Marginal iron deficiency without anemia impairs aerobic adaptation among previously untrained women

Abstract

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.