Is fatigue creeping in earlier and earlier on your run? Has your performance dropped off for no apparent reason? The culprit could well be low iron levels.
High intensity exercise increases iron loss by as much as 70%. Runners are particularly at risk - a 2003 study showed runners were four times more prone to red blood cell breakdown than cyclists. Iron deficiency is a prevalent yet significantly under-diagnosed problem in runners, especially in those running longer distances.
What does iron do?
Your bone marrow needs iron to make hemoglobin which is the oxygen carrying protein within red blood cells. Without adequate iron your body can't produce enough hemoglobin and without hemoglobin it can’t build enough red blood cells. Low levels of hemoglobin in the blood, or low levels of the iron storage protein ferritin, can have a significant negative effect on your ability to perform exercise due to your body’s decreased oxygen carrying ability.
RunStrong contains 5mg of Iron Bisglycinate per serving — the most bioavailable form of iron (90% absorption rate), and gentler on the stomach than iron sulfate. Formulated specifically for runners who lose iron through sweat and foot-strike haemolysis.
How am I losing iron?
There are multiple mechanisms through which iron loss in runners occurs, including sweating (iron is a constituent of sweat), blood loss in the urine and gastrointestinal tract* and foot strike hemolysis.
Bleeding from the GI tract during running is believed to either be caused by decreased blood flow to the GI tract or increased acidity directly breaking the GI tract down and causing bleeding. Whereas iron loss through urine is largely due to the mechanical trauma in the kidney or bladder wall in addition to breakdown of red blood cells in the bloodstream (hemolysis).
Foot strike hemolysis is caused by the repeated impact of feet hitting the ground and has been demonstrated in multiple studies to lead to the destruction of oxygen carrying red blood cells which the body will need to replace - requiring iron to do so.
Other causes of iron loss are muscle building or bleeding in the stomach or intestines due to frequent use of NSAIDs such as ibuprofen or aspirin. Haemorrhoids or piles can also impact your iron levels. Menstruating women need to pay extra care to their iron levels due to the blood lost during periods.
What are the symptoms of iron deficiency in runners?
Iron deficiency rarely announces itself with a single obvious symptom. Instead, it creeps in gradually — and because many of the signs overlap with overtraining, most runners blame their training before they think about their blood.
The runner-specific warning signs:
- Paces slipping for no reason — workouts that felt comfortable a month ago now feel hard. Your easy pace creeps up, your intervals slow down, and you can't explain why.
- Heavy legs from mile one — not the normal fatigue that builds during a run, but a deep heaviness that's there before you've even warmed up.
- Recovery taking longer than it should — you're sleeping well, eating right, and not overtraining, but your legs still feel flat 48 hours after a hard session.
- Breathlessness at lower intensities — running at conversational pace but finding yourself short of breath. Your body literally can't carry enough oxygen.
- Feeling like you're overtraining when you're not — this is the big one. Iron deficiency mimics overtraining syndrome so closely that many runners respond by training less, when the real fix is nutritional.
General signs of low iron:
- Persistent fatigue and low energy, even on rest days
- Pale skin, especially noticeable on the inner eyelids and nail beds
- Headaches and difficulty concentrating
- Heart palpitations during or after exercise
- Dry, damaged hair and brittle nails
- Low mood, irritability, or poor motivation
- Increased susceptibility to colds and infections
If several of these sound familiar — especially the performance-related ones — it's worth getting your ferritin levels tested. Don't settle for just a haemoglobin test; you can be iron-depleted with "normal" haemoglobin. Ask your GP specifically for serum ferritin, and note that athletes should aim for 30–50 ng/mL, not the standard clinical minimum of 15 ng/mL.
Iron supplementation has been shown to increase aerobic capacity (specifically VO2 max) even in runners who aren't clinically anaemic but have low iron stores. For a deeper dive into ferritin testing, athlete-specific thresholds, and iron deficiency without anaemia, see our full guide: Runner's Anaemia: The Hidden Performance Thief.
The iron in RunStrong
RunStrong contains 5mg of Iron Bisglycinate - known as the 'gentle iron' as it is kinder on the stomach and easier for the body to absorb. Iron Bisglycinate has around 90% bioavailability and is 2-4 times better absorbed by the body than the iron sulphate and fumarate which may be used in other supplements, where bioavailability is only around 27-28%.
