Endurance Athletes and Iron: Foot-Strike Loss, Testing and Food

Picture a runner in week ten of marathon training. The mileage is right, the sleep is decent, the diet is clean, and yet easy runs feel heavier than they should. Before blaming motivation, it is worth asking whether iron is part of the story. Endurance training strains iron balance in several ways at once. This guide walks through each mechanism, explains how sports medicine teams test for low iron, shows how to build an iron-aware plate, and is straightforward about what spirulina can and cannot do for athletes.

ENERGYbits® • Sports Nutrition Science

Why endurance athletes are a special case

A 2019 narrative review in the European Journal of Applied Physiology by Sim and colleagues lists the main routes by which athletes can end up short: hemolysis (red cell breakdown) exacerbated by ground impact, hematuria, gastrointestinal bleeding, sweating, muscle damage, and exercise-induced hepcidin responses, on top of whatever the diet provides [1]. The same review reports that deficiency affects roughly 15 to 35 percent of female and about 3 to 11 percent of male athlete cohorts, with some small cohorts showing higher figures [1].

• Higher loss: mechanical, sweat, gut and menstrual routes can add up over a training year [1].
• Lower absorption: hepcidin rises after exercise and blunts iron uptake for hours [1][3].
• Baseline needs: the RDA for women aged 19 to 50 is already 18 mg a day versus 8 mg for men [7].

Foot-strike hemolysis: the running-specific loss

Every running stride sends a shock through the foot, and the small blood vessels in the sole are squeezed repeatedly. Red cells passing through can rupture, releasing hemoglobin into plasma. This is called foot-strike hemolysis, and it is one reason runners have drawn more attention than cyclists in the iron literature.

The clearest evidence comes from a 2003 study in the Journal of Applied Physiology by Telford and colleagues. Ten male triathletes completed a one-hour run and a one-hour ride at the same relative intensity, 75 percent of peak oxygen uptake. Plasma free hemoglobin, a marker of hemolysis, rose after both, but the increase was fourfold greater after running. Haptoglobin, a protein that mops up free hemoglobin, fell after the run and not after cycling. The authors concluded that foot-strike is the major contributor to hemolysis during running [2].

It is a small, controlled study in men, and it measured hemolysis markers, not iron stores. A rise in free hemoglobin shows red cells are being damaged. It does not by itself show that an individual runner is losing enough iron to become deficient. Whether it matters depends on how much iron the body recovers, how much is eaten and absorbed, and what other losses are in play. That is exactly why testing beats guessing.

Hepcidin: why timing matters after a hard session

Hepcidin is the liver hormone that controls how much iron the gut absorbs and how readily stored iron is released. When it rises, absorption falls. Exercise is one of the triggers, working through inflammatory signaling, particularly interleukin-6 [1].

Peeling and colleagues measured this in a 2009 study in the International Journal of Sport Nutrition and Exercise Metabolism. Eight moderately trained athletes completed a 60-minute run and a seated-rest trial. After the run, interleukin-6 jumped immediately, and urinary hepcidin was 1.7 to 3.1 times higher than baseline at 3, 6 and 24 hours. At the 3-hour mark it was about three times higher than at the same time point on the rest day, an effect seen in every participant despite large differences in size [3].

What this means for meals and timing

The Sim review suggests iron intake should be as far from exercise as possible, with morning preferred over evening, and notes a possible window within an hour of finishing a morning session based on later work, while stressing that more research is needed [1]. Those suggestions were written mainly for supplement strategies managed by sports physicians. For food, the takeaway is modest: an iron-rich meal earlier in the day, away from a hard session, is reasonable, but there is no need to skip a post-run meal.

Sweat, gut and other losses

Foot-strike gets the headlines, but it is not the only drain. Reviews list sweat, gastrointestinal blood loss and hematuria (blood in urine) among the routes by which athletes lose iron [1]. Individually these may be modest, but they sit on top of the other routes, and the combined picture differs a lot from athlete to athlete.

• Sweat: a minor but continuous route, relevant in hot conditions and long sessions [1].
• Gastrointestinal: endurance exercise can irritate the gut, and trace bleeding is described in the sports literature [1]. Any visible blood or black stools belongs with a clinician, not a nutrition experiment.
• Urinary: hematuria after hard running is a recognized finding [1]. Dark or red urine that does not clear quickly warrants medical attention.
• Diet-related: low energy availability, restricted eating, vegetarian patterns and low-carbohydrate approaches can all reduce intake or increase post-exercise hepcidin signals [1].

Female athletes and menstrual losses

Female endurance athletes face an extra variable. The Sim review notes that a typical period involves about 30 to 50 mL of blood loss, with roughly 40 mL corresponding to about 1.6 mg of iron, and that cumulative loss above about 60 mL per cycle can compromise stores [1]. The RDA for women aged 19 to 50 is 18 mg a day, more than double the 8 mg for men, and for vegetarians the NIH Office of Dietary Supplements notes requirements are 1.8 times higher [7].

