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Iron

Iron

Iron is an essential mineral and a structural component of hemoglobin, an erythrocyte protein that transfers oxygen from the lungs to the tissues, and myoglobin, a protein in muscles that provides them with oxygen. Iron is also necessary to metabolize substrates for energy as a component of cytochromes and to dehydrogenase enzymes involved in substrate oxidation [144]. Iron deficiency impairs oxygen-carrying capacity and muscle function, and it limits people’s ability to exercise and be active [12,145]. Its detrimental effects can include fatigue and lethargy, lower aerobic capacity, and slower times in performance trials [146].

Iron balance is an important consideration for athletes who must pay attention to both iron intakes and iron losses. Teenage girls and premenopausal women are at increased risk of obtaining insufficient amounts of iron from their diets. They require more iron than teenage boys and men because they lose considerable iron due to menstruation, and they might not eat sufficient amounts of iron-containing foods [147,148].

Athletes of both sexes lose additional iron for several reasons [145,146,149,150]. Physical activity produces acute inflammation that reduces iron absorption from the gut and iron use via a peptide, hepcidin, that regulates iron homeostasis. Iron is also lost in sweat. The destruction of erythrocytes in the feet because of frequent striking on hard surfaces leads to foot-strike hemolysis. Also, use of antiinflammatories and pain medications can lead to some blood loss from the gastrointestinal tract, thereby decreasing iron stores.

The richest dietary sources of heme iron (which is highly bioavailable) include lean meats and seafood. Plant-based foods—such as nuts, beans, vegetables, and fortified grain products—contain non-heme iron, which is less bioavailable than heme iron.

Efficacy: Although iron deficiency anemia decreases work capacity, there is conflicting evidence on whether milder iron deficiency without anemia impairs sport and exercise performance [12,151,152]. One systematic review and meta-analysis to determine whether iron treatments (provided orally or by injection) improved iron status and aerobic capacity in iron-deficient but non- anemic endurance athletes identified 19 studies involving 80 men and 363 women with a mean age of 22 years. Iron treatments improved iron status as expected, but they did not guarantee improvement in aerobic capacity or indices of endurance performance [153]. Another systematic review and meta-analysis compared the effects of iron supplementation with no supplementation on exercise performance in women of reproductive age [149]. Most of the 24 studies identified were small (i.e., they randomly assigned fewer than 20 women to a treatment or control group) and had a risk of bias. Based on the limited data and heterogenicity of results, the study authors suggested that preventing and treating iron deficiency could improve the performance of female athletes in sports that require endurance, maximal power output, and strength.

Safety: Athletes can safely obtain recommended intakes of iron by consuming a healthy diet containing iron-rich foods and by taking an iron-containing dietary supplement as needed. High doses of iron may be prescribed for several weeks or months to treat iron deficiency, especially if anemia is present.

The UL for iron is 45 mg/day for men and women aged 14 and older and 40 mg/day for younger children [147]. Acute intakes of more than 20 mg/kg iron from supplements or medicines can lead to gastric upset, constipation, nausea, abdominal pain, vomiting, and fainting, especially if users do not consume food at the same time [147,150]. Individuals with hereditary hemochromatosis, which predisposes them to absorb excessive amounts of dietary and supplemental iron, have an increased risk of iron overload [154].

Implications for use: Correcting iron deficiency anemia improves work capacity, but there is conflicting evidence on whether milder iron deficiency without anemia impairs athletic performance. In a position statement, AND, DoC, and ACSM do not recommend routine supplementation of iron except in response to a healthcare provider’s instruction, and note that such supplementation is only ergogenic if the individual has iron depletion [12]. Furthermore, they warn that iron supplementation can cause gastrointestinal side effects.

The recommended dietary allowance (RDA) for iron is 11 mg for teenaged boys and 15 mg for teenaged girls [147]. The RDA is 8 mg for men and 18 mg for women aged 50 and younger, and 8 mg for older adults of both sexes. Individuals who engage in intense exercise might require 30% to 70% more iron than moderately active and sedentary people [147]. Recommended intakes of iron for vegetarians and vegans are 1.8 times higher than for people who eat meat [147].

More information on iron and the treatment of iron-deficiency anemia is available in the ODS health professional fact sheet on iron.