Iron is a required nutrient involved in a number of metabolic reactions in the body, including oxygen delivery to the tissues, energy metabolism, and thermoregulation (8). Iron depletion and iron-deficiency anemia can cause physiological changes in the body not only during exercise but also under resting conditions. The focus of this Nutritionist's View column focuses on the effects of iron on athletic performance.
BRIEF BACKGROUND ON IRON
Iron is a required trace mineral found in every living cell in the body. The normal amount of iron found in blood ranges from 20 to 52 μg/dL. Iron's main role is to transport oxygen in red blood cells and tissues, and it does so mainly through hemoglobin. Iron also acts as an antioxidant in the body (8).
IRON-DEFICIENCY ANEMIA
Iron-deficiency anemia can manifest itself quickly (e.g., due to gastrointestinal bleeding) or over time. When it manifests itself over time, it can develop over three main stages. The first stage is called "iron depletion," the second stage is called "iron deficiency without anemia," and the third stage is called "iron-deficiency anemia." Each of these stages is defined by changes in blood markers related to iron. With iron-deficiency anemia, the "classic" blood markers are below normal levels of hemoglobin and hematocrit, as well as low mean corpuscular volume. Some symptoms of iron-deficiency anemia include muscle weakness, fatigue (being tired with minimal exertion and poor physical performance), impaired body temperature, and impaired cognitive performance.
Iron-deficiency anemia is more common among female athletes than male athletes. It is usually a result of one or more of the following: low iron intake, high demand for iron because of high training, and/or high iron loss (from foot strike hemolysis [blood cells breaking from running], menstruation, or injury).
RECOMMENDED IRON INTAKES AND FOOD SOURCES
The Recommended Dietary Allowance for iron is 18 mg/day for female subjects, 19 to 50 years of age, and 8 mg/day for male subjects, 19 to 70 years of age or older. The upper limit for iron is 45 mg/day (e.g., a person should not strive to take in this much and should not consume amounts above this level) (2).
A person can obtain iron from food in two different forms: heme and nonheme. Heme iron is derived from hemoglobin, the protein in red blood cells that delivers oxygen to cells. Foods of animal origin have heme iron because these foods contained hemoglobin originally. Dietary iron from plant foods is called nonheme iron. Heme iron is absorbed better than nonheme iron because heme iron can be absorbed directly into the small intestine, whereas nonheme iron requires a carrier. See Tables 1 and 2 for a list of heme and nonheme iron foods (3,6).
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TABLE 1: Selected Food Sources of Heme Iron
TABLE 2: Selected Food Sources of Nonheme Iron
Factors that enhance iron absorption include consuming foods high in iron, consuming vitamin C foods with low- or high-iron foods, and high body demands for iron (e.g., training at high altitude, blood loss, exercise training, pregnancy, and low body stores of iron).
Factors that will inhibit iron absorption include consuming tea or coffee with a meal (wait at least an hour before or after consuming a meal to drink coffee or tea, if you are iron deficient anemic); full body stores of iron; and consumption (supplementation) with high levels of zinc, calcium, or manganese and some antacids.
IRON AND ATHLETIC PERFORMANCE
Di Santolo et al. (1) and Ostojic and Ahmetovic (7) reported a high prevalence of iron-deficiency anemia and iron depletion among elite and recreational female athletes of different sports. In a randomized, double-blind, placebo-controlled study, McClung et al. (4) examined if iron supplementation would improve physical performance in female soldiers during basic training. The researchers found that basic training affected iron status, and iron supplementation corrected the problem. The female soldiers, who had iron-deficiency anemia upon starting basic training, had improved running scores because of the supplementation. Although McClung etal. (4) reported improvements in physical performance and iron status in their sample, it is important to note that, when individuals of normal iron status are supplemented with iron, no improvement in iron status or performance is seen. Furthermore, Mettler and Zimmermann (5) reported iron excess in 170 male and female recreational marathon runners who participated in the Zurich marathon. Although they found that 28% of the women were iron depleted, less than 2% of the men were iron depleted. They also found that iron supplementation was common among the athletes with the hope of improving performance and that iron excess was present in many of the male runners. Iron excess can be equally as detrimental to health as iron-deficiency anemia. It is important to tell clients that "more is not better" when it comes to nutritional supplements.
SUMMARY
Iron is a required trace mineral involved in numerous reactions in the body. Its main function is in oxygen transport; thus, if a person has iron-deficiency anemia, athletic performance will suffer. Iron supplementation will only be of help to a person who is in one of the stages of iron-deficiency anemia. Iron supplementation will not help performance in a person of normal iron status and may even lead to iron overload.
References
1. Di Santolo M, Stel G, Banfi G, Gonano F, Cauci S. Anemia and iron status in young fertile non-professional female athletes.
Eur J Appl Physiol. 2008;102(6):703-9.
2. Food and Nutrition Board of the Institute of Medicine. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington (DC): National Academy Press; 2000.
3. Hurrell RF. Preventing iron deficiency through food fortification.
Nutr Rev. 1997;55:210-22.
4. McClung JP, Karl JP, Cable SJ,
et al. Randomized, double-blind, placebo-controlled trial of iron supplementation in female soldiers during military training: effects on iron status, physical performance, and mood.
Am J Clin Nutr. 2009;90(1):124-31.
5. Mettler S, Zimmermann MB. Iron excess in recreational marathon runners.
Eur J Clin Nutr. 2010. [Epub ahead of print].
6. Miret S, Simpson RJ, McKie AT. Physiology and molecular biology of dietary iron absorption.
Annu Rev Nutr. 2003;23:283-301.
7. Ostojic SM, Ahmetovic Z. Weekly training volume and hematological status in female top-level athletes of different sports.
J Sports Med Phys Fitness. 2008;48(3):398-403.
8. Rosenzweig PH, Volpe SL. Iron, thermoregulation, and metabolic rate.
Crit Rev Food Sci Nutr. 1999;39(2):131-48.
9. U.S. Department of Agriculture, Agricultural Research Service. 2003. USDA Nutrient Database for Standard Reference, Release 16. Nutrient Data Laboratory Home Page [Internet]. Available from:
http://www.nal.usda.gov/fnic/foodcomp.
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