Low ferritin with normal hemoglobin indicates depleted iron stores and impairs exercise capacity.

Background

Low ferritin in the presence of a normal hemoglobin concentration represents non-anemic iron deficiency (NAID), also known as latent iron deficiency. This clinical state represents a stage of iron depletion where tissue-level iron needs are compromised before circulating red blood cell production is significantly altered.

Clinical evidence and exercise capacity

• Diagnostic Accuracy: Clinical evidence confirms that serum ferritin is the most sensitive and specific biomarker for total body iron stores. While a threshold of $<15\text{ }\mu\text{g/L}$ has a specificity of up to 99% for depleted bone marrow iron, clinical consensus frequently utilizes a threshold of $<30\text{ }\mu\text{g/L}$ to maximize diagnostic sensitivity for symptomatic patients.
• Exercise Performance: Randomized controlled trials (RCTs) demonstrate that correcting NAID in active women improves aerobic conditioning and muscle performance. For example, in a double-blind RCT of non-anemic iron-depleted women (ferritin $<15\text{ }\mu\text{g/L}$, hemoglobin $\ge 12.0\text{ g/dL}$), iron supplementation significantly reduced muscle fatigue and improved submaximal work efficiency compared to placebo.

Mechanistic explanations

• Mitochondrial Dysfunction: Iron is an indispensable cofactor for iron-sulfur (Fe-S) clusters and heme groups. Depleted iron stores directly compromise the activity of mitochondrial electron transport chain complexes, particularly Complex I (NADH dehydrogenase) and Complex II (succinate dehydrogenase).
• Impaired Oxidative Phosphorylation: This enzymatic limitation impairs oxidative phosphorylation and ATP synthesis in skeletal muscle. This cellular energy deficit forces an earlier shift to anaerobic metabolism during physical exertion, leading to rapid lactic acid accumulation, reduced peak oxygen consumption ($\text{VO}_2$ max), and physical fatigue.

Bottom line

Low ferritin with normal hemoglobin reliably indicates depleted iron stores. This state of non-anemic iron deficiency compromises iron-dependent mitochondrial enzymes and oxidative phosphorylation, directly contributing to clinically significant fatigue and impaired exercise performance.