Iron
Evidence: strong
A genuine and common limiter, especially in female runners. Correct a blood-test-diagnosed deficiency under medical supervision; do not supplement when iron-replete.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
Iron deficiency is one of the few nutritional problems that genuinely and commonly limits distance runners, and one of the few where supplementation has a clear, evidence-based role, but only when there is a real deficiency. Around 15 to 35% of female athletes are iron-deficient, against roughly 5 to 11% of males, with endurance and female athletes most affected (Sim et al. 2019).
Running drives iron losses by several routes. Exercise raises the hormone hepcidin, which suppresses intestinal iron absorption for hours afterward (Sim et al. 2019), and foot-strike haemolysis, sweat and gastrointestinal losses, and menstruation add to the drain.
Iron-deficiency anaemia clearly impairs aerobic performance. The damage starts earlier, though: iron deficiency without anaemia, where haemoglobin is still normal but stores are low, already impairs training adaptation and submaximal endurance, so the problem begins before anaemia appears (Sim et al. 2019). Sports practice flags deficiency below a ferritin of roughly 30 to 35 µg/L, well above the clinical anaemia threshold, though the exact cut-off is a matter of expert consensus rather than firm agreement.
Supplementation reliably raises ferritin, but the performance benefit is concentrated in those who are genuinely deficient, with little gain once stores are adequate (Šmid et al. 2024). An emerging refinement is alternate-day dosing: a single dose raises hepcidin and suppresses absorption of the next dose for a day or two, so alternate-day morning dosing improves fractional absorption over consecutive-day dosing (Stoffel et al. 2017).
The firm rule from the IOC consensus is not to supplement iron-replete athletes: there is no performance benefit when stores are adequate, and real risks of gastrointestinal upset, oxidative stress and overload (Maughan et al. 2018). Correct a deficiency that has been diagnosed by blood test, under medical supervision; do not take iron on spec.
Test, do not guess
Do not start iron on the basis of tiredness alone. Tiredness has many causes, iron-replete athletes get no benefit, and excess iron is harmful. Get a blood test (ferritin plus haemoglobin), and supplement only a diagnosed deficiency under medical guidance. If treating one, alternate-day morning dosing absorbs better than taking it every day, and tablets are best kept away from coffee, tea and calcium, which blunt absorption.