A 2018 practical review in the European Journal of Sport Science by Pedlar and colleagues makes a point worth repeating: menstruating athletes, particularly those with heavy menstrual bleeding, are at elevated risk, and a fuller picture that includes menstrual pattern, training load and longitudinal blood results is more useful than a single snapshot [5]. Heavy periods are also a medical topic in their own right. If bleeding is heavy, a clinician can look for causes.

Test first: ferritin and what the thresholds mean

The most important sentence in this guide is also the simplest: test, do not guess. Fatigue, heavy legs and slow recovery overlap with overtraining, under-fueling, poor sleep and thyroid problems. NIH lists tiredness, weakness and poor concentration among deficiency signs [8], but none is specific. Ferritin, the protein that stores iron, is the usual first marker, interpreted alongside hemoglobin and transferrin saturation.

Thresholds used in sports medicine

There is no single universal cutoff. The Sim review describes a staged model used in athlete cohorts, shown below. Laboratory reference ranges and individual team protocols differ, so treat the table as an explanation of how clinicians think, not a self-diagnosis tool [1].

Thresholds as summarized in Sim et al. 2019 [1]. The review also describes about 30 µg/L as a borderline ferritin value. Cutoffs vary by lab, sport and guideline.

Why one ferritin number is not the whole story

• Inflammation raises ferritin. Ferritin is an acute-phase protein, so a hard session or a recent illness can make stores look better than they are [5].
• Plasma volume shifts. Training and heat expand plasma volume, which can dilute hemoglobin without any loss of red cell mass [5].
• Sport- and sex-specific norms are limited. Pedlar and colleagues note the absence of sport-, gender- and individual-specific normative data, and recommend looking at trends over time [5].
• Exercise affects iron markers. Hepcidin and inflammation shift after training [3], so ask your clinician when to have blood drawn.

What the treatment research shows, and why it is for clinicians

A 2015 systematic review and meta-analysis in the British Journal of Sports Medicine by Burden and colleagues pooled 17 studies of iron treatment in iron-deficient but non-anemic endurance athletes. Iron treatment improved ferritin, serum iron and transferrin saturation, and had a moderate effect on hemoglobin and on VO2max [4]. That is encouraging for athletes who are truly deficient, and it is also exactly why the decision should follow a test.

Iron taken when stores are adequate does not help. The NIH sets a tolerable upper intake level of 45 mg a day for adults and notes that people with hemochromatosis should avoid iron supplements [7][8]. The Sim review is explicit that the decision to supplement, and how, should be made by the sports physician with the support team [1]. The first step is a ferritin test and a conversation with a clinician, not self-treatment.

Food first: sources, absorption and timing

Diet is the foundation either way. The NIH lists lean meat, seafood and poultry, fortified cereals and breads, white beans, lentils, spinach, kidney beans, peas, nuts and some dried fruit as sources [8]. Meat, fish and poultry supply heme iron, which is absorbed more readily; plant foods supply non-heme iron, whose absorption is more easily swayed by what else is on the plate [7].

Sim and colleagues give approximate absorption ranges of 5 to 35 percent for heme iron and 2 to 20 percent for non-heme iron [1]. The NIH estimates overall bioavailability at roughly 14 to 18 percent from mixed diets that include meat, seafood and vitamin C, and 5 to 12 percent from vegetarian diets [7].

Plant-forward athletes can find more detail in our guide to plant-based iron absorption.

What helps and what blocks absorption

Timing, in practical terms

• Anchor iron-rich foods earlier in the day. Given the hepcidin rise after training [1][3], a lunch or early meal away from your session is a sensible default.
• Pair plant iron with vitamin C. Lentils with peppers, beans with tomatoes, fortified oats with strawberries [8].
• Keep tea and coffee away from the iron-heavy meal. You do not have to give them up [6].
• Do not skip recovery meals. Eating after a hard session still delivers protein, carbohydrate and other nutrients, even if it is not your best iron opportunity.
• Watch total energy. Under-fueling reduces iron intake and, in some approaches, raises post-exercise hepcidin signals [1].

Where spirulina fits

We make algae products, and we are fond of spirulina for good reasons: it is a whole food that is rich in protein, phycocyanin, chlorophyll, vitamins and minerals, and it is notably dense in iron for its weight. The USDA FoodData Central entry for dried spirulina (SR Legacy, FDC ID 170495) lists 28.5 mg of iron per 100 g [9].

What that means per serving, by calculation

Each ENERGYbits tablet is 250 mg, and a full serving is 30 tablets, or 7.5 g. Multiplying the USDA figure by 0.075 gives about 2.1 mg of iron per serving (28.5 mg per 100 g x 7.5 g = roughly 2.14 mg). This is our calculation from database values, not a laboratory analysis of a specific batch, and actual iron content varies by product and harvest.

