Iron and Endurance Athletes

By Jeff Rocco, MD

Iron is one of the most abundant mineral on earth and is essential to normal human physiology.  About 2/3 of the body’s iron is found in hemoglobin, the protein in red blood cells that carries oxygen to cells.  An adequate supply of iron is necessary for the body to produce hemoglobin and red blood cells.  If too little iron is available, fewer and/or smaller RBC’s are produced, leading to decreased oxygen carrying capacity of the blood.  This is called iron deficiency anemia and can cause fatigue, poor work capacity, and decreased immunity.

Endurance athletes depend on efficient oxygen delivery to working muscles, so even mild anemia can have deleterious effects on performance.  Some hypothesize that iron deficiency, even without anemia, can impair endurance.  However, one review showed that in seven of 8 studies that although iron supplementation does increase iron stores, as measured by ferritin levels, no increase in performance was found unless hemoglobin also increased. In the eighth study, there was an unexplainable drop in endurance performance in the control group which confounded the results. (Garza 1997)

Dietary iron comes in two forms:  heme and non-heme.  Heme iron is found in animal foods that originally contained hemoglobin, such as meat, fish, and poultry.  Non-heme iron comes in foods such as beans, spinach, raisins, and fortified cereals and breads.  Heme iron is better absorbed than non-hem iron, but most dietary iron is non-heme.  The recommended daily value for iron intake is 18 milligrams of elemental iron.  Three ounces of beef contains 3.2 mg of heme iron and one cup of lima beans contains 4.5 mg of non-heme iron.  Absorption of non-heme iron can be increased by consuming it with heme iron or with ascorbic acid (vitamin C).  Some substances that decrease iron absorption are the tannins and polyphenols found in tea and coffee, calcium, and phytates found in whole grains and legumes.  One study showed that taking 30-50 mg of ascorbic acid can overcome the inhibitory effects of tannins and phytate on iron absorption.  (Siegenberg 1991)

There is contradictory evidence on the prevalence of iron deficiency anemia in athletes.  Most studies show an increased prevalence of iron deficiency anemia in athletes.  One study showed that iron deficiency developed over an 11 week cross country season, with 17% of males and 45% of females becoming iron deficient.(Rowland 1987) Dietary choices may explain most of  this, but evidence exists for increased iron and red blood cell turnover. (Beard 2000)  Intense exercise may increase the need for iron by 30%.  This may be due to increase red blood cell turnover, effects of increased nitric oxide or other unknown mechanisms.   In sports, especially in endurance sports where low body weight can be an advantage, caloric restriction can result in inadequate dietary iron intake. Females, especially those with heavy menstrual periods or with eating disorders, are especially prone to iron deficiency.  However, the most common cause of a low hematocrit (a ratio of red blood cells to whole blood) in athletes is pseudo-anemia, which is a dilution of the blood caused by plasma expansion.  This is not true anemia, as the actual number of red blood cells is not decreased. (Portal 2003) Another study showed that endurance athletes were less likely than the general population to have iron deficiency.  Out of 52 controls, 50% were iron deficient, and out of 126 atheletes, 26% were iron deficient.  (Malczewska 2000)

Iron supplementation is indicated when an athlete has iron deficiency anemia and dietary sources are not sufficient to replace the necessary iron.  A low iron level with a low normal hemoglobin may be a relative indication for iron supplementation in order to increase performance.  Iron supplementation is available in ferrous and ferric forms, with ferrous forms being the best absorbed.  Ferrous fumarate contains 33% elemental iron, ferrous sulfate contains 20%, and ferrous gluconate contains 12%.  Therapeutic doses of iron supplements may cause side effects such as nausea, vomiting, constipation, diarrhea, dark colored stools and/or abdominal distress.  Taking the supplement in two or three divided doses and with food and titrating up to the recommended dose may help limit these symptoms.  Rarely, iron injections are needed, but require close medical supervision.

Should all athletes supplement with iron?  The answer is no.  Anemia has many causes.  It can be due to decreased production of red blood cells, blood loss, or increased destruction, such as heel-strike hemolysis in runners.  Decreased production can be due to bone marrow abnormalities, kidney problems, or more commonly, nutritional deficiencies.  Vitamin B12 and folate deficiencies can also cause anemia.  A physician can determine the presence or absence of anemia and do testing to determine the cause and direct appropriate treatment.   Iron overload can occur with excessive supplementation or in people with genetic diseases such as hereditary hemochromatosis.  Hemachromatosis affects one in 250 individuals of northern European descent and causes extremely efficient absorption and storage of iron.  The iron is stored in body organs, resulting in cirrhosis of the liver, heart failure, possibly cancer.(Fauci 1998)  Indiscriminate iron supplementation may induce hemochromatosis in susceptible individuals.(Zoller, 2004)   High iron stores have also been linked to ischemic heart disease.  Some have suggested a protective effect of iron depletion on cardiovascular disease.(Sullivan 2007)

Of particular concern is athletes taking iron in the hopes of increasing their performance and developing iron overload, eventually experiencing the above complications.  In 1999, the Union Cycliste Internationale (UCI) began systematically testing athletes in the wake of the doping scandals uncovered in the 1998 Tour de France.  Four times a year, all professional cyclists were required to submit to a battery of blood tests.  The most glaring abnormality found in these tests was a high ferritin level, indicating high iron stores.  The upper normal limit of ferritin is 300 ng/ml, and the average ferritin value in the 1000-plus athletes tested was 342 ng/ml.  Almost half the cyclists had ferritin levels over 300 ng/ml, and one fourth had levels over 500 ng/ml.  Iron supplementation has long been practiced by cyclists in the belief that iron enhances performance by increasing red blood cells.  This actually is true only in iron deficiency anemia.  Two concerns were raised:  First, high levels of iron storage cause organ damage and the high levels persisted even after supplementation ceased.  Second, excessive iron supplementation is linked with blood doping.  The use of EPO to boost red blood cell production increases the body’s need for iron.  The good news is that average ferritin values have decreased by 33% between 1999 and 2002.(Deugnier 2002, Zotter 2004)

Iron deficiency can occur in non-anemic individuals as well.  This condition is marked by normal hematocrit, normal hemoglobin, decreased serum  ferritin, and corresponding increased serum transferrin receptor.  Iron supplementation with 30 mg of elemental iron daily as Ferrous Sulfate for six weeks in a group of non-anemic, iron deficient athletes has been shown to correct iron deficiency, without increasing hematocrit beyond normal ranges.  Why is this important for athletes?  In that same iron deficient, non-anemic population, the iron supplement group demonstrated  an significant increase in ventilatory threshold, energetic efficiency, and progressive fatigue resistance, when compared to placebo.  (Hinton 2007, Brownlie 2004, Brutsaert 2003)

In summary, iron deficiency anemia decreases endurance performance and intense exercise may increase iron requirements.  Some groups of athletes, particularly menstruating female and vegetarian athletes, distance runners, and those on caloric restricted diets, are more prone to iron deficiency anemia.  Iron supplementation is common in endurance sports, and will help endurance performance in anemia caused by iron deficiency. However, because anemia can have many different causes, a low hemoglobin, especially if not responsive to iron supplementation, needs a complete medical workup. When iron supplementation is indicated, hemoglobin and ferritin levels should be checked regularly, to prevent complications from iron overload. 

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