is exactly the test that misses this.
| Test | What it tells you | Why ask for it |
|---|---|---|
| Ferritin | Your stored iron | The one that matters, and the one most often left off |
| Full blood count | Hemoglobin, red cell size | Shows whether deficiency has progressed to anemia |
| CRP or another inflammation marker | Background inflammation | Ferritin rises with inflammation and can look falsely reassuring without this |
| Transferrin saturation | Iron actively in circulation | Adds context when ferritin is borderline |
The CRP row is the one runners should care about most. Ferritin is an acute-phase reactant, which means it goes up during infection, injury, and inflammation. A ferritin drawn while you are fighting a cold, or a couple of days after a race, can read normal while your actual stores are depleted. Test when you are well and reasonably rested, ideally not straight after a hard block.
On numbers: labs commonly flag deficiency somewhere below 15 to 30 micrograms per litre, but many sports physicians work with a higher floor for endurance athletes because symptoms and performance often improve when stores are pushed above the strict laboratory cutoff. This is a genuine grey zone and a conversation to have with your clinician rather than a number to self-diagnose against. What is not grey: a result at the bottom of the range in a runner with textbook symptoms deserves discussion, not a shrug and a “technically normal”.
How do I actually raise my iron?
First, with a clinician, and with the cause addressed. Supplementing while heavy bleeding continues unexamined is bailing a boat without looking for the hole.
If you are supplementing:
- Alternate days often beats daily. Because a dose raises hepcidin for hours afterwards, taking iron every other day can absorb as well as or better than daily dosing, with fewer side effects. Discuss the protocol with whoever is prescribing it.
- Vitamin C helps. Take it with orange juice or a vitamin C tablet.
- Tea, coffee, and calcium hinder. Separate them from your dose by an hour or two. The morning cup of tea taken with an iron tablet is a common own goal.
- Avoid the hours right after hard training, when hepcidin is elevated.
- Expect it to be slow. Symptoms may ease in a few weeks, but refilling stores typically takes three to six months. Retest rather than assuming.
- Side effects are manageable. Constipation and stomach upset are common; different formulations and alternate-day dosing usually help. Tell your clinician instead of quietly stopping.
Through food, iron comes in two forms, and they are not equivalent:
| Form | Found in | Absorption |
|---|---|---|
| Heme | Red meat, liver, oysters, mussels, sardines | Efficient, and largely unaffected by the rest of the meal |
| Non-heme | Lentils, beans, tofu, fortified cereal, dark greens, seeds | Much less efficient, and easily blocked by tea, coffee, and calcium |
Food alone will maintain healthy stores and struggle to refill genuinely depleted ones. Use it to hold the gains, not as the rescue plan.
And do not supplement blind. Your body has no efficient route to excrete excess iron, some people carry a genetic condition that causes it to accumulate, and unnecessary supplements mostly buy you constipation. Test, then treat.
What this looks like inside your training
Iron is not a training variable you can feel your way through, which is what makes it dangerous. The natural response to feeling flat is to assume you are unfit and train harder, which raises hepcidin, spends more iron, and digs the hole deeper. The runners who suffer longest with this are usually the conscientious ones.
The habit worth building is noticing the shape of it: not one bad run, but weeks where effort and output have quietly decoupled, especially if your periods have changed. That is a pattern, and patterns are hard to see from inside a single week. It is exactly what Phaes is built to surface: a short daily check-in plus your cycle, so a slow slide shows up as a trend rather than a vague sense that you have lost your fitness. Phaes also tracks bloodwork, so you can put your ferritin results next to how your training actually felt over the same months and see whether the two moved together.
Being able to say “here are six months of my check-ins, here is when my periods got heavier, here is my ferritin” is also a much better start to a doctor’s appointment than “I have been feeling a bit tired”.
The bottom line
Low iron is common in female runners, more common again when perimenopausal periods get heavy, and routinely missed because ferritin falls long before a standard anemia test notices. If easy running has quietly turned hard and your heart rate is running high for the effort, ask specifically for ferritin, a full blood count, and an inflammation marker, and get them drawn when you are well.
Then treat it properly: fix the cause, supplement under guidance rather than on a hunch, give it months rather than weeks, and retest. Of all the reasons a woman’s running gets harder in her forties, this is one of the few with a blood test and a fix. It is worth ruling out before you conclude the problem is you.
Frequently asked questions
What is ferritin and why does it matter for runners?
Ferritin is the protein that stores iron, and the blood test for it estimates how much iron you have banked. Iron carries oxygen to working muscle, so when stores run down, endurance suffers first: your heart rate sits higher at an easy pace, your legs feel heavy, and recovery drags. Ferritin falls before hemoglobin does, which is why runners can feel awful while a basic anemia check still reads normal.
Can you have low iron without being anemic?
Yes, and this is the version that catches most runners out. Iron deficiency without anemia means your stores are depleted while your hemoglobin is still holding in the normal range. You can feel flat, breathless, and slow to recover with a completely unremarkable hemoglobin result. This is exactly why asking for ferritin specifically, rather than accepting "your bloods were fine", matters.
What are the symptoms of low iron in women runners?
The usual pattern is fatigue that does not match your training load, a higher heart rate at your normal easy pace, breathlessness on hills that never used to bother you, and recovery that takes days instead of hours. Non-running signs include cold hands and feet, hair shedding, restless legs at night, low mood, and occasionally a strange craving to chew ice. None of these are proof on their own, which is why you test.
Does perimenopause cause low iron?
It can, and it is badly under-recognized. Perimenopausal cycles often become heavier, longer, or unpredictable, and heavy menstrual bleeding is one of the biggest iron drains there is. A woman can be losing far more iron at 44 than she did at 34 while eating exactly the same way. If your periods have got heavier and your running has got harder, treat those two facts as related until a blood test says otherwise.
Should I just take an iron supplement?
No, test first. Iron is not a harmless top-up: your body has no efficient way to excrete excess, some people carry a genetic condition that causes iron overload, and supplements commonly cause constipation and stomach upset. Supplementing blind also hides the real question, which is why you are losing iron. Get ferritin measured, then supplement with a clinician's guidance and a plan to retest.
How long does it take to fix low ferritin?
Longer than most people expect. Symptoms often ease within a few weeks of consistent supplementation, but genuinely rebuilding stores usually takes three to six months, and it takes longer if the original cause, such as heavy bleeding, has not been addressed. Retesting is the only way to know you have actually refilled the tank rather than just felt slightly better for a while.