Iron Deficiency: Causes, Symptoms, Diagnosis — and What It Means for Athletes

Key Takeaways

• Iron deficiency (low iron stores, with or without anemia) affects roughly 2 billion people worldwide and 14% of US adults.
• You can be iron deficient with a normal hemoglobin. Symptoms such as fatigue, exercise intolerance and poor concentration can appear before anemia does.
• Ferritin is the key test, but it rises with inflammation and after hard exercise, so timing and context matter.
• Athletes, especially women and endurance athletes, are at higher risk because of exercise-related iron losses and post-exercise hepcidin rises that reduce absorption.
• In iron-deficient female athletes, endurance performance is reduced, and supplementation can improve it. Supplementing without a confirmed deficiency does not appear to improve endurance and may contribute to excess body iron.
• Single-dose and alternate-day regimens may improve iron absorption efficiency; morning timing may also be relevant, although evidence on long-term treatment outcomes is limited.
• Fatigue has many causes. A low-normal ferritin often does not explain the whole picture, so a careful differential diagnosis, including thyroid function, sleep, stress and mood, is worthwhile.
• In clinical practice, oral iron usually comes first, often as a two- to three-month course followed by a recheck. Alternate-day dosing or sucrosomial iron may help when tolerance is poor.
• Intravenous iron should not be used as a performance enhancer. Being an athlete is not in itself an indication for it.

Introduction: Why Iron Deficiency Deserves More Attention

Iron deficiency is one of the leading contributors to the global burden of disease.[1] A 2025 JAMA review estimated that absolute iron deficiency, meaning low iron stores with or without anemia, affects approximately 2 billion people worldwide and 14% of adults in the US.[2] Anemia itself affected 1.92 billion people globally in 2021 (24.3% of the population), and dietary iron deficiency was the most common cause of the disability that anemia produces.[3]

From a sports medicine perspective, iron deficiency has two faces: a general medical condition that deserves a proper work-up, and a performance problem that sits at the intersection of training, nutrition and blood work.

This article covers both. The first half explains what iron deficiency is, who gets it, how it shows up and how it is diagnosed and treated. The second half focuses on athletes: why their risk is higher, how it affects performance, how to test properly and how to supplement without causing harm.

In my clinical experience, iron deficiency comes up often, particularly in occupational health and primary care, and it is usually the patient who raises it. Many patients have already looked up ferritin reference ranges, often on online forums, and know that symptoms can occur even when ferritin is within the laboratory’s normal range. Iron deficiency is also, in many cases, more complex than a single number on a lab report.

What Is Iron Deficiency?

Iron deficiency comes in two broad forms. In absolute iron deficiency, the body’s total iron stores are too low. In functional iron deficiency, iron is present in the body but inflammation causes it to be withheld from the plasma and from red blood cell production, largely through the hormone hepcidin, the main regulator of iron balance.[1]

Absolute iron deficiency progresses from low iron stores to iron-deficiency anemia (IDA).[2] IDA, defined as low hemoglobin due to low iron stores, affects approximately 1.2 billion people worldwide.[2] Importantly, anemia is only one of many consequences of iron deficiency, and clinical and functional impairments can occur in the absence of anemia.[1]

Causes of Iron Deficiency and Who Is at Risk

According to the JAMA review, the most common causes of iron deficiency are bleeding (menstrual and gastrointestinal), impaired absorption (atrophic gastritis, celiac disease, bariatric surgery), inadequate dietary iron intake and pregnancy.[2] In high-income countries, about 38% of nonpregnant women of reproductive age have iron deficiency without anemia and about 13% have iron-deficiency anemia; based on data from high-income countries, iron deficiency affects up to 84% of women in the third trimester of pregnancy.[2]

Blood loss is central because each millilitre of blood contains 0.4–0.5 mg of iron, and a single whole blood donation costs the donor about 250 mg of iron.[1] In men and postmenopausal women, gastrointestinal blood loss is the most important cause of iron deficiency.[1] Absorption problems matter too: about 1 in 31 patients with IDA could have celiac disease, and vegetarians have lower iron stores than non-vegetarians.[1]

