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Brief Review

A Brief Review

The Implications of Iron Supplementation for Marathon Runners on Health and Performance

Zourdos, Michael C.1; Sanchez-Gonzalez, Marcos A.2,3; Mahoney, Sara E.4

Author Information
Journal of Strength and Conditioning Research: February 2015 - Volume 29 - Issue 2 - p 559-565
doi: 10.1519/JSC.0000000000000636
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Abstract

Introduction

The 42.2-km (26.2-mile) distance run known as the marathon is commonly recognized as one of the most physically demanding endurance events. Throughout marathon training and competition, an enormous amount of physiological stress is imposed over bodily structures, the metabolic machinery, and to various organ systems (50). For years, scientific evidence has consistently shown that marathoners are in need of rigorous training and specifically designed nutritional strategies to maintain good health and support peak performance. In fact, among competitive athletes, marathoners are at a greater risk of developing anemia, bone mineral density loss, and immunosuppression because of the prolonged and high-intensity training hours typically required for this sport (6,15,19,37,41,51). Because of these significant training strategies, appropriate nutrient intake may be a vital component to avoid clinical disorders and ultimately augment performance and maintain health. Accordingly, recent studies seem to suggest that a deficiency of the micronutrient iron may significantly impact the development of various clinically relevant syndromes that may in turn impair marathon performance and overall health.

The physiological importance of iron is remarkable in view of the fact that those with iron deficiency (ID) are known to have inefficient oxygen transport capacity, which affects energy use; this leads to a plethora of adverse health disorders such as immunosuppression and lack of bone strength (6,28,37,39). Moreover, ID has been associated with impaired performance in marathon runners (8,27,28), possibly because of inefficient oxygen transport. Although the typical marathoner's diet is high in caloric content (∼3,750 kcal·d−1), it is not often rich in iron (13.6 g·d−1) (63), and thereby, may not be adequate to satisfy the recommended dietary allowance (RDA) (60,63) and might accelerate ID. In addition, marathoners are known to have high erythrocyte fragility along with an augmented erythrocyte turnover (19,61); therefore, physiological iron requirements for marathoners are likely higher than those specified by the RDA. Moreover, there is a significantly increased demand of iron use among marathon runners. Therefore, it is reasonable to justify supplementation of the micronutrient iron in this particular athletic group (13,36,45,46,52,66). In fact, iron is one of the few nutrients recommended as a dietary supplement, for endurance athletes, by The American College of Sports Medicine (ACSM), the Academy of Nutrition and Dietetics (AND), and Dietitians of Canada (53,54).

The appropriate use of supplemental iron involves a high degree of complexity and a certain level of expertise partly because of its potential cardiovascular toxic effects (26,57). The sports dietitian who may implement this supplementation must be aware of individual variations and gender-specific factors, which modulate iron homeostasis and metabolism. When considering supplementation, gender differences are of particular importance because women are at a higher risk of developing ID, because of limited iron storage, augmented gastrointestinal iron loss (37,38), and the ID in premenopausal women being also associated with menstrual iron loss (∼1.4–1.6 mg·d−1) (29,70). Because of these genders-specific factors, in addition to the increased iron demand and use associated with the marathon run, female runners must be closely monitored to establish a successful training and dietary program. Because iron is important for preserving health and optimizing performance in marathoners, it is imperative to understand its physiological and nutritional implications. Therefore, the aim of this review is to discuss the role of iron on the marathoner runner's health and the possible impact of ID on endurance performance. Special emphasis will be given to the physiological mechanisms to account for the additional iron requirements in this group of athletes and the nutritional strategies intended to counteract ID. Additionally, gender-specific differences that may affect iron status will also be considered.

Physiology of Iron

For a 70-kg male individual, the total body iron is approximately 3.5 g (50 mg·kg−1); this makes iron the most abundant trace element present in the human body (67). Most of the body’s iron is found in blood (2,400 mg) and is distributed within erythrocytes as a component of the oxygen transport protein hemoglobin (Hb). Approximately 350 mg is located within muscle fibers and other tissues, such as enzymes and cytochromes, where iron is involved in oxygen storage (myoglobin) and metabolic processes, respectively. The remaining body iron is stored as ferritin bound in the liver (400 mg), in macrophages of the reticuloendothelial system (500 mg), in bone marrow (150 mg), and is transported by the protein transferrin (4 mg) (25,38).

A typical diet contains approximately 15–20 mg of iron; however, the body is able to absorb only a small portion of the ingested amount (1–2 mg·d−1) (1). In foods, iron adopts 2 different ionic forms known as heme (10%) and nonheme (90%). Because most of the nonheme iron primarily exists in a nonbioavailable oxidized Fe3+ form, it must be reduced to Fe2+ to be properly absorbed from the apical surface of duodenal enterocytes (1,2). Interestingly, the body has no effective mechanisms for excreting excessive iron (>1 mg·d−1); thus, regulation of absorption from the duodenum plays a pivotal role in iron homeostasis (38). Considering these factors (i.e., low absorption rates and heme and nonheme iron), the composition of an athlete's diet is quite important for maintaining iron status. Thus, and an endurance athlete who typically consumes a lower amount of iron from heme sources may need to consider iron supplementation.

