Iron

Iron is needed to form the oxygen-transporting compounds hemoglobin (in blood) and myoglobin (in muscle) and is also found in a number of other compounds involved in normal tissue function. Iron absorption is limited because there is no effective mechanism for excreting the excess once it is absorbed. To a large degree, the amount absorbed is driven by the amount of iron in storage (in ferritin and hemosiderin). The lower the iron storage level, the higher the rate of iron absorption; however, overall absorption rates rarely go above 10 to 15 percent of the iron content of consumed food. This variable absorption mechanism is aimed at maintaining a relatively constant level of iron while avoiding an excess iron uptake. Despite this variable absorption rate, people with marginal iron intakes are at risk of developing iron deficiency and, eventually, iron-deficiency anemia.

Iron-deficiency anemia is characterized by poor oxygen-carrying capacity, a condition that causes endurance problems in athletes. Iron deficiency is also associated with poor immune function, short attention span, irritability, and poor learning ability. In the United States, children experiencing fast growth, women of menstrual age, vegetarians, and pregnant women are at increased risk for developing iron-deficiency anemia. Periods of growth and pregnancy are associated with a higher requirement of iron because of a fast expansion of the blood volume, and iron is an essential component of red blood cells. Women of childbearing age have higher requirements because of the regular blood (and iron) losses associated with the menstrual period. For this reason, women of childbearing age have a higher DRI for iron (18 milligrams) than do men of the same age (8 milligrams). Some people are at risk of developing iron toxicity because they are missing the mechanisms for limiting absorption. Young children in particular may be at risk for iron toxicity if they ingest supplements intended for adults. Although the iron DRI for children (~7 to 10 milligrams per day) is similar to that for male adults, many iron supplements intended for adults contain iron levels more than 300 percent of the DRI. Iron overload disease is potentially fatal.

Iron is available in a wide variety of foods, including meats, eggs, vegetables, and iron-fortified cereals. Milk and other dairy products are poor sources of iron. The most easily absorbed form of iron is “heme” iron, which comes from meats and other foods of animal origin. Nonheme iron, which is not as easily absorbed, is found in fruits, vegetables, and cereals. However, nonheme iron absorption may be enhanced by consuming foods high in vitamin C. On the other hand, nonheme iron absorption may be inhibited by phytic acid (a substance associated with bran in cereal grains), antacids, and calcium phosphate. In general, red meats are considered to provide the most abundant and easily absorbable source of iron. For this reason, vegetarians are considered to be at increased risk of iron-deficiency anemia. Nevertheless, with proper planning; consumption of vegetables, iron-fortified grain products, and fruits high in iron; and sound cooking techniques that aid iron absorption, vegetarians can obtain sufficient iron.

Iron Status and Athletic Performance Athletes have good reason to be concerned about iron status because oxygen-carrying capacity and oxidative enzyme function are critical factors in physical endurance. Iron deficiency is one of the most common nutrient deficiencies of the general public, and it appears that iron deficiency and iron-deficiency anemia have the same incidence level in athletes.⁸⁰

Dietary Intake The dietary intake of athletes, particularly endurance athletes, typically focuses on carbohydrate foods and diminishes the intake of meats. This eating pattern is generally associated as ideal from the perspective of providing an optimal distribution of energy substrates, but it may marginalize the intake of iron. Meat clearly provides a higher concentration of iron than other foods, despite the fact that cereal grains are currently fortified with iron as a public health measure to lower iron-deficiency anemia prevalence. Vegetarian athletes, through their avoidance of foods that are highest in iron, are at greatest risk for deficiency.

Low Iron Absorption Iron absorption is relatively low (rarely more than 10 percent of total dietary consumption), even in those with the greatest need. The absorption of iron is enhanced by the intake of meat and diminished by the intake of nonmeat foods. In addition, certain components of vegetables (oxalic acid) and cereals (phytic acid) bind iron and other divalent minerals and make them unavailable for absorption.

Iron is competitively absorbed with other divalent minerals (most notably calcium, magnesium, and zinc), so an excessive intake of one or more of these minerals may diminish the absorption rate of iron. Given the common supplementary intake of calcium in particular, reduced iron absorption may be likely.

