ObjectiveIt is common practice to measure serum ferritin levels in endurance athletes because of the belief that low iron stores may compromise performance. The direct relationship between endurance performance and iron deficiency anemia is well known, but there are theoretical reasons to believe that endurance performance may be adversely affected by low iron stores even in the absence of frank anemia. The purpose of this article is to provide a critical review of the scientific evidence relating low iron stores to endurance performance.Data sourcesMedline was searched using MeSH for articles related to ferritin and endurance published since 1985. Additional references were reviewed from the bibliographies of the retrieved articles.Study selectionAll clinical study designs were reviewed as well as relevant animal studies. Conclusions regarding endurance performance in humans were limited to data from clinical studies.Data extraction and synthesisIn reviewing the literature, the relative strengths of the study designs were examined carefully. Particular attention of the effectiveness of each study in isolating ferritin as the key independent variable. Dependent measures of endurance capacity were also evaluated.Main resultsEight studies isolated serum ferritin as the experimental variable. Only one study reported a significant improvement in endurance performance (time to exhaustion) in subjects with low ferritin levels treated with oral iron, but this finding may have been magnified by an unexplained decrease in time to exhaustion in the control group. Iron dosages differed in the studies reviewed. Two additional studies that reported increases in performance parameters following increases in ferritin were confounded by concomitant increases in hemoglobin levels.ConclusionsIron supplementation can raise serum ferritin levels, but increases in ferritin concentration, unaccompanied by increases in hemoglobin concentration, have not been shown to increase endurance performance. Of concern to the clinician is that athletes with low ferritin levels but hemoglobin in the low-normal range may have iron deficiency anemia responsive to iron supplementation. Low ferritin with hemoglobin in the mid- to upper normal range is at best a relative indication for iron supplementation; low ferritin with hemoglobin in the low normal range is a stronger, yet still relative, indication for iron supplementation in athletes.