In terms of bodily movement, iron is transferred across intestinal epithelial cells via the transport protein: divalent metal transporter 1. Ultimately, iron in the epithelial cells or enterocytes is released into the blood by ferroportin, and is then bound to transferrin in the bloodstream (21). The regulation of iron homeostasis has been associated with multiple factors including absorption, excretion, and use. Factors that may enhance iron duodenal absorption, including ascorbic acid, are known to have a positive influence on iron status (2). Conversely, those factors that evoke a high iron demand (hemolysis and hematopoiesis) may negatively influence iron status (2,58). Thereby, iron status must be consistently monitored to prevent the development of ID especially in populations at a high risk of developing iron imbalances such as marathoners (10,13,40).

Although there are several blood markers available for monitoring iron status, the specificity and clinical relevance of each marker are different. Widely used measurements include, but are not limited to, erythrocyte size, serum ferritin and transferrin concentration, plasma ascorbate, and the most recently developed soluble transferrin receptor (sTfr) expression and the hepcidin peptide assay (5,12,46,61). Interestingly, sTfr has been shown to be very sensitive for detecting and monitoring the iron status in marathon runners (61). In addition, ID in female runners may be caused by elevated hepcidin levels (12,42). The potential mechanisms that explain the correlation between ID and increased hepcidin levels are based on the fact that after an acute bout of endurance exercise (60–120 minutes), iron is retained by macrophages in response to fluctuating hepcidin levels (55,63). When properly conducted, laboratory tests may be valuable in determining iron status that may ultimately help in the prevention of ID in both male and female runners.

Effects of Marathon Run on Iron Homeostasis

After a high-intensity endurance run, such as the marathon, iron dynamics are significantly altered as a result of the strenuous effort characteristic of the event. In fact, serum iron concentration may be unchanged or increased immediately after a race and may remain elevated for as long as 2 weeks (10,45). However, a decline in erythrocytes, Hb, and hematocrit has been detected between the second and the ninth day after a marathon (45,55). Additionally, various markers of oxidative stress, such as homocysteine, DNA damage, antioxidant capacity in lymphocytes, and plasma and ferritin levels, are increased in response to a long distance run (7,19,61,69). Some reports have shown that the high-intensity marathon run may cause hemoglobinuria (high levels of free plasma Hb), pulmonary hemorrhage, gastrointestinal blood loss, and most importantly foot strike associated hemolysis (16,24,33,68). In fact, the forces that promote hemolysis owing to foot strikes is considered a pivotal factor associated with iron imbalance in marathon runners, independent of gender (46).

Although exercise-induced hemolysis (the rupturing and subsequent loss of erythrocytes) has been described in the literature in regard to swimmers, cyclists, and marathoners, the underlying mechanisms that may explain the development of this specific hemolysis have not yet been fully elucidated (46,48,52). However, some of the potential factors associated with erythrocyte destruction in runners may include, but are not limited to, increased mechanical load, elevated intramuscular pressure, and the pounding of feet on solid surfaces (14,16). Running is considered as a high impact exercise in view of the fact that the body weight is considerably amplified at the time the foot hits the ground. Indeed, the mechanical energy that is imposed on the heel may increase from 0.24 J (walking) to 3.99 J (running) while causing a transient but significant deformation of the foot (14). In addition, intramuscular pressure has been reported to reach values >300 mm Hg, which are likely generated by local muscle tissue deformations in response to muscle force development or impact force absorption (3). These mechanical deformations may increase the susceptibility of hemolysis because red blood cells of marathon runners have high osmotic fragility resulting from reduced concentration of the protein spectrin within the cell membrane (30,71). Interestingly, the rate of hemolysis is more reliant on the running speed and exercise intensity than the running surface itself (46). Taken together, these acute responses seem to suggest that high-intensity aerobic exercise along with the mechanical loads imposed during a marathon race are important stressors that may induce organ damage. Given that the extreme efforts associated with a marathon alter the iron balance toward deficiency (30,62), an increased iron homeostasis may place marathoners at an increased risk of developing anemia.

Anemia is a clinical syndrome characterized by an impaired systemic oxygen-carrying capacity because of a reduced erythrocyte mass and/or Hb deficiency (17). Although the development of anemia in runners may be attributed to blood volume expansion (dilutional anemia), ID anemia is the most prevalent regardless of age or gender (20). In marathon runners, ID anemia has been implicated as a causative factor of immunosuppression and high incidence of upper respiratory tract infections (30,37). Moreover, it is important to consider the fact that if the iron status of the runner is not monitored regularly, the likelihood of ID anemia may be increased. Therefore, a runner with ID anemia may compromise his or her maximal aerobic capacity, and this can consequently result in deleterious effects on endurance running performance (8).

