Iron
What it is
Iron is an essential mineral required for a range of fundamental physiological processes. Its most well-known role is in haemoglobin, the protein in red blood cells that carries oxygen from the lungs to tissues throughout the body. Iron is also a core component of myoglobin, which stores oxygen in muscle, and is required for mitochondrial respiration, neurotransmitter synthesis (including dopamine and serotonin), and DNA replication. The body has no regulated mechanism for iron excretion; instead, iron balance is controlled almost entirely through absorption in the small intestine, regulated by the peptide hormone hepcidin.
Dietary iron exists in two forms. Haem iron, found in meat and fish, is absorbed efficiently at around 15–35% regardless of iron status. Non-haem iron, found in plant foods, fortified cereals, and most supplements, is absorbed at a much lower and more variable rate of around 2–20%, depending heavily on iron status, enhancers such as vitamin C, and inhibitors such as phytates, calcium, and polyphenols. This difference in bioavailability makes dietary source, not just total intake, an important variable in iron status.
Supplemental iron is available in several chemical forms. Ferrous salts (ferrous sulfate, ferrous fumarate, ferrous gluconate) are the most widely studied oral forms and are considered clinically equivalent in terms of efficacy, though they differ in elemental iron content and tolerability profile. Ferrous sulfate is the most commonly used and lowest-cost option. Intravenous preparations including ferric carboxymaltose, iron sucrose, and ferric derisomaltose are used in clinical settings where oral iron is ineffective or contraindicated. Dosing strategy matters: evidence supports alternate-day dosing for improved net absorption in iron-depleted individuals, as daily dosing elevates hepcidin and transiently suppresses absorption the following day.
What the evidence shows
Iron deficiency and iron-deficiency anaemia
The evidence for iron supplementation in confirmed deficiency is among the most consistent in clinical nutrition. Multiple randomised controlled trials and meta-analyses demonstrate that oral iron therapy reliably increases haemoglobin by approximately 10–20 g/L within two to four weeks in responsive patients, with continued therapy restoring iron stores measured as serum ferritin. Symptom improvement follows haematological recovery. This evidence base is large, consistent, and of low to moderate risk of bias, justifying a Strong rating for this indication. The effect is not a modest statistical signal but a clinically meaningful change that resolves the condition being treated. Response depends on adherence, the underlying cause of deficiency, and the presence of inflammation, which can suppress response through hepcidin-mediated mechanisms.
Fatigue without anaemia
Iron deficiency without anaemia, low ferritin in the presence of normal haemoglobin, is a distinct clinical state that may cause fatigue independently of anaemia. A small number of randomised trials, most notably in premenopausal women with low ferritin, report modest improvements in self-reported fatigue with iron supplementation. The effect is real but smaller than in iron-deficiency anaemia, and findings are not consistent across all trials. The evidence does not support a general claim that iron supplementation reduces fatigue in the absence of confirmed deficiency; it supports a more specific claim that correction of low iron stores in a defined population may improve fatigue symptoms.
Cognitive function in iron-deficient children
Meta-analyses of randomised trials in iron-deficient children report improvements in cognitive outcomes, particularly attention and learning performance. The direction of effect is generally consistent, though the magnitude varies across studies and the populations are specific, effects are concentrated in children with established deficiency. There is no consistent evidence of cognitive benefit in iron-replete children or in adults.
Restless legs syndrome
Randomised trials, including trials of both oral and intravenous iron, demonstrate that iron supplementation reduces the severity of restless legs syndrome (RLS) symptoms in patients with low ferritin. The effect appears to be ferritin-dependent, with most evidence of benefit in individuals with ferritin below commonly cited thresholds of approximately 50–75 µg/L, though these cutoffs are not universally agreed across guidelines. The mechanism is plausible, iron is required for dopamine synthesis and dopaminergic signalling, which is implicated in RLS pathophysiology, though the clinical evidence does not rest on mechanism alone. In patients with normal iron status, iron supplementation is not an established treatment for RLS.
Heart failure with iron deficiency (intravenous iron)
Several large, well-designed randomised trials have shown that intravenous iron supplementation improves functional capacity and quality of life in iron-deficient patients with chronic heart failure. The FAIR-HF and CONFIRM-HF trials demonstrated improvements in six-minute walk test performance and NYHA class. No consistent survival benefit has been established. This evidence is specific to intravenous preparations in this clinical setting and does not generalise to oral iron for the same indication or to iron deficiency without heart failure.
Exercise performance
A small number of trials in iron-deficient individuals, particularly female distance athletes, report modest improvements in aerobic capacity and physical performance with iron supplementation. Effect sizes are small, findings are inconsistent across studies, and populations are specific. There is no evidence supporting performance enhancement from iron supplementation in iron-replete athletes.
The five questions
Does low status cause harm that supplementation corrects?