Iron Deficiency Symptoms — With and Without Anemia

People with iron deficiency, whether or not they are anemic, may have no symptoms at all. Others experience fatigue, irritability, depression, difficulty concentrating, restless legs syndrome (32%–40%), pica (40%–50%), shortness of breath, lightheadedness, exercise intolerance or worsening heart failure.[2]

When true iron-deficiency anemia has developed, patients in my practice typically describe fatigue and breathlessness, and sometimes palpitations. It often develops gradually, and the causes vary: in women it may be heavy menstrual bleeding, while in other cases gastrointestinal bleeding can go unnoticed. A very low hemoglobin can occasionally require emergency treatment. The lowest I have personally seen was 48 g/L, which had developed slowly in a patient with ongoing bleeding and associated iron deficiency; the patient was referred to the emergency department, received a red blood cell transfusion and underwent further investigations. Symptom prevalences vary with age, comorbidities and the severity and rate of development of iron deficiency.[2]

The fatigue link is supported by randomized evidence even in people with normal hemoglobin. In a trial of 198 menstruating women aged 18–53 years with unexplained fatigue, ferritin below 50 µg/L and hemoglobin above 12.0 g/dL, 80 mg of elemental iron daily for 12 weeks reduced fatigue scores by 47.7%, compared with 28.8% for placebo.[4] Iron had no significant effect on quality of life, depression or anxiety, and the trial was sponsored by an iron-supplement manufacturer.[4] A Lancet review similarly notes that iron reduces self-reported fatigue in adults with non-anemic iron deficiency, but trials in women without fatigue have generally not shown improvement.[1]

From a clinical perspective, iron deficiency has become something of a fashionable explanation for fatigue. In my experience, much of the fatigue I assess turns out to have other or additional explanations. A common scenario is a normal hemoglobin with a ferritin that is low but still within the reference range. In selected patients, I may consider a monitored trial of oral iron after reviewing the clinical context, possible causes of iron deficiency and potential adverse effects, but I also explain that low iron alone often does not account for the whole picture. In my practice, fatigue and exhaustion warrant a careful differential diagnosis: I first exclude somatic causes, including checking thyroid function, and then take a thorough history covering work stress, burnout, insomnia, low mood and depression, often with a psychological assessment.

Sleep evaluation can also be important, for example to look for sleep apnea or restless legs. Restless legs in particular can genuinely be linked to iron deficiency, and in my clinical experience, patients with confirmed restless legs syndrome fairly often also have low iron values. The Finnish Current Care (Käypä hoito) guidance on restless legs recommends considering iron supplementation when ferritin is below 75 µg/L or transferrin saturation is below 20%, with a ferritin target of at least 75–100 µg/L.[5] In my practice, I use the guideline’s ferritin target of at least 75–100 µg/L rather than a particular position within the laboratory reference range, because reference ranges vary between laboratories.

How Iron Deficiency Is Diagnosed

The JAMA review recommends testing anyone with anemia or symptoms of iron deficiency, and considering it in people with risk factors such as heavy menstrual bleeding, pregnancy or inflammatory bowel disease.[2] Iron deficiency is diagnosed by low serum ferritin (typically below 30 ng/mL, equivalent to 30 µg/L) in people without inflammatory conditions, or by transferrin saturation below 20%.[2]

Why 30 and not the lower cut-offs found on many lab reports? In a study of healthy women cited in the Lancet review, ferritin below 15 µg/L identified absent bone marrow iron with 75% sensitivity and 98% specificity, while a 30 µg/L threshold raised sensitivity to 93% at the cost of lower specificity (75%).[1] Because ferritin is an acute phase protein, inflammation can mask a deficiency.[1] The WHO guideline states that in individuals with infection or inflammation, a ferritin below 70 µg/L may be used to indicate iron deficiency in adults; this is a conditional recommendation based on low-certainty evidence.[6]

Finding iron deficiency is only half the job. The Lancet review recommends looking for a cause in every patient: celiac serology is recommended for everyone with IDA, and men and postmenopausal women with IDA should be considered for upper and lower gastrointestinal endoscopy.[1]

In practice, when iron deficiency is suspected, my basic work-up includes a full blood count and ferritin, often with transferrin saturation, transferrin and serum iron, as well as vitamin B12 and folate. Reticulocytes are sometimes added, and haptoglobin if hemolysis is suspected. Checking inflammatory markers is also useful, because inflammation can affect iron absorption. If the blood count shows clear abnormalities in other cell lines, I usually refer the patient to hematology for further investigation.

