Background: We previously showed that iron supplementation significantly improves iron status and maximal work capacity in previously untrained, marginally iron-deficient women with a baseline serum transferrin receptor concentration > 8.0 mg/L. However, the effect of transferrin receptor status on adaptation in endurance capacity after aerobic training in these subjects has not been fully explored. Objective: Our objective was to examine the effect of baseline serum transferrin receptor status on adaptations in endurance capacity. Design: Forty-one untrained, iron-depleted, nonanemic women were randomly assigned to receive either 100 mg FeSO₄ or a placebo for 6 wk in a double-blind trial. All subjects trained on cycle ergometers 5 d/wk for the last 4 wk of the study. Endurance capacity was assessed at baseline and after treatment by using a 15-km time trial conducted on a cycle ergometer. Results: Significant treatment effects were observed for time to complete the 15-km time trial, work rate, and percentage of maximal oxygen uptake in subjects with a baseline serum transferrin receptor concentration > 8.0 mg/L. No significant treatment effects were observed in subjects with a normal baseline transferrin receptor concentration. Conclusions: Our findings suggest that, in the presence of overt tissue iron deficiency, iron deficiency without anemia impairs adaptation in endurance capacity after aerobic training in previously untrained women. This impairment can be corrected with iron supplementation.

Our objective was to investigate the effects of iron depletion on adaptation to aerobic exercise, assessed by time to complete a 15-km cycle ergometer test. Forty-two iron-depleted (serum ferritin 12 g/dl) women (18–33 yr old) received 100 mg of ferrous sulfate (S) or placebo (P) per day for 6 wk in a randomized, double-blind trial. Subjects trained for 30 min/day, 5 days/wk at 75–85% of maximum heart rate for the final 4 wk of the study. There were no group differences in baseline iron status or in 15-km time. Iron supplementation increased serum ferritin and decreased transferrin receptors in the S compared with the P group. The S and P groups decreased 15-km time and respiratory exchange ratio and increased work rate during the 15-km time trial after training. The decrease in 15-km time was greater in the S than in the P group (P = 0.04) and could be partially attributed to increases in serum ferritin and Hb. These results indicate that iron deficiency without anemia impairs favorable adaptation to aerobic exercise.

Background: Iron deficiency without anemia has been shown to reduce both muscle-tissue oxidative capacity and endurance in animals. However, the consequences of iron deficiency in humans remain unclear. Objective: We investigated the effects of iron supplementation on adaptation to aerobic training among marginally iron-depleted women. We hypothesized that iron supplementation for 6 wk would significantly improve iron status and maximal oxygen uptake (V̇O₂max) after 4 wk of concurrent aerobic training. Design: Forty-one untrained, iron-depleted, nonanemic women were randomly assigned to receive either 50 mg FeSO₄ or a placebo twice daily for 6 wk in a double-blind trial. All subjects trained on cycle ergometers 5 d/wk for 4 wk, beginning on week 3 of the study. Results: Six weeks of iron supplementation significantly improved serum ferritin and serum transferrin receptor (sTfR) concentrations and transferrin saturation without affecting hemoglobin concentrations or hematocrit. Average V̇O₂max and maximal respiratory exchange ratio improved in both the placebo and iron groups after training; however, the iron group experienced significantly greater improvements in V̇O₂max. Both iron-status and fitness outcomes were analyzed after stratifying by baseline sTfR concentration (> and ≤8.0 mg/L), which showed that the previously observed treatment effects were due to iron-status and fitness improvements among subjects with poor baseline iron status. Conclusions: Our findings strongly suggest that iron deficiency without anemia but with elevated sTfR status impairs aerobic adaptation among previously untrained women and that this can be corrected with iron supplementation.

Iron is a trace mineral that is highly significant to endurance athletes. Iron is critical to optimal athletic performance because of its role in energy metabolism, oxygen transport, and acid-base balance. Endurance athletes are at increased risk for suboptimal iron status, with potential negative consequences on performance, because of the combination of increased iron needs and inadequate dietary intake. This review paper summarizes the role of iron in maximal and submaximal exercise and describes the effects of iron deficiency on exercise performance. Mechanisms that explain the increased risk of iron deficiency in endurance athletes, including exercise-associated inflammation and hepcidin release on iron sequestration, are described. Information on screening athletes for iron deficiency is presented, and suggestions to increase iron intake through diet modification or supplemental iron are provided.