Does iron deficiency without anemia cause fatigue and reduced exercise capacity?

Clinical and performance evidence

• Subjective fatigue reduction: Clinical evidence from randomized controlled trials demonstrates that iron deficiency without anemia (IDWA)—defined by serum ferritin ≤50 µg/L with normal hemoglobin levels—is strongly associated with subjective fatigue. Repletion with oral iron therapy in fatigued, non-anemic women has shown significant improvements in self-reported fatigue scales, highlighting that iron stores impact systemic vitality before affecting red blood cell counts.
• Impaired aerobic capacity: In active populations and athletes, IDWA is associated with a 3% to 4% decrement in endurance performance. Epidemiological and clinical trials demonstrate that depleted iron stores impair maximal aerobic capacity ($\text{VO}_2\text{max}$), which can be partially or fully restored with targeted iron supplementation.

Mechanistic explanations

• Mitochondrial dysfunction: Because iron is an essential cofactor for iron-sulfur clusters and heme groups, IDWA directly compromises mitochondrial respiration. It reduces the activity of Complex I, II, and III of the electron transport chain and impairs critical tricarboxylic acid (TCA) cycle enzymes, such as aconitase.
• Impaired skeletal muscle energetics: Intracellular iron depletion down-regulates mitochondrial biogenesis and limits the synthesis of myoglobin, the primary oxygen-binding protein in skeletal muscle. This restriction in intramuscular oxygen transport and oxidative ATP generation forces a premature reliance on anaerobic glycolysis, leading to rapid lactate accumulation, muscle fatigue, and diminished physical work capacity.

Bottom line

• Iron deficiency without anemia is a clinically significant condition that impairs physical performance and increases subjective fatigue. These deficits are driven by mitochondrial respiration failure and reduced intramuscular oxygen transport, which occur independently of hemoglobin levels and are highly responsive to targeted iron repletion.