Iron for endurance athletes: what your ferritin number actually tells you
You can train well, eat well, sleep enough and still feel flat for six weeks straight. Low iron does that. It's one of the few nutrition problems in endurance sport that's common, measurable and fixable, and most people racing Hyrox have never had it checked.
Here's what the number on your blood test means, why training itself makes iron harder to absorb, and what to do about it.
Why endurance athletes lose more iron than everyone else
Four things drain iron faster in athletes than in the general population.
Foot-strike haemolysis. Repeated impact through the feet destroys a small number of red blood cells every session. Running volume matters here, and Hyrox has plenty: 8km split across eight runs, most of it on hard indoor flooring.
Sweat and gut losses. Small amounts of iron leave in sweat, and hard training can cause minor bleeding in the gut lining. Neither is dramatic on its own. Over a race season they add up.
Menstrual losses. This is the biggest single reason female athletes turn up with low ferritin far more often than men. Heavy periods can push monthly iron loss well past what a normal diet replaces.
Low intake. Your body absorbs plant iron far less efficiently than iron from meat. If you've cut red meat and upped your training volume in the same year, you've pulled both levers the wrong way.
Hepcidin: why training makes absorption worse
Most athletes have never heard of this bit.
Hard exercise raises interleukin-6, an inflammatory signal. IL-6 raises hepcidin, a hormone that shuts down iron absorption in the gut. Peeling and colleagues have tracked this repeatedly: hepcidin climbs after training and peaks about 3 hours later, blocking absorption in exactly the window when most people take their supplement.
So the athlete training hardest is also the one absorbing least. Volume goes up, absorption goes down, and ferritin drifts.
The practical fix is timing. Take iron first thing in the morning before training, or within about 30 minutes of finishing a session, before hepcidin climbs. Avoid a mid-afternoon dose after a lunchtime session, which lands in the worst possible window.
Read more: Peeling et al. on hepcidin and iron status in athletes (PubMed).
What your ferritin number actually tells you
Ferritin measures stored iron. It drops long before haemoglobin does, which is why you can feel awful with a normal full blood count.
Sim and colleagues set out a three-stage model in a 2019 review that most sports dietitians now work from:
- Stage 1: ferritin below roughly 35µg/L. Stores are depleted and haemoglobin is still normal. Performance can already suffer.
- Stage 2: ferritin below 20µg/L with reduced transferrin saturation. Red blood cell production is now limited.
- Stage 3: iron deficiency anaemia. Haemoglobin has fallen and aerobic capacity falls with it.
Two things to know about the test itself.
Ferritin rises with inflammation. A blood draw the morning after a hard interval session or a race can read falsely high and hide a real deficiency. Test rested, ideally 48 hours clear of anything hard, fasted and in the morning.
A single ferritin reading tells you less without transferrin saturation alongside it. If your GP will run both, take them up on it.
Raising ferritin without guessing
Test before you supplement. This one's non-negotiable. Iron overload is a real risk, and roughly 1 in 200 people of northern European descent carry the genes for haemochromatosis. Supplementing on a hunch is a bad trade.
Sort the food side first. Haem iron from red meat, liver and shellfish absorbs at around 15-35%. Non-haem iron from lentils, spinach, tofu and fortified cereal absorbs at around 2-20%, and that range shifts depending on what else is on the plate.
Pair non-haem iron with vitamin C. Lentils with peppers and tomato, spinach with lemon. Vitamin C converts non-haem iron into a form you absorb more easily, and the effect is big enough to matter.
Move the tea and coffee. Polyphenols in tea and coffee bind iron and cut absorption significantly. Calcium does the same. Leave an hour either side of iron-rich meals, and wash your supplement down with water instead of a milky coffee.
Consider alternate-day dosing. Work by Stoffel and colleagues in Lancet Haematology found that taking iron every other day gave better fractional absorption than daily dosing, because a daily dose keeps hepcidin permanently raised. If you're supplementing under medical guidance, raise this with whoever prescribed it.
What this looks like across a training block
Iron stores rebuild slowly. Expect 8-12 weeks before a retest shows a meaningful change, and longer if you keep training hard through it.
If you're going to test, do it in the off-season or early in a build. Three weeks out from a race, there's nothing useful you can do with the result.
And keep the rest of the picture honest. Low ferritin sometimes explains flat legs. Poor sleep, under-fuelling and a block with no easy days explain it more often. Check iron, then check whether you've actually been recovering. Our guide to recovery supplements for Hyrox athletes covers that second half, and our breakdown of micronutrients for Hyrox athletes puts iron in context with the rest.
The short version
Get a blood test once a year, or twice if you're female or training high volume. Ask for ferritin and transferrin saturation. Test rested. Fix the food and the timing before you reach for a supplement, and only supplement with a number in front of you.
Interval makes clearly dosed supplements for athletes who read the label. Everything we make lists exactly what's in it and how much. See the full range.