Iron is an essential mineral that functions as the catalytic centre of haemoglobin (the oxygen-carrying protein of red blood cells), myoglobin (the oxygen-storage protein of muscle), and the cytochromes of the mitochondrial electron transport chain. It is also a cofactor for the enzymes of the TCA cycle (including aconitase) and for the enzymes required for the synthesis of dopamine, noradrenaline, and other neurotransmitters. Iron deficiency — the most common nutritional deficiency in the world, affecting approximately 25% of the global population — produces a characteristic pattern of symptoms that includes fatigue, exercise intolerance, cognitive impairment, restless leg syndrome, and immune dysfunction, all of which are directly attributable to the impaired oxygen-carrying capacity of the blood, the impaired mitochondrial energy production in tissues with high metabolic demands, and the impaired synthesis of dopamine and other neurotransmitters that require iron as a cofactor. This comprehensive iron deficiency syndrome is one of the most treatable causes of fatigue and cognitive impairment in clinical practice, yet it remains chronically underdiagnosed in developed countries where iron-deficient diets and iron-loss menstruation create a persistent prevalence of iron deficiency in women of reproductive age and in athletes.
Iron Metabolism and the Haematocrit System
Iron metabolism is tightly regulated at the level of absorption (in the duodenum), transport (in plasma bound to transferrin), storage (in ferritin), and recycling (by macrophages that phagocytose aged red blood cells). The average adult body contains approximately 3-4 grams of iron, of which approximately 2.5 grams is in haemoglobin, 0.5 grams is in storage (ferritin and haemosiderin), 0.3 grams is in myoglobin, and the remainder is in the cytochromes and enzymes of various tissues. The body recycles iron with extraordinary efficiency — approximately 95% of the daily iron requirement (approximately 20-25mg) is supplied by the recycling of iron from aged red blood cells by macrophages in the spleen, liver, and bone marrow, with only approximately 1-2mg absorbed from the diet each day to balance menstrual and dermal losses.
The key regulatory hormone is hepcidin — a 25-amino acid peptide produced by the liver that is the master regulator of iron homeostasis. When iron stores are adequate, hepcidin levels increase and suppress further iron absorption by degrading the ferroportin exporter on enterocytes and macrophages, trapping iron in storage sites. When iron stores are depleted (as in iron deficiency anaemia) or when erythropoietic demand increases (as during recovery from blood loss, during high-altitude acclimatisation, or in response to erythropoietin-stimulating agents), hepcidin levels fall, ferroportin levels increase, and iron absorption and recycling are increased to meet the demand. This regulatory system explains why iron supplementation is less effective when hepcidin is elevated (as in chronic inflammation, which upregulates hepcidin and traps iron in storage) and why iron deficiency is so common in people with chronic inflammatory conditions including obesity, metabolic syndrome, and autoimmune disease.
Stages of Iron Deficiency
Iron deficiency progresses through three clinically distinct stages before it manifests as frank anaemia. The first stage is iron depletion — characterised by low ferritin (the storage form of iron) with normal serum iron and haemoglobin. At this stage, there are typically no symptoms, but iron stores are exhausted and the stage is set for further progression. The second stage is iron-deficient erythropoiesis — characterised by low serum iron and low transferrin saturation (the percentage of transferrin that is carrying iron) with normal haemoglobin. At this stage, functional impairments begin to appear even though anaemia has not yet developed: reduced exercise tolerance (the limiting factor in aerobic performance at altitude and in iron-deficient athletes), impaired cognitive function (particularly attention and concentration), reduced work capacity, and early signs of fatigue. The third stage is frank iron deficiency anaemia — characterised by low haemoglobin, low haematocrit, and low MCV (mean corpuscular volume, indicating microcytic or small red blood cells). The clinical manifestations at this stage include severe fatigue, pallor, shortness of breath on exertion, rapid heart rate, impaired immune function, impaired cognitive function, pica (the consumption of non-nutritive substances), restless leg syndrome, and in children, impaired neurodevelopmental outcomes.
Iron and Athletic Performance
Iron is critical for athletic performance through multiple mechanisms. Haemoglobin carries oxygen from the lungs to the working muscles; myoglobin stores oxygen in the muscle for use during high-intensity effort; and the cytochromes of the mitochondrial electron transport chain require iron for the final step of aerobic energy production. Athletes have higher iron requirements than sedentary individuals due to the increased destruction of red blood cells during repeated impact (foot-strike haemolysis in runners), increased sweat losses, and increased erythropoiesis in response to training. Female athletes and athletes in endurance sports (where the oxygen-carrying demands are highest) are at the greatest risk of iron deficiency. Studies in elite athletes show that iron deficiency without anaemia (low ferritin with normal haemoglobin) is associated with a measurable decline in aerobic capacity (VO2 max) and time trial performance — and that iron supplementation in iron-deficient athletes improves performance even when haemoglobin is in the normal range.
Clinical Evidence for Iron Supplementation
Iron supplementation in iron-deficient individuals produces consistent and clinically meaningful improvements in multiple outcomes. A meta-analysis of 32 RCTs in women with iron deficiency anaemia found that iron supplementation significantly increased haemoglobin and ferritin levels, with corresponding improvements in fatigue, exercise capacity, and cognitive function that were apparent within 2-4 weeks of initiating treatment. Studies in women with suboptimal iron levels (ferritin below 50mcg/L but haemoglobin in the normal range) found that iron supplementation improved cognitive function (particularly attention and working memory), reduced fatigue, and improved exercise performance — even in the absence of frank anaemia. Iron supplementation has also been studied for its effects on restless leg syndrome (RLS), which is strongly associated with low ferritin levels: a double-blind RCT in 58 patients with RLS found that iron supplementation at 1,000mg daily (as ferrous sulfate) for 12 weeks significantly reduced RLS symptom severity scores compared to placebo, with the greatest benefits in patients with the lowest baseline ferritin levels. A follow-up study found that maintaining ferritin above 75mcg/L was associated with sustained RLS symptom control, suggesting that long-term iron maintenance may be necessary for RLS management in iron-deficient patients.
Practical Application
For iron deficiency, the evidence-based dose is 60-100mg of elemental iron daily, taken on an empty stomach for optimal absorption. The most commonly used forms are ferrous sulfate (20% elemental iron by weight) and ferrous gluconate (12% elemental iron). Iron bisglycinate is a chelated form that is better absorbed and produces less GI upset than the inorganic forms, making it preferable for people with sensitive stomachs or for those who require long-term maintenance supplementation. Iron should always be taken with vitamin C (which enhances iron absorption by keeping iron in the ferrous state and acting as a chelator that prevents the formation of insoluble iron complexes) and should not be taken with calcium, coffee, or tea (which inhibit iron absorption by forming insoluble complexes with iron). For comprehensive iron management, ferritin should be measured before starting supplementation — iron supplementation is only indicated when ferritin is below approximately 50-75mcg/L, and should not be used when ferritin is in the normal or elevated range. The goal of iron supplementation is to raise ferritin to the optimal range of 75-150mcg/L and then transition to a maintenance dose or to dietary iron management alone.
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