['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.', '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 (VO(2)max) after 4 wk of concurrent aerobic training.', 'Forty-one untrained, iron-depleted, nonanemic women were randomly assigned to receive either 50 mg FeSO(4) 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.', '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 VO(2)max and maximal respiratory exchange ratio improved in both the placebo and iron groups after training; however, the iron group experienced significantly greater improvements in VO(2)max. Both iron-status and fitness outcomes were analyzed after stratifying by baseline sTfR concentration (> and < or = 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.'] Does marginal iron deficiency without anemia impair aerobic adaptation among previously untrained women?