Yes, clearly. Iron deficiency is one of the most prevalent nutritional deficiencies globally and causes a well-characterised spectrum of harm. At mild levels of deficiency, tissue iron stores are depleted without anaemia, which may impair energy metabolism and cause fatigue. As deficiency progresses to iron-deficiency anaemia, oxygen delivery to tissues is compromised, producing symptoms including fatigue, reduced exercise tolerance, impaired cognitive function, and in severe cases cardiovascular strain. In pregnancy, iron deficiency is associated with adverse maternal and neonatal outcomes including preterm birth and low birth weight. Iron supplementation in deficient individuals reliably corrects both iron stores and, where present, anaemia, with large and reproducible clinical benefit. This is one of the most clearly established status-dependent effects in nutritional medicine.
Does supplementation prevent disease in at-risk populations?
Yes, in defined at-risk groups. Supplementation in iron-deficient pregnant individuals reduces the risk of maternal anaemia and associated adverse outcomes. Supplementation in infants and young children in iron-deficient populations reduces the incidence of iron-deficiency anaemia and supports normal cognitive development. Routine supplementation in premenopausal women with confirmed low ferritin may prevent the development of iron-deficiency anaemia. These are prevention effects in populations with established risk of deficiency, not broad preventive effects in the general population.
Does iron produce meaningful biomarker effects?
Yes, with direct clinical relevance. Serum ferritin and haemoglobin are the primary biomarkers of iron status and both respond consistently to supplementation in deficient individuals. These are not exploratory biomarkers, they are validated measures that track the clinical condition being treated. In the context of iron, biomarker improvement (ferritin normalisation, haemoglobin increase) is directly and causally linked to clinical recovery. This is a distinction from many supplement categories where biomarker changes are measured as proxies for hoped-for clinical effects.
Does iron improve outcomes in clinical populations?
Yes, in specific clinical settings. Iron-deficient patients with heart failure show improvements in functional capacity and quality of life with intravenous iron. Patients with restless legs syndrome and low ferritin show symptom improvement. Iron-deficient children show cognitive improvements. Iron-deficient pregnant individuals have better maternal and neonatal outcomes with adequate iron repletion. In each case, the benefit is restricted to populations with confirmed deficiency or iron-dependent pathology, and the route of administration and clinical context determine which evidence applies.
Does iron benefit healthy, replete adults?
No. There is no evidence that iron supplementation benefits adults with normal iron status. In replete individuals, supplementation does not improve fatigue, cognitive function, exercise performance, or any other meaningful outcome. Beyond the absence of benefit, supplementation in replete individuals carries risk: excess non-transferrin-bound iron is mechanistically associated with oxidative stress, though clinical relevance outside frank overload states remains uncertain, and in individuals with undiagnosed haemochromatosis, supplementation can accelerate iron overload and organ damage. Self-supplementation with iron in the absence of confirmed deficiency is not supported by evidence and is not recommended.
Individual variation
Iron status and response to supplementation are substantially influenced by individual biology, life stage, and lifestyle. Premenopausal women lose iron through menstruation and are at meaningfully higher risk of deficiency than men of the same age, particularly with heavy menstrual bleeding. Pregnant individuals have substantially increased iron requirements to support fetal development and expanded blood volume, making deficiency common without adequate dietary intake or supplementation. Distance runners and endurance athletes face iron losses through haemolysis, sweat, and gastrointestinal microbleeding, and female athletes in particular carry a high prevalence of iron deficiency.
People following plant-based or vegan diets consume iron exclusively as non-haem iron, which is absorbed at much lower rates. While dietary adequacy is achievable with careful planning, low-ferritin states are more common in this group and monitoring is warranted. Post-bariatric surgery patients frequently develop iron deficiency due to reduced gastric acid and bypassed duodenum, the primary site of iron absorption; this population often requires ongoing supplementation and monitoring.
Genetic variation in iron regulation is clinically significant. Hereditary haemochromatosis, most commonly caused by mutations in the HFE gene (C282Y homozygosity), impairs hepcidin-mediated regulation and leads to progressive iron overload. Affected individuals must not supplement without medical supervision. HFE mutation carrier status (C282Y/H63D compound heterozygotes) confers lower but not negligible risk. Separately, inflammatory conditions including chronic infection, autoimmune disease, and malignancy elevate hepcidin and impair iron absorption, creating a state of functional iron deficiency where standard supplementation strategies may be less effective and where intravenous iron may be required.
Response to oral iron also varies by gastrointestinal function. Individuals with coeliac disease, inflammatory bowel disease, or atrophic gastritis may absorb iron poorly and may require higher doses, intravenous administration, or treatment of the underlying condition before deficiency can be corrected.
