There's a particular kind of tired that runners learn to ignore. Your legs feel like sandbags on runs that used to be easy. Your heart rate climbs for no reason. You finish sessions wrung out, sleep badly, and start them flat again. And the story you tell yourself is the obvious one: you've lost fitness, you're not training hard enough, you're getting older, you're just not tough.
I spent the better part of a year believing exactly that. I thought I'd simply gone backwards, that the answer was to push harder and stop making excuses, until a blood test put a number on it. My ferritin was on the floor. The problem was never effort. It was iron.
Iron deficiency is one of the most common reasons a runner feels persistently flat, and one of the most frequently missed. If you menstruate, you're at higher risk, and worth knowing why.
Why runners who menstruate run low
Iron does a job nothing else can. It's the core of haemoglobin, the molecule in your red blood cells that carries oxygen to your muscles. It sits in myoglobin, which holds oxygen inside the muscle itself. It's woven into the machinery your cells use to turn fuel into energy. Run low on iron and every one of those systems works at a discount.
Running quietly drains it. Every footstrike damages a small number of red blood cells. You lose tiny amounts through sweat and through the gut. And hard training raises a hormone called hepcidin, which has the awkward side effect of reducing how much iron you absorb from food in the hours afterwards, so the harder you train, the harder iron can be to top up.
On top of all of that, a monthly period is a monthly iron loss. None of these things is dramatic on its own. Stacked together, month after month, they are why iron deficiency shows up so often in female runners, and why it's worth taking seriously rather than running through.
What it actually feels like
Low iron rarely announces itself. It feels like ordinary fatigue, which is exactly why it gets blamed on everything else.
The signs to notice: tiredness that's out of proportion to your training; breathlessness on easy efforts that never used to make you puff; heavy, dead legs; a heart rate that sits higher than usual at the same gentle pace; recovery that drags on for days. Away from running you might feel cold, notice brittle nails or more hair than usual in the brush, feel low or short-fused, or struggle to concentrate. One genuinely odd sign worth knowing: a strong craving to chew ice.
You don't need all of these. Two or three that have crept in over a few months are reason enough to look properly.
Why a "normal" blood test can still miss it
This is the part that catches people out. If you've had blood taken and been told it's fine, that may not be the whole story.
A standard test usually checks haemoglobin (whether your blood is carrying enough oxygen right now), but your body protects haemoglobin for as long as it can by draining its stored iron first. That stored iron is measured by ferritin, and it isn't always tested unless someone asks.
So there's a stage (iron deficiency without anaemia) where your ferritin is genuinely low, your stores are empty, and you feel every bit of it, while your haemoglobin still reads as normal. The test says fine. But you feel far from fine. Both things are true. The fix is simply to ask specifically for a ferritin test.
What to do (and what not to do)
If this sounds like you, the single most useful step is a blood test. Book a GP appointment, describe the fatigue plainly, and ask whether they can check your ferritin alongside a full blood count. It's a routine request.
What not to do is start high-dose iron tablets on your own. It's tempting, they're sold over the counter, but it's the wrong move. Too much iron is genuinely harmful, the symptoms of overload can mimic the symptoms of deficiency, and the right dose, form and timing depend on numbers you don't have yet. There's also good evidence that how you take iron matters as much as how much, which is a conversation to have with a GP or sports doctor once you know where you stand.
Eating for iron, without the diet talk
This is not about restriction or "clean eating." It's about a few simple pairings.
Iron from meat, poultry and fish, haem iron. is absorbed more easily by your body. Iron from plants (think lentils, beans, chickpeas, tofu, dark leafy greens, pumpkin seeds, fortified cereals) is absorbed less readily, but you can help it along. Vitamin C consumed alongside it makes a real difference, so a squeeze of lemon, some peppers or a piece of fruit with the meal does useful work. Tea and coffee do the opposite, the tannins blunt absorption, so it's worth keeping them between meals rather than with them. Whatever your way of eating, you can build it in. It's pairing, not punishing.
Give it time to work
One thing to know before you start: fixing low iron is not fast. If a blood test confirms it and you and your GP put a plan in place, your symptoms usually improve before your stores are fully rebuilt, you may feel noticeably better within a few weeks, while ferritin itself can take several months to climb back to a healthy level. That gap matters in two directions. Don't stop early just because you feel human again: the job isn't done until the stores are. And don't expect a transformation overnight and quietly conclude it isn't working, iron is slow, restorative work, not a switch you flip. Your GP will usually want to retest your bloods after a few months, and that follow-up is the test that actually tells you whether you're topped back up.
Tired is information
The reason this matters goes beyond performance. Feeling exhausted, flat and joyless every time you run is not the price of being a runner, and it is not evidence of a weak character.
Tired is information. Sometimes it means rest. Sometimes it means a real, measurable, fixable thing, and you deserve to find out which. Pushing harder through iron deficiency doesn't make you tougher. It just keeps you tired. The braver move is to get the blood test.
This article is general information, not medical advice or a diagnosis. If you recognise yourself in it, please see a GP, fatigue has several possible causes, and a blood test is the way to know.