You train regularly, you sleep reasonably well, and yet you’re dragging yourself around: heavy legs, short of breath at paces that used to feel easy, tiredness that rest doesn’t fix. Before blaming your training plan, think about iron. Among endurance athletes, and even more so among women, iron deficiency is common, and it often goes unnoticed because a basic blood test comes back “normal”.
In short: iron deficiency develops in three stages, and only the last one (anaemia) shows up on a standard blood count. Low ferritin, even without anaemia, can reduce your endurance performance by 3 to 4%. The right approach: a full blood test (including ferritin) before any supplementation, then, if your doctor prescribes it, a well-tolerated iron supplement taken at the right time relative to training.
Definition: iron deficiency without anaemia (IDNA) is a state where iron stores are too low to support performance, while haemoglobin is still within the normal range. It’s the most common and least visible stage in endurance athletes.
Key points
- In female athletes, ferritin below 40 µg/L is associated with a 3 to 4% drop in endurance performance (Pengelly et al., 2025).
- Three stages: low stores, deficiency without anaemia, then anaemia. Only anaemia is picked up by a basic blood test.
- Iron bisglycinate causes far fewer digestive problems than ferrous sulphate (Fischer et al., 2023).
- Hepcidin, a hormone that blocks iron absorption, rises 3 to 6 hours after hard training: timing matters.
- No supplementation without a blood test and medical advice.
The 3 stages of iron deficiency in athletes
Iron deficiency isn’t just anaemia. It develops in three stages:
| Stage | What happens | What the blood test shows |
|---|---|---|
| 1. Low stores | iron stores fall | low ferritin, normal haemoglobin |
| 2. Deficiency without anaemia | iron available to the tissues becomes insufficient | low ferritin and transferrin saturation, normal haemoglobin |
| 3. Iron-deficiency anaemia | red blood cell production is affected | low haemoglobin |
A blood test that only measures haemoglobin therefore misses the first two stages. That’s why so many athletes put up with unexplained tiredness for months. At the deficiency-without-anaemia stage, muscles and brain are already short of iron: slower recovery, poorer concentration, a “brain fog” feeling during hard training blocks (Borówka et al., 2026).
What low ferritin costs an endurance athlete
A systematic review by Pengelly, Pumpa, Pyne and Etxebarria (2025, Journal of Sport and Health Science), covering 23 studies and 669 female athletes from 16 sports, shows that ferritin below 40 µg/L is associated with a 3 to 4% drop in endurance performance. Supplementation of about 100 mg of elemental iron per day, prescribed and monitored, led to gains of 2 to 20% depending on the study, the larger the initial deficiency.
Three to four percent may not sound like much. Over a 3:30 marathon, though, it’s 6 to 8 minutes. The authors stress, however, that most studies involve small groups of athletes: the figures are orders of magnitude, not promises.
Why endurance athletes, especially women, are at risk
Endurance athletes lose more iron than sedentary people, and absorb less of it:
- Foot-strike haemolysis: in runners, the repeated impact of the foot on the ground destroys some red blood cells. Trail runners and marathon runners are the most affected.
- Sweat and the gut: small amounts of iron are lost in sweat and, during long efforts, through the gut.
- The hepcidin spike: hard exercise releases interleukin-6, which stimulates the liver to produce hepcidin. This hormone blocks iron absorption for 3 to 6 hours after training (Peeling et al., 2008). A recent review suggests absorption is reduced by about a third during that window (Borówka et al., 2026).
Women face additional factors:
- Periods, which mean a regular loss of iron, especially if they’re heavy.
- Often insufficient intake: women’s iron needs are higher than men’s, while their food intake is often lower.
- Too little energy for the training load (relative energy deficiency in sport, or RED-S): eating too little also means eating too little iron.
- Vegetarian or plant-based diets, if they aren’t well planned (see below).
Warning signs
- persistent tiredness despite rest;
- an unexplained drop in performance, especially on long efforts;
- unusual breathlessness at easy paces;
- slower recovery between sessions;
- difficulty concentrating;
- in some people, paleness, brittle nails or hair loss.
These signs aren’t specific: they can have other causes (overtraining, lack of sleep, insufficient energy intake). Only a blood test can tell.
The blood test: why haemoglobin isn’t enough
The tests to ask your doctor for are: ferritin, haemoglobin, haematocrit, transferrin saturation and CRP. CRP is there to detect inflammation, which artificially raises ferritin and can hide a deficiency. For the same reason, have your blood taken when rested, ideally after a day or two without hard sessions.
To interpret ferritin (1 µg/L equals 1 ng/mL), here are benchmarks commonly used in sport. They don’t replace your doctor’s interpretation:
| Ferritin | Benchmark |
|---|---|
| Below 30 µg/L | likely deficiency: discuss with your doctor, supplementation is often indicated |
| 30 to 50 µg/L | low stores for an endurance athlete: optimise your diet and retest in 3 months |
| Above 50 µg/L | stores generally sufficient for training |
During periods of hard training, a blood test every 3 to 6 months helps you spot a drop before it affects your performance.
Diet: the first line of defence
Dietary iron comes in two forms:
- Haem iron (red meat, offal, poultry, fish, seafood), the best absorbed.
- Non-haem iron (pulses, tofu, fortified cereals, seeds, green vegetables), absorbed 2 to 3 times less well, but its absorption can be improved.
