Iron deficiency in runners: signs and how to fix it

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You’ve been running consistently for months. Your plan is solid, your shoes are decent, you’re putting the sessions in — and yet something is off. Your easy runs feel hard. Your legs feel heavy before you’ve even warmed up. You’re finishing runs that should feel routine completely wiped out, and your pace has quietly drifted backwards over the last few weeks.

Before you blame overtraining, poor sleep, or just a bad patch, there’s something worth ruling out: iron deficiency. It’s one of the most common and most commonly missed reasons runners hit a wall that has nothing to do with their training.

This isn’t a niche problem for elite athletes eating 800 calories a day. Iron deficiency affects everyday runners — especially women, those training on plant-based diets, and anyone who’s ramped up their mileage in the last couple of months. The frustrating part is that it can look and feel like a dozen other things. Here’s how to recognise it, confirm it, and sort it out.

Why runners are particularly at risk

Iron plays a central role in making haemoglobin — the protein in red blood cells that carries oxygen to your working muscles. When iron is low, your blood becomes less efficient at delivering oxygen. For a runner, that means your aerobic capacity drops, your perceived effort goes up, and performance suffers even if your fitness hasn’t changed.

Runners face a specific set of risk factors that the general population doesn’t:

• Foot-strike haemolysis: the repeated impact of running literally breaks apart red blood cells in your feet. The harder or longer you run, the worse this gets.
• Increased iron loss through sweat: not a massive amount per session, but it adds up over weeks of training.
• GI blood loss: hard running can cause minor gastrointestinal bleeding, particularly in longer efforts.
• Increased demand: building fitness and blood volume means your body needs more iron, not less.

Add to this the fact that many runners are also not eating enough — either because they’re watching their weight, because they’re too busy to cook properly, or because they’ve shifted to a diet lower in red meat — and you have a recipe for depletion.

The signs that should make you suspicious

The tricky thing about iron deficiency is that the symptoms are vague enough to explain away. You might chalk it up to a tough week at work, poor sleep, or just a bad run. Here’s what to look for:

• Persistent fatigue that doesn’t improve with rest days
• Breathlessness on efforts that used to feel easy — running at 6:00/km and feeling like you’re pushing hard
• Elevated heart rate at paces that were previously comfortable
• Noticeably slower times despite consistent training
• Headaches, particularly after runs
• Pale inner eyelids or skin (pull down your lower eyelid — it should be dark pink/red, not pale)
• Difficulty concentrating or brain fog
• Restless legs at night — underreported but surprisingly common with iron deficiency
• Increased susceptibility to illness — your immune function takes a hit too

None of these in isolation means you’re iron deficient. But if you’re ticking three or four of them and your running has tanked, it warrants a blood test.

Getting tested: what to actually ask for

A standard GP blood test will often just check your haemoglobin level. That’s useful — but it can look normal even when you’re iron depleted. By the time haemoglobin drops, you’ve been running on empty for a while.

What you actually want checked:

Ask your GP specifically for serum ferritin. The NHS considers anything above 12–15 µg/L technically “normal,” but sports medicine research suggests runners may need ferritin above 40–50 µg/L to perform optimally. If your result comes back at 18 µg/L and you’re told it’s fine, push back — or at least understand that for a runner in training, that’s on the low end.

How to fix iron deficiency through diet first

If your ferritin is borderline low or you just want to protect your stores, diet is the first port of call. Iron comes in two forms:

Haem iron (from animal sources) — absorbed at roughly 15–35% efficiency:
– Red meat (beef, lamb) — best source
– Liver and organ meats — extremely high, but limit to once a week
– Dark poultry meat
– Oily fish

Non-haem iron (from plant sources) — absorbed at only 2–10% efficiency:
– Lentils, chickpeas, kidney beans
– Tofu and tempeh
– Dark leafy greens (spinach, kale)
– Fortified cereals
– Pumpkin seeds, cashews

If you’re vegetarian or vegan, you need roughly 1.8x the iron intake of a meat-eater to compensate for the difference in absorption. That’s not impossible, but it does require some deliberate meal planning.