Treating Iron Deficiency

Oral iron (ferrous sulfate 325 mg daily or on alternate days) is typically first-line therapy.[2] Intravenous iron may be indicated for selected patients who cannot tolerate or absorb oral iron, those with ongoing blood loss or chronic inflammatory conditions such as chronic kidney disease, heart failure, IBD and cancer, and in selected cases during the second and third trimesters of pregnancy.[2]

More iron per dose is not always better. In iron-deficient non-anemic women, elemental iron doses of 60 mg or more raise hepcidin for 24 hours, reducing absorption from the next dose.[1] In a randomized stable-isotope trial in iron-depleted women (ferritin ≤25 µg/L), 60 mg given on alternate days for 28 days produced a cumulative fractional absorption of 21.8% versus 16.3% with the same dose on consecutive days for 14 days, and total absorbed iron of 175.3 mg versus 131.0 mg.[7] Splitting the dose twice daily raised hepcidin without improving absorption.[7] The authors noted that these results still need confirmation in patients with iron-deficiency anemia.[7]

Gastrointestinal side effects limit adherence and can lead people to stop oral iron.[1] A systematic review cited in the Lancet review found that ferrous salts increased gastrointestinal symptoms, particularly constipation (12%), nausea (11%) and diarrhea (8%).[1]

When iron deficiency is mild, or the anemia is not severe, oral iron is usually prescribed. In my clinical practice, I often prescribe a 100-tablet pack of 100 mg iron tablets, to be taken over two to three months, then recheck the values and decide on further treatment. In my view, taking iron continuously is often not appropriate, at least not without checking the values at regular intervals.

Many patients tolerate oral iron poorly. In my clinical experience, it can cause stomach upset and dark stools, and some patients develop diarrhea. For these patients, I may suggest sucrosomial iron as a second-line option. In my experience, these products are often better tolerated, although the doses are often lower; for example, a commonly used product contains 30 mg of sucrosomial iron per capsule. In my experience, patients usually have to pay for these products themselves, and although they are more expensive, many patients find them tolerable. I am not certain whether the better tolerability comes from the sucrosomial formulation itself or simply from the smaller amount of iron per tablet.

For some patients who tolerate iron poorly, I may instead advise taking the iron tablet every other day. In my clinical experience, this works better for many patients and reduces stomach symptoms. The total amount of iron taken over the same period is somewhat lower, but the course can be continued for longer.

Iron Deficiency in Athletes: Why the Risk Is Higher

Athletes, especially female and endurance athletes, are commonly diagnosed with iron deficiency. A narrative review in the European Journal of Applied Physiology reports it in roughly 15–35% of female athlete cohorts and about 5–11% of male athlete cohorts.[8] A 2025 systematic review states in its background that up to 60% of female athletes experience iron deficiency.[9]

Several exercise-related mechanisms drive this. Proposed iron losses include hemolysis from foot strike and muscle contraction, blood in the urine (hematuria), gastrointestinal bleeding and sweating.[8] The review cites an estimate that an additional ~1–2 mg of iron per day may be required to replace exercise-related iron losses.[8] Low energy availability, vegetarian diets and menstrual losses add to the burden.[8]

The newer piece of the puzzle is hepcidin. Exercise triggers an inflammatory response (via interleukin-6) that transiently raises hepcidin for 3–6 hours after exercise, reducing iron absorption from the gut and iron recycling from macrophages.[8] An individual participant data meta-analysis found exercise to be associated with a 1.5–2.5-fold increase in hepcidin, and exercise duration explained ~44% of the variance in the size of the rise.[10]

Menstrual blood loss is another important factor. In a survey of 1,073 runners at the 2015 London Marathon, 36% reported heavy menstrual bleeding (HMB), as did 37% of elite athletes in the study’s sub-analysis.[11] Across both survey groups (online and at the marathon), 32% of the exercising women reported a history of anemia and 50% had previously supplemented with iron, yet only 22% had sought medical advice for heavy periods.[11] These were self-reported data.

