Iron losses in runners
The best-known explanation is foot-strike haemolysis: red blood cells broken by the repeated impact of the foot on the ground. The evidence for it is real but small.
A 2024 scoping review pooled nine studies of 267 marathon and ultramarathon runners, 88 per cent of them men. After a race, reticulocytes (young red cells) rose by 16 per cent, though they stayed within normal limits, and haptoglobin, which falls when red cells break down in the blood, dropped by 21 per cent. Haemoglobin and haematocrit did not change notably. The authors read this as transient haemolysis.
Foot strike is also not the whole story. An earlier study of male athletes noted that this kind of red cell breakdown also happens in swimmers and cyclists, and found that runners' red cells were no younger on average than those of other athletes or untrained men, so mechanisms other than impact have to be considered.
Whatever the route, an iron deficiency in a runner still needs its cause looked for. The 2021 Lancet review of iron deficiency advises that an underlying cause should be sought in everyone who presents with it, and that screening for coeliac disease should be considered routinely. The same review lists heavy menstrual bleeding among the causes of iron deficiency in athletes, and notes that menstrual blood loss shapes iron stores in premenopausal women more than diet does. Iron depletion over time covers periods, training and pregnancy.
Hepcidin after a run
Hepcidin is the main hormone controlling iron. When it is high, less iron is absorbed from the gut and less is released from the body's own recycling.
Exercise raises it. A 2014 review described post-exercise increases in hepcidin, reported to be driven by interleukin-6 (IL-6), an inflammatory signal released during exercise, and noted that iron absorption and recycling may be compromised while hepcidin is elevated.
In a 2019 trial, a 90-minute run raised hepcidin three hours afterwards. In the afternoon trial, hepcidin was already higher at rest before the run started, then rose again after it.
Timing of iron around training
The same 2019 trial studied 16 endurance-trained runners, 10 men and 6 women, all with serum ferritin below 50 micrograms per litre. Each ran 90 minutes in the morning and, on another occasion, in the afternoon, and took an iron-fortified drink with a meal 30 minutes after each run or rest period. Iron absorption was measured with stable isotopes.
Iron was best absorbed at breakfast after the morning run, better than in the rested condition and better than at dinner on the day of the morning run. The authors suggested a short window after exercise may favour absorption despite the rise in hepcidin.
A 2020 review by the same research group brought the athlete studies together and concluded with a practical strategy: oral iron in the morning, ideally within 30 minutes after morning exercise, and for athletes with gut sensitivity, on alternate days or at lower doses.
Daily or alternate days
The alternate-day idea comes from two randomised trials in iron-depleted women, with ferritin of 25 micrograms per litre or less, published in The Lancet Haematology in 2017.
Splitting 120 mg into two daily doses of 60 mg made no difference to absorption, and it raised hepcidin.
Taking the same 14 doses on alternate days, the body absorbed more of the iron: 175 mg against 131 mg. These women were not recruited as runners, and the authors asked for the finding to be confirmed in people with anaemia, so it is a reason to ask about the schedule, not a schedule in itself.
Questions for your doctor
- Ask for ferritin together with transferrin saturation, not ferritin alone. Ferritin targets for female athletes explains why the number on your slip may sit below the threshold athlete research uses.
- Ask whether the cause of the low iron has been looked for, including coeliac screening, rather than only topping it up.
- If tablets upset your stomach, ask whether a morning dose after training, or alternate-day dosing, suits you.
- Ask when to retest, and keep the test day consistent so results can be compared.
If your running has gone flat and your bloods were called normal, the number is worth a second look.
Related articles
Sources
The study figures on this page come from these seven papers.
- Gartner A, Dombrowski N, Lowe N, Behzadpour V, Zackula R. Foot-strike hemolysis: a scoping review of long-distance runners. Kansas Journal of Medicine 2024;17(5):119-124. doi:10.17161/kjm.vol17.22146
- Robinson Y, Cristancho E, Böning D. Intravascular hemolysis and mean red blood cell age in athletes. Medicine and Science in Sports and Exercise 2006;38(3):480-3. doi:10.1249/01.mss.0000188448.40218.4c
- Pasricha SR, Tye-Din J, Muckenthaler MU, Swinkels DW. Iron deficiency. The Lancet 2021;397(10270):233-248. doi:10.1016/S0140-6736(20)32594-0
- Sim M, Dawson B, Landers G, Trinder D, Peeling P. Iron regulation in athletes: exploring the menstrual cycle and effects of different exercise modalities on hepcidin production. International Journal of Sport Nutrition and Exercise Metabolism 2014;24(2):177-87. doi:10.1123/ijsnem.2013-0067
- McCormick R, Moretti D, McKay AKA, et al. The impact of morning versus afternoon exercise on iron absorption in athletes. Medicine and Science in Sports and Exercise 2019;51(10):2147-2155. doi:10.1249/MSS.0000000000002026
- McCormick R, Sim M, Dawson B, Peeling P. Refining treatment strategies for iron deficient athletes. Sports Medicine 2020;50(12):2111-2123. doi:10.1007/s40279-020-01360-2
- Stoffel NU, Cercamondi CI, Brittenham G, et al. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials. The Lancet Haematology 2017;4(11):e524-e533. doi:10.1016/S2352-3026(17)30182-5
This article is educational and not medical advice. Never begin iron supplementation without testing, and always discuss changes to supplementation or testing with a qualified clinician who knows your full history.