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Iron is important for an athlete because it is a component of hemoglobin, which transports oxygen to muscle cells. Some athletes, especially women, do not get enough iron in their diet. In addition, for reasons that are unclear, endurance athletes, such as marathon runners, frequently have low body-iron levels.1, 2, 3 However, anemia in athletes is often not due to iron deficiency and may be a normal adaptation to the stress of exercise.4 Supplementing with iron is usually unwise unless a deficiency has been diagnosed. People who experience undue fatigue (an early warning sign of iron deficiency) should have their iron status evaluated by a doctor. Athletes who are found to be iron deficient by a physician are typically given 100 mg per day until blood tests indicate they are no longer deficient. Supplementing iron-deficient athletes with 100 to 200 mg per day of iron increased aerobic exercise performance in some,5, 6, 7 though not all,8, 9 double-blind studies. A recent double-blind trial found that iron-deficient women who took 20 mg per day of iron for six weeks were able to perform knee strength exercises for a longer time without muscle fatigue compared with those taking a placebo.10
Caution: Iron (ferrous sulfate) is the leading cause of accidental poisonings in children.11, 12, 13 The incidence of iron poisonings in young children increased dramatically in 1986. Many of these children obtained the iron from a child-resistant container opened by themselves or another child, or left open or improperly closed by an adult.14 Deaths in children have occurred from ingesting as little as 200 mg to as much as 5.85 grams of iron.15 Keep iron-containing supplements out of a child's reach.
Hemochromatosis, hemosiderosis, polycythemia, and iron-loading anemias (such as thalassemia and sickle cell anemia) are conditions involving excessive storage of iron. Supplementing iron can be quite dangerous for people with these diseases.
Supplemental amounts required to overcome iron deficiency can cause constipation. Sometimes switching the form of iron (see "Which forms of supplemental iron are best?" above), getting more exercise, or treating the constipation with fiber and fluids is helpful, though fiber can reduce iron absorption (see below). Sometimes the amount of iron must be reduced if constipation occurs.
Some researchers have linked excess iron levels to diabetes,16cancer,17 increased risk of infection,18systemic lupus erythematosus (SLE),19 exacerbation of rheumatoid arthritis,20 and Huntington's disease.21 The greatest concern has surrounded the possibility that excess storage of iron in the body increases the risk of heart disease.22, 23, 24 Two analyses of published studies came to different conclusions about whether iron could increase heart disease risk.25, 26 One trial has suggested that such a link may exist, but only in some people (possibly smokers or those with elevated cholesterol levels).27 The link between excess iron and any of the diseases mentioned earlier in this paragraph has not been definitively proven. Nonetheless, too much iron causes free radical damage, which can, in theory, promote or exacerbate most of these diseases. People who are not iron deficient should generally not take iron supplements.
Patients on kidney dialysis who are given injections of iron frequently experience "oxidative stress". This is because iron is a pro-oxidant, meaning that it interacts with oxygen molecules in ways that can damage tissues. These adverse effects of iron therapy may be counteracted by supplementation with vitamin E.28
Supplementation with iron, or iron and zinc, has been found to improve vitamin A status among children at high risk for deficiency of the three nutrients. 29
People with hepatitis C who have failed to respond to interferon therapy have been found to have higher amounts of iron within the liver. Moreover, reduction of iron levels by drawing blood has been shown to decrease liver injury caused by hepatitis C.30 Therefore, people with hepatitis C should avoid iron supplements.
Interactions with Supplements, Foods, & Other Compounds
Many foods, beverages, and supplements have been shown to affect the absorption of iron.32
Foods, beverages and supplements that interfere with iron absorption include
Green tea(Camellia sinensis).33, 34, 35, 36 This effect may be desirable for people with iron overload diseases, such as hemochromatosis. The inhibitory effect of green tea on iron absorption was 26% in one study.37
Red wine, particularly the polyphenol component (also found in tea).41, 42 Since wine is also a dietary source of iron, it is not clear whether drinking red wine would lead to a deficiency of iron.
Phytate (phytic acid), found in unleavened wheat products such as matzoh, pita, and some rye crackers; in wheat germ, oats, nuts, cacao powder, vanilla extract, beans, and many other foods, and in IP-6 supplements.43, 44, 45
Whole wheat bran, independent of its phytate content, has been shown to inhibit iron absorption.46
Calcium from food and supplements interferes with heme-iron absorption.47, 48
Although vitamin C increases iron absorption,58, 59, 60, 61 the effect is relatively minor.62
Taking vitamin A with iron helps treat iron deficiency, since vitamin A improves the absorption and/or utilization of iron.63, 64
Although soy protein has been shown to decrease iron absorption (see above), certain soy-containing foods (e.g. tofu, miso, tempeh) have significantly improved iron absorption.65 Some soy sauces may also enhance iron absorption.66
Alcohol, but not red wine, has been reported to increase the absorption of ferric, but not ferrous, iron.67, 68
Iron has been reported to potentially interfere with manganese absorption. In one trial, women with high iron status had relatively poor absorption of manganese.69 In another trial studying manganese/iron interactions in women, increased intake of "non-heme iron"--the kind of iron found in most supplements--decreased manganese status.70 These interactions suggest that taking multiminerals that include manganese may protect against manganese deficiencies that might otherwise be triggered by taking isolated iron supplements.
The information presented in Aisle7 is for informational purposes only. It is based on scientific studies (human, animal, or in vitro), clinical experience, or traditional usage as cited in each article. The results reported may not necessarily occur in all individuals. Self-treatment is not recommended for life-threatening conditions that require medical treatment under a doctor's care. For many of the conditions discussed, treatment with prescription or over the counter medication is also available. Consult your doctor, practitioner, and/or pharmacist for any health problem and before using any supplements or before making any changes in prescribed medications. Information expires June 2015.
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