Spirulina iron is non-heme, and we are not aware of data showing how well athletes absorb iron from spirulina specifically, so we do not apply the general absorption percentages to it. It is a helpful contribution within a varied diet. It is not a replacement for medical treatment of diagnosed deficiency. Our existing guide, is spirulina good for iron deficiency, covers that question in detail.

Spirulina and hemoglobin in cyclists: what the trials found

There is an intriguing, if early, line of research from a UK group led by Tom Gurney. Two small randomized crossover trials in cyclists reported higher hemoglobin after spirulina, though their performance results differ, and neither measured iron stores in the parts we reviewed.

• Recreationally active cyclists (2023). In a double-blind, randomized crossover trial in the Journal of Dietary Supplements, 17 recreationally active adults took 6 g a day of spirulina or placebo for 14 days. Hemoglobin was 4.8 g/L higher after spirulina (145.6 versus 150.4 g/L, p = 0.047). There were no significant differences in VO2max, maximal power, time to fatigue, heart rate, oxygen uptake or blood lactate. The authors concluded spirulina raised hemoglobin but provided no meaningful ergogenic benefit in this group [10].
• Trained cyclists (2022). In a double-blind crossover trial in Applied Physiology, Nutrition, and Metabolism, 15 trained male cyclists took 6 g a day for 21 days. Hemoglobin was higher with spirulina (152.6 versus 143.2 g/L), submaximal heart rate and blood lactate were lower, and peak and average power were higher during repeated sprints. There was no difference in time trial performance, oxygen consumption or lactate threshold [11].

If you would like to try algae as part of a whole-food routine, ENERGYbits spirulina is a single ingredient with no fillers or additives, freshwater-grown, gently dried at low temperatures and third-party tested for purity and contaminants. The VITALITYbits spirulina and chlorella blend pairs spirulina with chlorella, which has a cracked cell wall for improved digestibility. For more on the athletic side of spirulina, see our guide to spirulina for athletes.

Precautions and when to see a clinician

See a doctor, not a food plan, for fatigue that does not improve with rest, shortness of breath, dizziness, very heavy periods, black or bloody stools, or red or cola-colored urine. Do not start iron supplements without a result. The NIH notes that iron supplements can interact with medicines such as levothyroxine and levodopa, that people with hemochromatosis should avoid them, and that the adult upper limit is 45 mg a day from all sources [8].

For algae: spirulina and chlorella are generally well tolerated, but check with your healthcare provider first if you are pregnant or nursing, have phenylketonuria, have an autoimmune condition, have a bleeding disorder or take blood thinners or immunosuppressants, or have thyroid or iodine concerns (relevant to chlorella). ENERGYbits products are third-party tested for purity and contaminants.

Conclusion

Iron and endurance training are tightly linked, and the link runs in both directions: training can drain stores and limit absorption, and low stores can make training feel harder than it should. The good news is that the path forward is clear. Get a ferritin test, with context from a clinician. Build a plate that makes the most of the iron you eat, with vitamin C alongside plants and tea and coffee kept away from the main iron meal. Leave supplement decisions to the person who has seen your results.

Spirulina can be a pleasant, nutrient-dense addition to that food-first approach. It offers a modest amount of iron by calculation, along with protein, phycocyanin, chlorophyll and other nutrients. The early hemoglobin findings in cyclists are interesting, and they are not a treatment plan. We will keep following the research, and we will tell you plainly when it changes.

Frequently asked questions

Why are endurance athletes at higher risk of low iron? +

Endurance training can increase iron loss through foot-strike hemolysis, sweat, gastrointestinal blood loss and menstruation, and can reduce absorption because exercise raises hepcidin for roughly 3 to 6 hours [1]. Deficiency is reported in about 15 to 35 percent of female and 3 to 11 percent of male athlete cohorts [1].

What is foot-strike hemolysis? +

It is the breakdown of red blood cells caused by repeated ground impact while running. In a study of 10 male triathletes, free hemoglobin rose fourfold more after an hour of running than after an hour of cycling at equivalent effort, identifying foot-strike as the major contributor [2].

How long does hepcidin stay elevated after exercise? +

Reviews describe elevation for about 3 to 6 hours after exercise [1]. In one study of eight athletes, urinary hepcidin was still 1.7 to 3.1 times higher than baseline at 3, 6 and 24 hours after a 60-minute run [3].

When should an athlete take iron relative to training? +

That is a question for your clinician. Researchers have suggested iron intake should be as far from exercise as possible, with morning preferred over evening, but they stress more research is needed, and decisions about supplementing belong with a sports physician [1].

