Standard blood panels declare iron status normal when haemoglobin falls within range. But haemoglobin is the last thing to drop when iron becomes depleted — it is a downstream marker that only falls after stores are exhausted. By the time anaemia appears on a standard panel, you have been functionally iron deficient for months or years.
The Three Compartments of Iron
Iron in the body exists in three pools. The first is haemoglobin — approximately 2g circulating in red blood cells, delivering oxygen to tissues. The second is myoglobin — roughly 300mg in muscle cells, storing oxygen for immediate use. The third is storage iron, bound to ferritin in the liver, spleen, and bone marrow. The transferrin saturation percentage represents iron in transit between these compartments.
Ferritin is the storage compartment. When ferritin falls below approximately 50 ng/mL (the lower end of the reference range), iron supply to tissues begins to be compromised — even before haemoglobin changes. Symptoms appear: fatigue, exercise intolerance, cognitive fog, impaired thermogenesis, reduced immune function, and restless legs syndrome.
Why Reference Ranges Are Misleading
The reference range for ferritin typically spans 20-300 ng/mL for men and 20-200 ng/mL for women. But these are population averages, not optimal values. They include people with chronic inflammation, infection, and subclinical disease — all of which elevate ferritin — dragging the range upward and masking the low end that represents functional deficiency in healthy people.
Functional medicine practitioners generally consider ferritin suboptimal below 50-75 ng/mL for energy and cognitive function, and optimal in the 100-150 range for athletes and women of reproductive age. Iron deficiency without anaemia — sometimes called “ferritin deficiency” — responds to iron supplementation at doses that would never move a haemoglobin needle.
Who Is at Risk
Athletes, particularly runners and cyclists, lose iron through foot-strike haemolysis (repeated impact destroying red blood cells in the feet), sweating, and GI microbleeding. Women of reproductive age lose iron monthly through menstruation. People on plant-based diets absorb only 2-3mg of iron per day from food even when intake is adequate, because plant iron (non-heme) has approximately 15% the bioavailability of heme iron from meat. Anyone who has ever taken proton pump inhibitors long-term has impaired iron absorption because stomach acid is required for iron solubilisation.
Testing and Interpreting
Request serum ferritin, transferrin saturation, and serum iron — not just haemoglobin. A ferritin below 50 ng/mL with normal haemoglobin is iron deficiency without anaemia. The fix is straightforward: 15-30mg of elemental iron daily as ferrous bisglycinate or ferrous sulfate with vitamin C. Ferrous bisglycinate is gentler on the gut and better absorbed, making it preferable for anyone with sensitive digestion. Vitamin C at 200-500mg dramatically enhances iron absorption by keeping iron in the ferrous (absorbable) state.
Recheck after three months. Do not take iron with calcium, polyphenols (coffee, tea, red wine), or phytates (grains, legumes) — all inhibit absorption. Take in the morning on an empty stomach for maximum absorption.
Iron Role in Brain Energy Metabolism
Iron is essential for brain function far beyond its role in haemoglobin and oxygen transport. The brain consumes approximately 20% of the body oxygen despite accounting for only 2% of body weight, and iron is critical in this energy metabolism — particularly in the electron transport chain within mitochondria, where iron-sulfur clusters are essential components of Complexes I, II, and III. Iron is also a cofactor for tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, and for ribonucleotide reductase, the enzyme required for DNA synthesis. These roles mean that iron deficiency — even without frank anaemia — can impair dopaminergic signalling, reduce neural energy production, and compromise myelin formation, with measurable effects on attention, memory, and executive function.
Why Iron Deficiency Is So Common
Iron deficiency is the most common nutritional deficiency worldwide, affecting an estimated 2 billion people. In menstruating women, iron deficiency is particularly prevalent due to monthly menstrual blood loss — even a “normal” menstrual iron loss of 30-40ml per cycle can gradually deplete iron stores over months to years. In men and post-menopausal women, iron deficiency should always be investigated as it can signal occult gastrointestinal blood loss. The symptoms of iron deficiency extend well beyond fatigue and pallor: restless legs syndrome (strongly associated with brain iron deficiency), impaired thermoregulation, reduced exercise tolerance, and cognitive impairment in both children and adults.
Iron Status: Not Just Haemoglobin
The standard diagnostic marker for iron deficiency is haemoglobin — but this misses the majority of iron-deficient people, because haemoglobin only falls after iron stores (ferritin) are already significantly depleted. Ferritin is the storage form of iron, and a level below 30 ng/mL indicates depleted stores, while anything below 15 ng/mL indicates frank deficiency. Optimal ferritin for cognitive function appears to be in the range of 50-100 ng/mL. Iron supplementation should always be guided by ferritin testing, not haemoglobin alone, and excessive iron (from over-supplementation or haemochromatosis) carries its own serious risks including liver cirrhosis and increased infection risk through iron-dependent pathogen growth.
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