*Your GI tract may also bleed from regular pain relievers, especially NSAIDs such as high dose aspirin or ibuprofen (see our article on Curcumin on how it can help). Blood loss in urine and stool should not be visible. If you see blood in your waste products, please make an appointment with your doctor.
Here's the terminology to help you decipher the science behind iron and iron deficiency:
Iron is an essential element for blood production. About two thirds of the iron in your body is found in hemoglobin.
Hemoglobin, or red blood cells, is the oxygen carrier in your blood and hence essential for your muscles to function properly. It's also what gives blood its red colour. Iron is a building block of hemoglobin, so when you don’t have enough iron, you won’t be able to produce enough hemoglobin and your muscles and other tissues will not receive enough oxygen to work effectively.
Red blood cell is a type of blood cell that is made in the bone marrow and found in the blood. Red blood cells contain a protein called hemoglobin, which carries inhaled oxygen from the lungs to all parts of the body. They then return to the lungs with carbon dioxide to be exhaled.
Ferritin is the storage protein for iron in your body. It plays a vital function in the absorption, storage and release of iron. Ferritin levels are directly proportionate to the stored iron in your body and hence are used as an indicator when testing for iron levels. Low ferritin levels indicate your body’s iron stores are low and could be a sign of iron deficiency anaemia.
Anaemia is a condition in which you lack enough healthy red blood cells to carry adequate oxygen to your body’s tissues. There are various types of anaemia but the most common one is Iron deficiency anaemia.
Iron deficiency anaemia is caused by a lack of iron in your body. Iron is required for the body to make red blood cells (which carry oxygen to the body’s tissues). As a result, a deficiency can leave you tired and short of breath.
This article is part of our runner's nutrition series. For the complete picture of how all five key nutrients work together, see our guide: RunStrong Daily Supplement for Runners: A Complete Guide.
References
Clénin, G., Cordes, M., Huber, A., Schumacher, Y. O., Noack, P., Scales, J., & Kriemler, S. (2015). Iron deficiency in sports - definition, influence on performance and therapy. Swiss medical weekly, 145, w14196. https://doi.org/10.4414/smw.2015.14196
Parks, R. B., Hetzel, S. J., & Brooks, M. A. (2017). Iron Deficiency and Anemia among Collegiate Athletes: A Retrospective Chart Review. Medicine and science in sports and exercise, 49(8), 1711–1715. https://doi.org/10.1249/MSS.0000000000001259
Halvorsen, F. A., Lyng, J., & Ritland, S. (1986). Gastrointestinal bleeding in marathon runners. Scandinavian journal of gastroenterology, 21(4), 493–497. https://doi.org/10.3109/00365528609015168
Telford, R. D., Sly, G. J., Hahn, A. G., Cunningham, R. B., Bryant, C., & Smith, J. A. (2003). Footstrike is the major cause of hemolysis during running. Journal of applied physiology (Bethesda, Md. : 1985), 94(1), 38–42. https://doi.org/10.1152/japplphysiol.00631.2001
B. J. Miller, R. R. Pate, W. Burgess (1988). Foot Impact Force and Intravascular Hemolysis During Distance Running (Department of Physical Education, University of South Carolina, Columbia, South Carolina)https://www.thieme-connect.com/products/ejournals/abstract/10.1055/s-2007-1024979
Burden, R. J., Morton, K., Richards, T., Whyte, G. P., & Pedlar, C. R. (2015). Is iron treatment beneficial in, iron-deficient but non-anaemic (IDNA) endurance athletes? A systematic review and meta-analysis. British journal of sports medicine, 49(21), 1389–1397. https://doi.org/10.1136/bjsports-2014-093624
Bagna, R., Spada, E., Mazzone, R., Saracco, P., Boetti, T., Cester, E. A., Bertino, E., & Coscia, A. (2018). Efficacy of Supplementation with Iron Sulfate Compared to Iron Bisglycinate Chelate in Preterm Infants. Current pediatric reviews, 14(2), 123–129. https://doi.org/10.2174/1573396314666180124101059