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. Iron for endurance athletes is one of the few nutrition problems 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 with every session. Running volume matters here, and Hyrox has plenty of it: 8 kilometres 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. Across a race season they add up.
Menstrual losses. This is the biggest single reason female athletes show up with low ferritin far more often than male athletes. Heavy periods can push monthly iron loss well past what a normal diet replaces.
Low intake. Plant-based iron is absorbed far less efficiently than iron from meat. If you've cut red meat and increased training volume in the same year, you've moved both levers in the wrong direction.
Hepcidin: why training makes absorption worse
This is the part most athletes don't know about.
Hard exercise raises interleukin-6, an inflammatory signal. IL-6 raises hepcidin, a hormone that shuts down iron absorption in the gut. Research from Peeling and colleagues has tracked this repeatedly: hepcidin climbs after training and peaks roughly 3 hours later, blocking absorption during exactly the window when most people take their supplement.
So the athlete training hardest is also the athlete 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 the 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 proposed a three-stage model in a 2019 review that most sports dietitians now work from:
- Stage 1: ferritin below roughly 35 µg/L. Stores are depleted, haemoglobin is still normal. Performance can already be affected.
- Stage 2: ferritin below 20 µg/L with reduced transferrin saturation. Red blood cell production is now limited.
- Stage 3: iron deficiency anaemia. Haemoglobin has fallen and aerobic capacity goes 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 artificially high and hide a real deficiency. Test rested, ideally 48 hours clear of anything hard, fasted, in the morning.
And a single ferritin reading without transferrin saturation is limited. If your GP will run both, take that.
Raising ferritin without guessing
Test before you supplement. This one isn't optional. Iron overload is a real risk, and roughly 1 in 200 people of northern European descent carry the genes for haemochromatosis. Supplementing blind on a hunch is a bad trade.
Get the food side right first. Haem iron from red meat, liver and shellfish absorbs at around 15 to 35 per cent. Non-haem iron from lentils, spinach, tofu and fortified cereal absorbs at around 2 to 20 per cent, and that range moves 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 more absorbable form and the effect is large enough to matter.
Move the tea and coffee. Polyphenols in tea and coffee bind iron and cut absorption significantly. So does calcium. Leave an hour either side of iron-heavy meals, which also means not washing your supplement down with a milky coffee.
Consider alternate-day dosing. Work published by Stoffel and colleagues in Lancet Haematology found that taking iron every other day produced better fractional absorption than daily dosing, because a daily dose keeps hepcidin permanently raised. If you're supplementing under medical guidance, this is worth raising with whoever prescribed it.
What this looks like across a training block
Iron stores rebuild slowly. Expect 8 to 12 weeks before a retest shows a meaningful move, and longer if you're still training hard through it.
If you're going to test, test in the off-season or early in a build, not three weeks out from a race when there's nothing useful you can do with the result.
And keep the rest of the picture honest. Low ferritin explains flat legs sometimes. Poor sleep, under-fuelling and a training 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 the second half of that, and our breakdown of micronutrients for Hyrox athletes puts iron in context with the rest.
The short version
Iron for endurance athletes is worth a blood test once a year, 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 never supplement without a number in front of you.
Interval builds transparently-dosed supplements for athletes who read the label. Everything we make lists exactly what's in it and how much. See the full range.