If your running has gone flat for no obvious reason — heavy legs, breathless at paces that used to be easy, tired all day — iron deficiency is one of the few genuinely common, genuinely treatable explanations. Estimates of how common it is in female athletes range from about 15 to 35% up to 60% depending on the threshold used and the population studied, and you can be deficient without being anaemic, which is why a normal haemoglobin does not rule it out.
Why are runners prone to iron deficiency?
The two that matter most are menstrual losses and simply not eating enough, particularly on a vegetarian diet where the iron is largely non-haem and less well absorbed — which describes a great many Indian runners. Layered on top, running adds exercise-induced hepcidin release that blunts absorption for hours after a hard session, small amounts of gastrointestinal blood loss — which is roughly doubled by taking anti-inflammatories before a long run, the one part of this you directly control — foot-strike haemolysis, and a little iron in sweat. That last one gets quoted a lot and is the smallest of them: roughly 1 to 2 mg over a couple of hours of exercise, a few percent of your daily requirement.
The number that matters is ferritin, not haemoglobin
Haemoglobin is the last thing to fall. You can have empty iron stores, feel terrible, and still have a haemoglobin the lab calls normal. The store measure is serum ferritin:
- Below 15 µg/L — stores essentially empty
- 15 to 30 µg/L — low stores
- 30 µg/L is a commonly used cut-off in athletes — but there is little consensus, and other frameworks use 35 or even 40 µg/L, so do not treat a result just above 30 as automatically fine if you feel awful
One important trap: ferritin is an acute-phase reactant, so infection or inflammation pushes it up and can mask a deficiency. If you are unwell when tested, the number may not mean what it appears to mean. Ask for CRP alongside it.
Does iron supplementation actually improve performance?
Here is where to be careful, because this is a topic where enthusiasm outruns the data.
Iron deficiency itself is associated with endurance performance decrements of roughly 3 to 4%. In reviews of treatment, performance improved by a reported 2 to 20% when deficient athletes were supplemented — but read that range carefully, because it covers both 100 mg/day of oral elemental iron for up to 56 days and iron given by injection over 8 to 10 days. It is not a range you should expect from tablets alone.
But a meta-analysis that split the protocols found that iron supplementation did not significantly improve performance when time-trial and time-to-exhaustion tests were analysed separately. Only when both were pooled did a small but significant improvement appear, and it was driven particularly by studies including women who were actually anaemic — a subgroup of just 46 people out of 380, so hold it loosely. Iron status — ferritin, haemoglobin — reliably improves. Performance in non-anaemic deficiency is a weaker and less consistent finding than it is usually presented as.
There is a serious finding on the other side, and leaving it out would be the same selective reading we are warning about: pooling 17 studies in iron-deficient non-anaemic endurance athletes, iron supplementation improved VO2 max with a moderate effect size (g = 0.610, 95% CI 0.399 to 0.821, p < 0.001). Note also that the two null results above sat at p = 0.067 and p = 0.059 — which is as much a story about small samples as about no effect.
So: worth testing, worth correcting, and treat the performance question as genuinely unsettled rather than as a promise in either direction.
Practical
Do not supplement iron without a test. This is not a harmless vitamin. Iron overload is a real condition, and untargeted supplementation is a bad idea. Get ferritin, haemoglobin and CRP done, and take the result to a doctor.
Absorption is easy to get wrong. Tea and coffee are the big ones, and the effect is large — a cup of tea with a meal has been measured cutting non-haem iron absorption by 60% or more, so taking iron with your morning chai is close to the worst option available. For coffee the timing is asymmetric in a useful way: an hour before a meal is fine, while an hour after is about as bad as drinking it with the meal. Tea appears to behave differently, with some evidence that leaving an hour afterwards does reduce the effect — so the safe rule is simply to keep both away from your iron. Calcium and dairy are often listed alongside, but honestly the effect there is smaller and the longer-term studies have not shown it changes iron status — not a reason to cut dairy. Vitamin C is the standard advice and it does improve absorption in isotope studies, but be aware that a randomised trial of 440 people with iron-deficiency anaemia found adding vitamin C made no difference to haemoglobin, ferritin or transferrin saturation over eight weeks. Do not count on it doing the work.
Alternate-day dosing is worth asking your doctor about — with one crucial detail. It is clearly better tolerated, with substantially fewer gut side effects, and each dose absorbs better. But to deliver the same total iron you have to take roughly twice the daily dose on the days you take it — simply skipping every other day halves your weekly iron without you noticing. Be aware the head-to-head evidence is not settled: the absorption studies favour alternate-day, while at least one randomised trial in iron-deficient young women found daily dosing produced the greater haematological improvement. Treat it as a tolerability option to discuss with your doctor, not an upgrade.
Time it around your running, not just around breakfast. Hepcidin peaks roughly 3 to 6 hours after hard exercise and blocks absorption. So the windows are either within about 30 minutes of finishing, before the surge, or six hours or more afterwards. Note this matters especially here: we tell you elsewhere to run early in the Gurugram heat, and for an early runner “take it in the morning” lands squarely in the worst part of the day.
Food first if you are only mildly low. Rajma, chana, dal, ragi, green leafy vegetables, dates and jaggery all contribute; pair them with lemon or amla. Meat and eggs contribute haem iron, which is absorbed several times more readily.
Do not self-diagnose this
Fatigue has a long list of causes, several of which matter more than iron — thyroid disease, B12 deficiency, sleep apnoea, depression, and simply training too much. If you feel persistently exhausted, see a doctor and get investigated properly rather than working through a supplement aisle.
References
Pengelly M, Pumpa K, Pyne DB, Etxebarria N. Iron deficiency, supplementation, and sports performance in female athletes: a systematic review. J Sport Health Sci, 2025;14:101009. · Nakamoto FP, et al. Effect of iron supplementation on exercise performance of women with non-anaemic iron deficiency or iron deficiency anaemia: a systematic review and meta-analysis. J Trace Elem Miner, 2025. · Li N, et al. The efficacy and safety of vitamin C for iron supplementation in adult patients with iron deficiency anemia: a randomized clinical trial. JAMA Netw Open, 2020;3(11):e2023644. · Stoffel NU, et al. Iron absorption from oral iron supplements given on consecutive versus alternate days. Lancet Haematol, 2017;4(11):e524–33. · Burden RJ, et al. Is iron treatment beneficial in iron-deficient but non-anaemic (IDNA) endurance athletes? A systematic review and meta-analysis. Br J Sports Med, 2015;49(21):1389–97. · Clénin G, Cordes M, Huber A, et al. Iron deficiency in sports — definition, influence on performance and therapy. Swiss Med Wkly, 2015.