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Clinical Journal of Sport Medicine:
Sport Medicine Journal Club

Iron Supplementation to Improve Endurance in Iron-Depleted, Nonanemic Women

Hart, Lawrence E. MBBCh, MSc; Matheson, Gordon O. MD, PhD

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Stanford University School of Medicine Stanford, California, U.S.A.

Associate Editor:

Mark Tarnopolsky, MD, PhD

Editorial Associate:

Ann Lotter

Hinton PS, Giordano C, Brownlie T, Haas JD. Iron supplementation improves endurance after training in iron-depleted, nonanemic women. J Appl Physiol 2000;88:1103–1111.

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ObjectiveIron Supplementation to Improve Endurance in Iron-Depleted, Nonanemic Women

To determine whether iron supplementation is associated with an increase in endurance capacity in nonanemic women in response to aerobic training on a cycle ergometer.

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DesignIron Supplementation to Improve Endurance in Iron-Depleted, Nonanemic Women

Randomized double-blind placebo-controlled 6-week trial.

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SettingIron Supplementation to Improve Endurance in Iron-Depleted, Nonanemic Women

Community study that took place in an exercise laboratory in New York State, U.S.A.

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ParticipantsIron Supplementation to Improve Endurance in Iron-Depleted, Nonanemic Women

49 physically active, untrained, 19-to-35-year-old women, who were iron depleted (serum ferritin concentrations <16 μg/L) but not anemic (Hb concentrations >120 g/L), were selected from 271 women who were recruited and screened for iron depletion. Exclusion criteria were current or recent pregnancy; recent infectious illness, fever, history of eating disorders, or use of recreational drugs; hemolytic anemia, asthma, musculoskeletal problems, smoking, excessive alcohol consumption, consumption of medications that may interfere with dietary iron absorption, or participation in competitive athletics.

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InterventionIron Supplementation to Improve Endurance in Iron-Depleted, Nonanemic Women

Participants were randomized to an iron supplement of 50 mg of ferrous sulfate (10 mg Fe) or an identical placebo capsule twice a day for 6 weeks to be consumed with citrus juice at meals. They were asked to take no other supplements, to maintain their normal diet and level of physical activity, and not to do any strenuous activities for 2 days before the cycle time trials. A 4-week exercise training regimen (5 sessions per week) began after 2 weeks of supplementation. Training was done on a cycle ergometer and comprised a 4-min warm-up followed by a 25-min cycling session divided between workloads that allowed participants to achieve 75% and 85% of maximum heart rate.

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Main outcome measuresIron Supplementation to Improve Endurance in Iron-Depleted, Nonanemic Women

The primary measure was change in minutes required to cycle 15 km at an individually standardized resistance. Response to iron treatment was monitored after 3 and 6 weeks, and related to physical performance. Respiratory variables were calculated from expired gas measurements. 42 women (86%) were included in the analysis.

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Main resultsIron Supplementation to Improve Endurance in Iron-Depleted, Nonanemic Women

Mean time (SE) to complete the 15-km time trial after training decreased less from baseline in the placebo group (31.9 [0.5] to 30.3 [0.7] min) than in the iron supplementation group (33.0 [0.5] to 29.6 [0.6] min; p for difference < 0.05). The improved time in the supplemented group was primarily due to faster times in the second (p = 0.082) and third (p = 0.027) 5-km segments of the trial. Both groups improved V̇O2max, respiratory exchange ratio, work rate, and time to complete the trial, suggesting a positive effect of the 4-week training. Increases in mean levels of serum ferritin, serum iron, and transferrin saturation, and decreases in serum transferrin receptor, were greater for the supplementation group (p < 0.05). Multiple regression analysis showed that decrease in time in the 15-km trial was independently related to iron status indicators.

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ConclusionIron Supplementation to Improve Endurance in Iron-Depleted, Nonanemic Women

Iron supplementation was associated with an improved response to endurance training in iron-depleted, nonanemic women.

Source of funding (in part): Mead Johnson Research Fund and National Institute of Child Health and Human Development.

For article reprint: J. D. Haas, Division of Nutritional Sciences, 127 Savage Hall, Cornell University, Ithaca, NY 14853-6301, U.S.A. E-mail: jdh12@cornell.edu

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CommentaryIron Supplementation to Improve Endurance in Iron-Depleted, Nonanemic Women

The relationship between endurance performance and the oxygen carrying capacity of the blood is of concern to female athletes. Impaired endurance capacity as a result of frank anemia (low hemoglobin levels) has an obvious physiologic explanation. The question remains as to whether iron store deficiencies can reduce performance on the basis of concomitant reductions in iron-dependent mitochondrial enzyme and respiratory chain cytochrome activities. 1

In a well-designed study of women with low serum ferritin values, Hinton et al. found greater reductions in 15-km cycling time after a training regimen in the women who received iron supplementation. This requires a careful analysis of the factors that could have contributed to this result. One consideration is that most people can tell when they are taking iron supplements, thus the possibility of a voluntary effect exists. Another possible explanation is that the supplemented group were less fit and had longer 15-km cycle times at baseline and might be expected to show greater levels of improvement following training even without supplementation. Finally, the iron supplemented group had very slight increases in hemoglobin levels under treatment while the nonsupplemented group had slight decreases in hemoglobin levels. The greater reduction in 15-km cycle time after training and iron supplementation was small and it is difficult to judge the clinical significance of this result.

The study lends credibility to the hypothesis that low serum ferritin, in the absence of frank anemia, may contribute to reduced endurance performance but stops short of being able to confirm the hypothesis.

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Iron Supplementation to Improve Endurance in Iron-Depleted, Nonanemic Women

1. Garza D, Shrier I, Kohl HW III, et al. The clinical value of serum ferritin tests in endurance athletes. Clin J Sport Med;1997;7:46–53.

© 2001 Lippincott Williams & Wilkins, Inc.

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