How Iron Deficiency Affects Athletic Performance

Iron is needed for oxygen transport (hemoglobin and myoglobin) and for energy production in the mitochondria, and the magnitude of the expected performance reduction is related to the severity of the deficiency.[8]

Some of the most direct data come from female athletes. A 2025 systematic review of 23 studies comprising 669 athletes across 16 sports, all high-level and iron deficient (ferritin < 40 µg/L), reported endurance-performance decrements of approximately 3%–4% associated with iron deficiency in the included studies.[9] Across the included studies, improvements of 2%–20% in endurance performance were reported with 100 mg/day of oral elemental iron for up to 56 days or with parenteral iron given “bi-daily”, in the review’s wording, over 8–10 days, and individual studies reported 6%–15% improvements in maximal aerobic capacity with 16–100 mg/day over 36–126 days.[9] These percentages describe changes reported in individual studies rather than consistent benefits over placebo: in a trial of intramuscular iron injections in iron-depleted female athletes, for example, time to exhaustion increased within the iron group but was not significantly greater than in the placebo group.[12] Effects on strength and anaerobic power were inconsistent, and most included studies had 20 or fewer athletes per group.[9]

A meta-analysis in iron-deficient non-anemic endurance athletes found that iron treatment had a large effect on ferritin (Hedges’ g = 1.088) and a moderate effect on hemoglobin (g = 0.695) and VO₂max (g = 0.610).[13] The European Journal of Applied Physiology review adds important nuance: the largest aerobic gains in that meta-analysis came from less-trained individuals (VO₂max below 40 mL/kg/min), the effect of non-anemic iron deficiency on endurance remains equivocal, and supplementing athletes who are not iron deficient does not appear to improve endurance performance.[8]

Testing Iron Status in Athletes

An athlete classification proposed by Peeling and colleagues, as summarized in a narrative review, describes three stages:[8]

• Stage 1 — iron depletion: ferritin < 35 µg/L, hemoglobin > 115 g/L, transferrin saturation > 16%
• Stage 2 — iron-deficient non-anemia: ferritin < 20 µg/L, hemoglobin > 115 g/L, transferrin saturation < 16%
• Stage 3 — iron-deficiency anemia: ferritin < 12 µg/L, hemoglobin < 115 g/L, transferrin saturation < 16%

The Swiss Society of Sports Medicine consensus statement considers ferritin values below 15 µg/L to represent empty stores and 15–30 µg/L low stores, making 30 µg/L an appropriate cut-off for athletes over 15 years; before altitude training, adult elite athletes should reach a ferritin of 50 µg/L because iron demand increases.[14]

Sampling conditions matter, because ferritin rises with inflammation and after intense exercise, and hemoglobin is affected by plasma volume shifts (training-induced “sports anemia” does not appear to impair performance).[8] The review recommends blood samples in the morning, after an overnight fast, in a hydrated state, at least 24 hours after training and with no muscle-damaging (e.g. eccentric) exercise in the previous 2–3 days.[8] It suggests screening annually for athletes without risk factors and quarterly, or at least biannually, for at-risk athletes.[8]

In my clinical experience, athletes tend to suspect iron deficiency readily when their performance stalls or does not improve at the rate they expect. Because they notice changes in performance early, they often seek testing sooner, whereas in sedentary people fatigue can develop insidiously until it starts to affect daily work capacity. Athletes rarely seem to know the athlete-specific ferritin thresholds, however. A sports medicine consensus considers 30 µg/L an appropriate ferritin cut-off for athletes over 15 years,[14] and a systematic review of high-level female athletes used ferritin below 40 µg/L to define iron deficiency.[9]

Treating Iron Deficiency in Athletes

Diet. A narrative review describes a full dietary assessment by a qualified sports dietitian as the initial and most conservative strategy. Heme iron from meat is absorbed far better (~5–35%) than non-heme iron (~2–20%) from a single meal. Vitamin C, meat, poultry and fish enhance non-heme iron absorption, while polyphenols, phytates and calcium (in tea, coffee, whole grains, legumes and dairy) reduce it.[8]