Testing and status assessment
Serum ferritin is the primary test used to assess iron stores and is the most useful single marker in clinical practice. A ferritin below 30 µg/L is generally consistent with iron depletion; below 12–15 µg/L is diagnostic of absent iron stores. Values between 30 and 100 µg/L represent a zone where context matters, this range may indicate early depletion in symptomatic individuals, or adequate stores in asymptomatic ones. A ferritin above 100 µg/L makes iron deficiency unlikely in most circumstances.
Ferritin has a critical limitation: it is an acute phase reactant, meaning it rises during infection, inflammation, liver disease, and malignancy independently of iron stores. In the context of chronic inflammatory conditions, ferritin may appear normal or elevated even when iron stores are genuinely depleted. Where inflammation is suspected, additional markers are needed. Transferrin saturation below 20% alongside elevated ferritin may indicate functional iron deficiency or anaemia of chronic disease. Soluble transferrin receptor (sTfR) is less influenced by inflammation and can help distinguish true iron deficiency from inflammatory anaemia, though it is less commonly measured in routine practice.
Haemoglobin and a full blood count are used to identify and characterise anaemia. Iron-deficiency anaemia produces a microcytic, hypochromic pattern (low MCV, low MCH), though this is not universal in early deficiency. Where the picture is ambiguous, for example, combined iron and B12 deficiency, haemoglobin alone can be misleading.
For most people considering iron supplementation, at minimum serum ferritin and haemoglobin should be tested before starting, both to confirm deficiency and to establish a baseline for monitoring response. Testing should be repeated after eight to twelve weeks of supplementation to assess response. In populations at higher risk, pregnant individuals, those with heavy menstrual bleeding, distance athletes, plant-based eaters with symptoms, periodic testing is warranted even in the absence of symptoms, as deficiency can develop gradually.
Safety
Oral iron at standard doses (40–100 mg elemental iron daily) is generally well tolerated in deficient individuals, though gastrointestinal side effects are common. Constipation, nausea, abdominal discomfort, and dark stools affect a meaningful proportion of users and are a frequent cause of non-adherence. Taking iron with food reduces these effects but also reduces absorption by approximately 40–50%. Alternate-day dosing is supported by evidence as a strategy that may improve net absorption while reducing gastrointestinal burden. Ferrous gluconate is sometimes better tolerated than ferrous sulfate at equivalent elemental iron doses.
Iron overload is a serious risk with inappropriate or prolonged supplementation, particularly in individuals with hereditary haemochromatosis. Symptoms of iron overload include fatigue, joint pain, and in later stages liver cirrhosis, diabetes, and cardiac arrhythmia. Individuals with confirmed or suspected HFE mutations, or a family history of haemochromatosis, should not supplement without medical supervision and monitoring of ferritin and transferrin saturation.
Drug interactions require attention. Iron significantly reduces the absorption of levothyroxine, quinolone antibiotics (including ciprofloxacin and levofloxacin), tetracycline antibiotics, and levodopa; a minimum two-hour separation is recommended for all of these. Proton pump inhibitors and H2 receptor antagonists reduce gastric acidity and impair iron absorption with prolonged use, which can complicate management in patients who take both medications long-term. Antacids containing calcium or magnesium similarly impair absorption and should be taken separately.
Intravenous iron carries a small risk of hypersensitivity and, rarely, anaphylactic reactions. It should be administered only in clinical settings with appropriate monitoring capability. Pregnancy-specific safety considerations apply: high-dose supplementation in replete pregnant individuals is not recommended, and management should be guided by blood tests and clinical assessment throughout pregnancy and the postpartum period.
What can reasonably be concluded
Iron supplementation has a clearly established role in the correction of iron deficiency and iron-deficiency anaemia. The evidence base is large, consistent, and clinically meaningful: haemoglobin rises, iron stores replenish, and symptoms resolve in deficient individuals. For specific clinical populations, those with iron-deficient heart failure, restless legs syndrome with low ferritin, or iron deficiency in pregnancy, additional benefits are supported by moderate-quality evidence, though the route of administration and clinical context determine which evidence applies.
Beyond these indications, the evidence does not support iron supplementation. The absence of benefit in replete adults is not simply a gap in evidence, it reflects a well-understood physiological mechanism whereby the body tightly regulates iron absorption based on need. When that need is absent, supplementation neither improves nor protects; it exposes individuals to unnecessary risk. Iron is not a general energy, performance, or cognitive supplement, and should not be framed as one.
The most important clinical implication is that iron supplementation should always be guided by blood test results and clinical assessment. Symptoms associated with iron deficiency, fatigue, reduced exercise capacity, poor concentration, are common and non-specific. Supplementing based on symptoms alone, without testing, risks treating the wrong cause and missing an important diagnostic opportunity. Where deficiency is confirmed, the decision to supplement is well-supported by evidence. Where it is not, supplementation is not recommended.
Where evidence is limited or outcomes are uncertain, conclusions should be treated as provisional and subject to revision as the evidence base develops.