The most important dietary trick: pair non-haem iron sources with vitamin C. A glass of orange juice with your lentil soup, a squeeze of lemon on your spinach — vitamin C can increase non-haem iron absorption by up to 3–6 times. Conversely, avoid tea, coffee and calcium-rich foods (like dairy) for at least an hour either side of iron-rich meals — they block absorption significantly.

If you’re building toward a race and want to make sure your nutrition is working as hard as your training, the running nutrition plan for marathon training covers iron alongside carbohydrates and protein in the context of a full training diet.

When you need supplements (and which ones to take)

If your ferritin is confirmed low by a blood test, diet alone won’t rebuild stores fast enough. Supplementation is usually necessary — but the type and timing matter.

What to take:
– Ferrous sulfate (200mg tablet = 65mg elemental iron) — the most common prescribed form, effective but can cause GI side effects (constipation, nausea)
– Ferrous gluconate or ferrous fumarate — slightly gentler on the gut, slightly less elemental iron per tablet
– Iron bisglycinate — the form most commonly found in sports/health stores; better absorbed, far fewer side effects, but typically more expensive

How to take it:
– On an empty stomach, ideally first thing in the morning — absorption is significantly better than with meals
– With a glass of orange juice or vitamin C supplement
– Not with coffee, tea, dairy, or antacids
– Every other day may actually be as effective as daily dosing for some people, with better tolerance — research published in the Lancet Haematology supports alternate-day iron absorption

Timeline: don’t expect overnight results. It typically takes 4–8 weeks of supplementation to feel meaningfully better, and 3–6 months to fully replenish ferritin stores. Retest at 8–12 weeks to track progress.

If your GP prescribes iron and you find the GI side effects hard to manage, mention it — switching to ferrous gluconate or bisglycinate is a reasonable ask.

How iron deficiency affects your training — and what to do about it

Here’s the honest reality: if your ferritin is significantly depleted, your easy runs will feel hard, your interval sessions will feel almost impossible, and pushing through won’t fix it. Iron deficiency isn’t a fitness problem you can train your way out of.

The practical adjustments while you’re treating it:

• Reduce intensity, not volume — keep running, but dial back your hard efforts. Grinding through threshold sessions on depleted iron stores is counterproductive and demoralising.
• Use heart rate rather than pace — if your usual easy effort is 140bpm, run to that number rather than chasing 5:45/km. Your pace will be slower, and that’s okay.
• Don’t panic about lost fitness — the fitness is still largely there; the oxygen delivery isn’t. As iron rebounds, pace will return faster than you expect.
• Take rest days seriously — your body is working harder than usual at every level of effort while iron-deficient. Overtraining on top of deficiency is a fast route to illness.

If you’re unsure whether what you’re experiencing is iron-related fatigue or something else, it’s also worth reading about how to avoid overtraining while marathon training — the symptoms overlap significantly and one can compound the other.

Who’s at highest risk and how to stay on top of it

Some runners should probably test their ferritin every 6–12 months as standard, not just when symptoms appear:

• Women of menstruating age — monthly blood loss significantly increases iron requirements
• Runners covering more than 50km per week — foot-strike haemolysis becomes a real factor at higher mileage
• Plant-based runners — reduced dietary availability means stores deplete faster under training load
• Runners who have recently significantly increased mileage — the body’s demand outpaces intake during rapid build phases
• Anyone who has had iron deficiency before — it tends to recur if the underlying cause isn’t addressed

Routine testing isn’t always easy to get from a GP without symptoms, but private blood tests (ferritin-specific panels are widely available for £25–£50) are a practical option if you want to stay ahead of it.

The honest takeaway

• The key test is serum ferritin, not just haemoglobin. Ask for it specifically. A result technically in the “normal” range (below 40 µg/L) may still be too low for a runner in training.
• Diet can maintain iron, but if stores are already depleted, you’ll need supplements. Take iron on an empty stomach with vitamin C. Avoid tea and coffee immediately before or after.
• It takes time — usually 4–8 weeks to feel better, 3–6 months to fully restore stores. Adjust your training expectations while you’re treating it.
• During deficiency, train by effort or heart rate, not pace. The pace will come back once iron does.
• Certain runners should test proactively every 6–12 months rather than waiting for performance to fall off a cliff — especially women, high-mileage runners, and those eating plant-based diets.

Next read: Signs you are overtraining as a runner (and what to do)