Increased Red Blood Cell Breakdown A number of studies have documented higher rates of intravascular hemolysis in athletes than in nonathletes.⁸¹ Hemolysis occurs when exertional forces cause a ballistic and premature breakdown of red blood cells (RBCs). Athletes have RBCs with a life expectancy of approximately 80 days, while in nonathletes RBCs last approximately 120 days. Runners, because of frequent foot strike, and other athletes involved in concussive sports may be at increased risk of hemolysis, but hemolysis has also been documented in swimmers and dancers.⁷⁶ The importance of foot strike on hemolysis is evident-he phrase “foot-strike hemolysis” is commonly used to describe this condition. Typically, the harder the surface the runner works out on, the greater the potential for hemolysis.^{81, 82}

Loss of Iron in Sweat The concentration of iron in sweat is low (approximately .2 milligrams per liter of sweat), but the sweat loss in long-duration activities may be sufficiently high (possibly more than 2 liters of sweat per hour) that a significant amount of iron can be lost.⁸³ Although it appears that those athletes engaging in extremely long-duration training sessions are at risk of losing a substantial amount of iron through this route, other athletes are likely to have negligible iron losses through sweat.⁸⁴

Loss of Iron From Blood Loss Loss of blood is typically from menstrual losses or through the gastrointestinal tract. Of course, athletes who donate blood also lose a significant amount of iron. Blood loss through the GI tract appears to be significant and has been found in up to 85 percent of athletes engaged in intense endurance events.⁸⁵ It seems likely that nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin and ibuprofen, frequently taken by athletes to control muscle pain, may result in a degree of GI tract irritation and blood loss.⁸¹

Dilutional Pseudoanemia (Sports Anemia) Sports anemia is experienced by most athletes, typically at the beginning of an intensive training period. The initiation of intensive training is associated with an enlargement of the blood volume, which causes a dilution of the blood constituents. Because there is no reduction in blood constituents, as occurs with any form of blood loss, oxygen-carrying capacity remains at previous levels (thus the name pseudoanemia). After several weeks, the constituents of the blood (including red blood cells) have an opportunity to increase so as to normalize their concentrations. Exhaustive exercise typically results in a reduction in plasma volume, which experiences a positive recovery and expansion after rehydration in the postexercise period.⁸⁶ Harder training, particularly in endurance activities, is associated with the greatest plasma volume increase and will persist for up to 5 days after exercise cessation.⁸¹ True iron-deficiency anemia is associated with a smaller red cell volume (i.e., lower mean cell volume, or MCV) and lower stored iron (i.e., lower ferritin), but athletic pseudoanemia is not associated with either of these biomarkers.

Iron Deficiency and Iron-Deficiency Anemia Athletes should make every effort to avoid an iron-deficient state because oxygen-carrying capacity is of central importance for athletic endurance. Besides its obvious importance in oxygen transport, iron is also important in a large number of energy-transport enzymes and is also involved in normal nerve and behavioral function and in immune function.⁸⁷

Iron deficiency is seen in approximately 20 percent of females of childbearing age and has a much smaller incidence (1 to 5 percent) in postmenopausal females and males.⁸⁸ Iron deficiency with anemia (i.e., low hemoglobin, low hematocrit, low MCV, low ferritin) has a lower prevalence (1 to 3 percent of the population). Athletes may have a higher prevalence of iron deficiency but not iron-deficiency anemia. Further, athletes might respond differently to the presence of frank anemia (reduction in the number and size of red blood cells) versus iron-deficiency anemia (low serum iron and low stored iron but normal red blood cells).⁸⁹ Those at highest risk for iron-deficiency anemia appear to be elite endurance runners, although the condition has been reported in virtually every athlete group that has been assessed.^{90, 91}

Although iron-deficient athletes are known to experience a performance deficit, there appears to be no benefit in providing iron supplements to athletes who have a normal iron status.⁹² Further, iron supplementation is often associated with nausea, constipation, and stomach irritation. However, in athletes with blood tests that demonstrate either an anemia or a marginal level of stored iron, iron supplementation is warranted. The usual iron replacement therapy is to provide oral ferrous sulfate, but in athletes with GI distress, ferrous gluconate can be used and appears to be better tolerated. Intermuscular injections of iron are generally not recommended because of their association with potentially serious side effects.⁸¹

The frequency of iron supplement intake remains a topic of ongoing debate. Some suggest that the best means of providing iron supplements to reduce the chance of potential negative side effects is to take 25 to 50 milligrams every third or fourth day instead of daily doses.⁹³ This approach may prevent GI distress and may impart the same benefits seen with daily supplementation. Of course, taking iron supplements in the absence of iron deficiency or iron-deficiency anemia should be avoided. Besides increasing the risk of hemochromatosis (iron overload disease), which may affect 1 percent of people of northern European descent, iron supplementation may mask celiac disease and colon cancer.⁹⁴ Excess iron stores may be observed among professional road cyclists who habitually consume excessive iron supplements.⁹⁵