Iron Supplementation and Running Performance

The use of iron supplementation with the intent to increase aerobic capacity has been studied for many years. Early studies showed that after intervention using iron supplementation, patients who were previously anemic were able to improve their Hb levels and erythrocyte mass (22). A study conducted by Schoene et al. (59) demonstrated that discrete reductions in iron and Hb levels may induce significant reductions in maximal aerobic capacity. In addition Lyle et al. (34) showed that a diet high in meat was as effective as a 50-mg·d−1 oral iron supplement in protecting the ferritin status in iron-deficient (serum ferritin level of 25 mg·L−1), previously sedentary, young women. Interestingly, those participants that did not receive the iron treatment and performed moderate aerobic exercise during the 12-week period significantly decreased their iron and Hb levels (34). Long-term studies (≥3 months) have shown that a treatment with 100 mg of ferrous iron per day is sufficient to significantly increase the values of serum ferritin from 34 to 54 μg·L−1 and liver iron from 105 to 227 μg·g−1 of liver (13,27). These studies were critical in supporting the theory that exercise performance, which requires a high oxygen delivery, is significantly affected by an individual's Hb level, while at the same time iron supplementation is needed to overcome such reductions (Table 1).

T1-34
Table 1-a:
A collection of previous results of iron supplementation for health and endurance performance.*
T2-34
Table 1-b:
A collection of previous results of iron supplementation for health and endurance performance.*

Several of the studies designed to test the effect of iron supplementation on parameters related to physical fitness have produced mixed results. In most cases, runners with ID anemia or poor iron status improve performance in response to the supplement intervention. For example, iron supplementation improves iron status and endurance capacity in iron-deficient, nonanemic male and female runners (28). Further, individuals with abnormally high sTfr (>8.0 mg·L−1) improved 15-km time trial performance, work rate, and maximal oxygen consumption (

) after 4 weeks of iron supplementation (8). Additionally, in slightly anemic women, iron supplementation has been shown to improve

, and to reduce blood lactate concentration after submaximal endurance exercise (32). Iron supplementation may also improve performance parameters that are specific to long distance runs including improved energy efficiency and reduced respiratory exchange ratio (28). These results seem to suggest that in iron-deficient athletes both with and without anemia may benefit from iron supplementation as an effective way to augment performance. It is also worth mentioning that female runners are likely better responders to iron supplements than are male runners; however, the exact mechanisms require further investigation.

Conversely, iron supplementation in athletes without ID seems to be ineffective for improving athletic performance (18,39,49). If these findings are conclusive, they may represent an argument in opposition to the use of iron supplementation in marathon runners without any ID. Further, the risk of developing hemochromatosis along with iron toxicity and even cancer may be increased by the excessive use of iron supplements (57,64,65,70). Therefore, although iron supplementation may be beneficial in certain situations, the incorrect or unsupervised use of iron supplementation may be harmful and may result in serious health complications.

Iron Dietary Guidelines and Recommendations

The decision to use iron supplementation in marathoners should be based on carefully performed and analyzed laboratory tests. It is commonly recommended to proceed with supplementation in athletes with basal serum ferritin concentrations of <35 mg·L−1 (53). The measurement of serum ferritin level should be made once or twice per year, and supplementation should aim to restore the serum ferritin level to a target value of approximately 60 mg·L−1 (45). Most studies have shown that 100 mg·d−1 of heme iron is an adequate amount to replenish iron stores within 2–3 months (11,28,37,44). Although iron supplementation has not been shown to be effective in athletes without ID, controlled supplementation may be helpful to prevent ID. Moreover, dietary sources of heme iron, such as meat products along with ascorbic acid, facilitate iron absorption and iron bioavailability (47). Finally, it has been reported that women (43) may consume a lower than normal amount of heme iron and that vegetarians (4) may have an increased affinity for ID as a result of decreased levels of heme iron intake. Therefore, monitoring iron status is of greater importance in the case of women and vegetarians, and iron supplementation may become an attractive strategy.

Practical Applications

Running the distance of a marathon and marathon-type training is known to evoke responses, which may eventually lead to improper iron balance and consequentially to ID. These responses that may lead to disruption of iron homeostasis include mechanical, hematological, and musculoskeletal damage. Moreover, female runners are particularly susceptible to iron imbalance because of increased gastrointestinal and menstrual blood losses, and abnormally high hepcidin levels after exercise. Additionally, nutritional considerations may further contribute to ID; therefore, marathon runners should monitor heme iron intake in an effort to consume enough iron in the diet. Marathoners, coaches, and sport dietitians should be attentive to serum markers of iron status such as ferritin, Hb, and sTrf to avoid the development of ID anemia. Ultimately, iron supplementation can be used as a preventative strategy against ID and in some cases has been shown to improve performance. In fact, iron is even a supplement recommended by the ACSM, AND, and the Dietitians of Canada. Prospective studies should consider variations in iron status throughout a racing season in an attempt to understand iron dynamics so as to properly adjust dietary intakes, if necessary. In conclusion, the marathoner may experience beneficial effects from the supplemental use of iron, which are not only related to performance but are also associated with the maintenance of an adequate health status.

Acknowledgments

No funding was received for the preparation of this article. The authors have no professional relationships to disclose. The authors would like to thank Alex Klemp for his revisions to this manuscript.

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Keywords:

iron supplementation; marathon; running performance

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