Up to 35% of female runners are iron deficient. Most have no idea. The symptoms — fatigue, slow recovery, a heart rate that spikes for no reason — look exactly like overtraining, underperformance, or just being tired. Doctors miss it. Athletes run through it for months.
If your easy runs feel harder than they should, and your training isn't responding the way it used to, iron is worth investigating before you assume it's fitness.
Why female runners lose iron faster
Three things work against you simultaneously.
Sweat. You lose small amounts of iron in sweat. For anyone running significant weekly volume, this adds up.
Foot-strike haemolysis. The repeated impact of running literally destroys red blood cells in your feet. This is called haemolysis, and it releases iron that your kidneys then excrete. The harder and faster you run, the more you lose.
Menstruation. Most women lose between 10–35mg of iron per menstrual cycle. Add that to sweat loss and haemolysis and the deficit compounds quickly, especially if diet doesn't compensate.
Postpartum runners face an additional challenge. Blood loss during birth — particularly C-sections — depletes iron stores significantly. Many women return to running while still recovering their baseline, without realising their iron hasn't caught up.
What iron deficiency actually looks like
The textbook symptom is fatigue. But runners are already tired. Here's what makes iron deficiency specifically recognisable:
- Your easy runs feel hard. Zone 2 effort registers as Zone 3 or 4. Heart rate at the same pace is higher than it used to be.
- You're recovering slowly. Legs that should feel fresh 48 hours after a run still feel heavy.
- Your performance has plateaued or gone backwards despite consistent training.
- You feel short of breath on efforts that didn't used to wind you.
- You're cold more than usual, particularly in your hands and feet.
- Persistent low-grade headaches, particularly after runs.
- Your motivation to train has dropped — not mentally, but physically. You want to run but your body resists.
None of these symptoms are diagnostic on their own. But if three or more of them resonate and you've never had your iron tested, that's enough reason to book a GP appointment.
The tests to ask for — and why a standard blood test misses it
This is important. A standard full blood count shows whether you're anaemic — meaning your haemoglobin is low. But iron deficiency often shows up long before you become anaemic. By the time your haemoglobin drops, you've been running on depleted stores for months.
Ask your GP for these specific tests:
- Serum ferritin. This measures your iron stores. For athletes, optimal is 50–100 ng/mL. Many labs use a normal reference range starting at 15 ng/mL — which is enough to not be anaemic, but not enough to perform. A result of 18 ng/mL will get a 'normal' flag from the lab. For a runner, it isn't.
- Serum iron and transferrin saturation. This gives a fuller picture of how much iron is actively in circulation versus what's stored.
- Full blood count. Checks for anaemia and MCV — if MCV is low, iron-deficiency anaemia has already set in.
If your ferritin is below 30–40 ng/mL and you're experiencing the symptoms above, most sports medicine doctors would recommend supplementation even without full anaemia.
What to do if your levels are low
Talk to your GP before starting iron supplementation — too much iron is harmful, and dosing depends on how deficient you are. That said, here's what the evidence shows works:
Ferrous sulphate is the most common prescribed form. It's effective but causes GI issues in some people. Taking it with orange juice (vitamin C improves absorption) and away from coffee or tea (tannins block absorption) makes a meaningful difference.
Ferrous bisglycinate is a gentler alternative with better bioavailability and lower GI upset — worth asking about if ferrous sulphate is causing problems.
Dietary sources alone are rarely enough if ferritin is already low, but they matter for maintenance. Red meat is the most bioavailable source. For non-meat eaters: lentils, tofu, spinach, and fortified cereals are good sources, but pair them with vitamin C to maximise absorption.
Expect 2–3 months of supplementation before levels normalise and you start feeling the difference in training. It's slow. Don't stop early because you feel better.
When to revisit
Test ferritin every 6–12 months if you're a high-volume runner, postpartum and back to training, or have had low levels before. Iron isn't a one-time fix — ongoing monitoring is part of the picture.
If symptoms persist despite normal ferritin, look at B12, vitamin D, and thyroid function. They overlap in symptoms and are often worth checking at the same time.