Oral iron. The same review reports ferritin increases of 40–80% in athlete cohorts taking ~100 mg per day over 8–12 weeks.[8] Alternate-day dosing may improve absorption efficiency, as in the trial above in iron-depleted women.[7]

Timing. In a crossover study of 16 endurance-trained runners with ferritin below 50 µg/L, fractional iron absorption from an iron-fortified drink consumed with a standardized meal 30 minutes after a 90-minute morning run was significantly greater than in the rested condition, and greater than at dinner on the same day.[15] The authors interpreted this as a possible transient post-exercise mechanism that favours absorption despite the rise in hepcidin.[15] It is a small study that used an isotope-labelled, iron-fortified drink with a meal rather than iron tablets, so it suggests, but does not establish, that the morning shortly after a morning session may be a practical time to take iron.

Parenteral iron. The review reports 200–400% increases in ferritin in athletes after 300–550 mg of parenteral (intramuscular or intravenous) iron; parenteral iron avoids gastrointestinal upset, but it carries a small risk of adverse reactions and must comply with sport and anti-doping rules.[8]

In practice, oral iron is my first step when iron-deficiency anemia is confirmed and the situation is not severe. If the situation is more difficult, or oral supplementation has demonstrably not worked, an iron infusion can be justified in selected cases. In my clinical practice in Finland, athlete status alone is not a reason to prescribe intravenous iron: I apply strictly the same medical indications as for other patients, rather than using intravenous iron solely to enhance performance.

Don’t Supplement Blindly: The Case for Testing First

Iron supplements are commonly used by athletes, and their use is not always justified.[16] Among 170 recreational runners at the Zürich marathon, iron depletion was found in 28.0% of women but only 1.6% of men, whereas 15.0% of men (about one in six) had ferritin above 200 µg/L, the study’s definition of iron overload after excluding runners with elevated CRP; a high ferritin alone does not confirm tissue iron overload. The authors concluded that supplements should be used only when tests confirm deficiency.[16] The Swiss consensus statement likewise warns that long-term daily oral or intravenous iron in the presence of normal or high ferritin makes no sense and may be harmful.[14]

Conclusion: Iron Deficiency Is Common, Measurable and Treatable

Iron deficiency affects roughly 2 billion people and can cause fatigue, reduced concentration and exercise intolerance long before anemia appears. Ferritin below 30 µg/L, interpreted in context, is the key diagnostic signal, and every case deserves a search for its cause.

In my clinical experience, iron deficiency is also sometimes used as a convenient explanation for fatigue that has other causes, so a careful differential diagnosis is worth the effort. When a deficiency is confirmed, oral iron is usually the first step, with a recheck after two to three months rather than open-ended supplementation. For athletes, intravenous iron is reserved for the same indications as for anyone else.

For athletes, the risk is higher and the stakes include performance: a systematic review reported endurance decrements of about 3%–4% in the iron-deficient high-level female athletes it examined, and correcting a confirmed deficiency can improve performance. The flip side is just as important: supplementing athletes with healthy iron stores does not appear to improve performance, and unnecessary supplementation should be avoided because excess iron may be harmful. Test properly, dose sensibly and treat the cause.

References

1. https://doi.org/10.1016/S0140-6736(20)32594-0
2. https://doi.org/10.1001/jama.2025.0452
3. https://doi.org/10.1016/S2352-3026(23)00160-6
4. https://doi.org/10.1503/cmaj.110950
5. https://www.kaypahoito.fi/nix03635
6. https://www.ncbi.nlm.nih.gov/books/NBK569876/
7. https://doi.org/10.1016/S2352-3026(17)30182-5
8. https://doi.org/10.1007/s00421-019-04157-y
9. https://doi.org/10.1016/j.jshs.2024.101009
10. https://doi.org/10.1007/s40279-023-01874-5
11. https://doi.org/10.1371/journal.pone.0149881
12. https://doi.org/10.1123/ijsnem.17.3.221
13. https://doi.org/10.1136/bjsports-2014-093624
14. https://doi.org/10.4414/smw.2015.14196
15. https://doi.org/10.1249/MSS.0000000000002026
16. https://doi.org/10.1038/ejcn.2010.16