Can low iron stores cause fatigue and reduced exercise tolerance even without anemia?

Iron is a fundamental cofactor for both systemic oxygen delivery and cellular energy production. Depleted iron stores, indicated by low serum ferritin, can impair these processes and contribute to symptoms like fatigue and reduced physical capacity, even when hemoglobin levels remain within the normal range (non-anemic iron deficiency).

Mechanistic basis

Iron is biochemically indispensable for mitochondrial function and oxygen transport.

• Electron Transport Chain (ETC): Iron-sulfur (Fe-S) clusters are required for the one-electron transfer reactions in Complexes I, II, and III of the ETC. These reactions are essential for electron flow from NADH and FADH₂ toward oxygen.
• ATP Synthesis: Complex IV (cytochrome c oxidase) requires iron in the form of heme $a$ and $a_3$ to reduce molecular oxygen to water, which drives the proton pump necessary for ATP production.
• Cellular Impact: Research indicates that iron depletion reduces the activity of respiratory pathways and can transcriptionally repress genes necessary for mitochondrial biogenesis, leading to decreased oxidative phosphorylation capacity.

Clinical evidence and fatigue

There is robust evidence that low ferritin levels are a clinically valid driver of fatigue, even without anemia.

• Fatigue Reduction: Meta-analyses of randomized controlled trials (RCTs) involving adults with non-anemic iron deficiency (IDNA) show that iron supplementation significantly reduces subjective fatigue. The effect size (standardized mean difference) is approximately -0.41, with the most pronounced benefits seen in individuals with ferritin levels below 50 μg/L.
• Exercise Tolerance: While the link to fatigue is strong, evidence for objective improvements in exercise tolerance is more complex. While iron is a critical cofactor for enzymes involved in aerobic metabolism, meta-analyses suggest that supplementing non-anemic individuals does not consistently result in measurable increases in objective physical performance, despite improvements in subjective energy levels.

Bottom line

Iron is essential for the mitochondrial enzymes that produce energy and the hemoglobin that transports oxygen. Depleted iron stores (low ferritin) are a proven cause of fatigue in non-anemic individuals, likely due to impaired mitochondrial efficiency. While correcting these stores often improves subjective energy, objective improvements in exercise capacity are less consistently observed in clinical studies.