What ferritin level is low for an endurance athlete? +

There is no universal cutoff. Sports medicine literature describes below 35 µg/L as depleted stores, below 20 µg/L with normal hemoglobin as non-anemic iron deficiency, and below 12 µg/L with low hemoglobin as iron-deficiency anemia [1]. Lab ranges vary, so ask your clinician to interpret.

Should I take an iron supplement for fatigue? +

Not without testing. Fatigue has many causes, and iron supplements carry risks, including interactions with medicines such as levothyroxine and a 45 mg daily upper limit for adults [8]. A ferritin test and a clinician's advice should come first [1].

Does iron treatment help non-anemic iron-deficient athletes? +

A meta-analysis of 17 studies found iron treatment improved ferritin, serum iron and transferrin saturation in iron-deficient but non-anemic endurance athletes, with moderate effects on hemoglobin and VO2max [4]. It applies to confirmed deficiency, under clinical guidance.

Do tea and coffee affect iron absorption? +

Yes. In a human study, black tea reduced iron absorption from a bread meal by 79 to 94 percent, and coffee and cocoa were also strongly inhibitory; adding milk made little difference [6]. Having them between meals rather than with iron-rich food is a simple fix.

How much iron is in spirulina? +

USDA FoodData Central lists 28.5 mg of iron per 100 g of dried spirulina [9]. A 7.5 g serving (30 ENERGYbits tablets of 250 mg) therefore contains about 2.1 mg by our calculation. Actual content varies by product.

Does spirulina raise hemoglobin or improve cycling performance? +

In two small crossover trials, 6 g a day of spirulina was followed by higher hemoglobin in cyclists [10][11]. The 14-day trial in recreational cyclists found no performance benefit [10]. The 21-day trial in trained cyclists reported lower submaximal heart rate and higher repeated-sprint power [11]. Both are preliminary.

Can spirulina replace an iron supplement? +

No. A serving provides about 2.1 mg of non-heme iron by calculation [9], and no trial we reviewed shows spirulina treats iron deficiency. If a clinician diagnoses deficiency, follow their plan; spirulina can sit alongside as a whole food.

This article is for educational purposes only and is not medical advice. These statements have not been evaluated by the FDA. ENERGYbits® products are whole-food supplements and are not intended to diagnose, treat, cure, or prevent any disease. Talk with your doctor before starting any supplement, especially if you are pregnant, nursing, taking medication, or managing a medical condition.

References

1. Sim M, Garvican-Lewis LA, Cox GR, Govus A, McKay AKA, Stellingwerff T, Peeling P. Iron considerations for the athlete: a narrative review. Eur J Appl Physiol. 2019;119(7):1463-1478. https://doi.org/10.1007/s00421-019-04157-y
2. Telford RD, Sly GJ, Hahn AG, Cunningham RB, Bryant C, Smith JA. Footstrike is the major cause of hemolysis during running. J Appl Physiol. 2003;94(1):38-42. https://doi.org/10.1152/japplphysiol.00631.2001
3. Peeling P, Dawson B, Goodman C, Landers G, Trinder D. Effects of exercise on hepcidin response and iron metabolism during recovery. Int J Sport Nutr Exerc Metab. 2009;19(6):583-597. https://doi.org/10.1123/ijsnem.19.6.583
4. Burden RJ, Morton K, Richards T, Whyte GP, Pedlar CR. 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):1389-1397. https://doi.org/10.1136/bjsports-2014-093624
5. Pedlar CR, Brugnara C, Bruinvels G, Burden R. Iron balance and iron supplementation for the female athlete: a practical approach. Eur J Sport Sci. 2018;18(2):295-305. https://doi.org/10.1080/17461391.2017.1416178
6. Hurrell RF, Reddy M, Cook JD. Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages. Br J Nutr. 1999;81(4):289-295. https://doi.org/10.1017/S0007114599000537
7. National Institutes of Health, Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/
8. National Institutes of Health, Office of Dietary Supplements. Iron: Fact Sheet for Consumers. https://ods.od.nih.gov/factsheets/Iron-Consumer/
9. U.S. Department of Agriculture, FoodData Central. Seaweed, spirulina, dried (FDC ID 170495, SR Legacy). https://fdc.nal.usda.gov/food-details/170495/nutrients
10. Ali Y, Aubeeluck R, Gurney T. Fourteen-days spirulina supplementation increases hemoglobin, but does not provide ergogenic benefit in recreationally active cyclists: a double-blinded randomized crossover trial. J Diet Suppl. 2024;21(2):261-280 (online 2023). https://doi.org/10.1080/19390211.2023.2263564
11. Gurney T, Brouner J, Spendiff O. Twenty-one days of spirulina supplementation lowers heart rate during submaximal cycling and augments power output during repeated sprints in trained cyclists. Appl Physiol Nutr Metab. 2022;47(1):18-26. https://doi.org/10.1139/apnm-2021-0344