To absorb more iron from your meals:
- pair a source of vitamin C (citrus fruit, kiwi, peppers, parsley) with iron-rich foods;
- avoid coffee, tea and calcium-rich dairy in the hour around iron-rich meals;
- for pulses and grains, soaking, sprouting and fermenting (sourdough bread) improve absorption.
If you’re vegetarian or vegan, these levers are your priority.
Supplementation: a medical decision, not a reflex
Don’t take iron without a blood test first. Unnecessary supplementation does nothing, and too much iron can be harmful: it promotes oxidative stress and can worsen an undiagnosed iron overload (haemochromatosis). The decision, dose and duration are for your doctor.
If supplementation is prescribed:
- The form: iron bisglycinate is generally better tolerated. A meta-analysis by Fischer et al. (2023, Nutrition Reviews) shows it causes 64% fewer digestive problems than ferrous sulphate. Haem iron supplements also appear to be better tolerated (Gallo Ruelas et al., 2024), although the evidence is still weak.
- Timing relative to training: avoid taking iron in the 3 to 6 hours after a hard session, when hepcidin blocks absorption. The simplest option: in the morning, before training, or on rest days.
- Morning or evening? A study in dancers (Attwell et al., 2023) found no clear difference: over 8 weeks, ferritin rose by about 26 µg/L with a morning dose and 22 µg/L with an evening dose. What matters most is taking it consistently and avoiding the post-exercise window.
- What to take it with: water or orange juice (vitamin C helps absorption), away from coffee, tea and calcium.
- Follow-up: have another blood test after 8 to 12 weeks to check the effect and adjust.
Frequently asked questions
What ferritin level should an endurance athlete aim for?
Below 40 µg/L, studies report a drop in endurance performance in female athletes (Pengelly et al., 2025). Many sports doctors aim for at least 50 µg/L in athletes who train a lot. The exact target depends on your profile: talk to your doctor.
Bisglycinate or ferrous sulphate: which form?
Bisglycinate causes far fewer digestive problems (Fischer et al., 2023). It’s often the form offered to athletes, but your doctor should choose the form and dose.
Can foot-strike haemolysis really drain a runner’s iron stores?
It contributes, especially in runners who log a lot of kilometres on hard surfaces. It adds to losses through sweat and the gut and to reduced absorption after exercise. Together, these factors explain why deficiency is so common in endurance runners.
Are vegetarian athletes at greater risk?
Yes, because plant iron is less well absorbed. With well-chosen combinations (vitamin C with meals, soaked or sprouted pulses, coffee and tea kept apart), a vegetarian diet can meet your needs, but monitoring your ferritin is advisable.
Should I take iron “just in case” before a race season?
No. Without a proven deficiency, iron doesn’t improve performance, and too much can be harmful. Get a blood test first.
Look after yourself and your biology
Iron deficiency is one of the most common, and most easily corrected, causes of poor form in endurance athletes. The keys: a full blood test including ferritin, a diet rich in well-absorbed iron, and supplementation only on medical advice, taken away from hard sessions.
To keep your eating in step with your training day after day, create your free Ograal account. And if you need one-to-one support, Ingrid Gallerini, sports dietitian, offers video consultations at nutrisport.diet.
Go further
- Vegetarian Diet for Endurance Athletes: Marathon, Cycling, Triathlon Without Deficiencies
- Gut Microbiome and Endurance Performance: What Your Gut Says About Recovery
- Wout van Aert and Creatine: What His Classics Profile Tells Cyclists
- Ashwagandha for Endurance Athletes: VO2max, Cortisol and Recovery with Kristian Blummenfelt
- Cordyceps and Endurance: VO2max, Performance and Cycling
- Spirulina for Endurance Athletes: Hayden Wilde's Recovery Secret
Sources
- Pengelly, M., Pumpa, K., Pyne, D. B., Etxebarria, N. (2025). Iron deficiency, supplementation, and sports performance in female athletes: A systematic review. Journal of Sport and Health Science. DOI: 10.1016/j.jshs.2024.101009
- Fischer, J. A. J., Cherian, A. M., Bone, J. N., Karakochuk, C. D. (2023). The effects of oral ferrous bisglycinate supplementation on hemoglobin and ferritin concentrations in adults and children: a systematic review and meta-analysis of randomized controlled trials. Nutrition Reviews. DOI: 10.1093/nutrit/nuac106
- Attwell, C., McKay, A. K. A., Sim, M., Dugan, C., Nicholas, J., Hopper, L., Peeling, P. (2023). Timing is everything, but does it really matter? Impact of 8-weeks morning versus evening iron supplementation in ballet and contemporary dancers. European Journal of Sport Science, 23(12), 2275-2282. DOI: 10.1080/17461391.2023.2224285
- Peeling, P., Dawson, B., Goodman, C., Landers, G., Trinder, D. (2008). Athletic induced iron deficiency: new insights into the role of inflammation, cytokines and hormones. European Journal of Applied Physiology, 103(4), 381-391. DOI: 10.1007/s00421-008-0726-6
- Borówka, K., et al. (2026). Iron Deficiency and Beyond: Implications for Cognitive Function and Recovery in Female Endurance Athletes. Quality in Sport, 52. DOI: 10.12775/qs.2026.52.69538
- Gallo Ruelas, M., et al. (2024). A comparative analysis of heme vs non-heme iron administration: a systematic review and meta-analysis of randomized controlled trials. European Journal of Nutrition. DOI: 10.1007/s00394-024-03564-y
