Iron deficiency is the most common single nutrient deficiency disease in the world and is a major concern for approximately 15 percent of the world's population. The commonly used definition for anemia, for whatever cause, is a low hemoglobin concentration (Hb). If iron deficiency is an underlying etiology, then by definition the individual must have depleted iron stores, a low plasma ferritin or decreased stainable iron in bone marrow, and an inadequate delivery of iron to tissues, as characterized by a low transferrin saturation, a high erythrocyte protoporphyrin concentration, and an elevated transferrin receptor concentration.1,2
Iron deficiency can be defined as that moment in time when body iron stores become depleted and a restricted supply of iron to various tissues becomes apparent.3 The process of depletion of iron stores can occur rapidly or very slowly and is dependent on the balance between iron intake and iron requirements. Clearly, iron intake is dependent on food composition and quantity of iron therein, with a number of inhibitors and a smaller number of enhancers of iron absorption now known to exist. Iron absorption increases in individuals who have depleted iron stores; it is this internal regulator of absorption that may be more important than any particular constituents of the food supply.4 Basal obligatory iron losses in humans are approximately 1 mg/day and must be replaced by an equivalent amount of iron derived from the diet. The typical Western diet provides an average of 6 mg of heme and nonheme iron per 1,000 kcals of energy intake.
Consequences of Poor Iron Status
Many organs show morphological, physiological, and biochemical changes with iron
deficiency in a manner related to the turnover of essential iron-containing
proteins. Sometimes this occurs even before there is any significant drop in
Hb concentration.5 Iron deficiency is associated with altered metabolic
processes; among them are mitochondrial electron transport, neurotransmitter synthesis, protein synthesis, organogenesis, and others.
It is also important to delineate whether exercise itself may alter iron status, and whether such alterations are detrimental to athletic performance or to the health of an athlete. Although a multitude of laboratories worldwide have contributed to a very broad-based accumulation of knowledge in these areas, an analysis of more than two decades of research illustrates four central points:
1. Reductions in heme and nonheme iron can detrimentally alter exercise performance.
2. Iron status is altered in certain populations of chronically exercising individuals.
3. Women may have an increased prevalence for exercise-related alterations in body iron.
4. It is reasonable to question whether these manifestations are specifically detrimental to the health or the athletic performance of the individual afflicted.
Iron and Exercise Performance
The role of heme and nonheme iron in biologic function and work performance has
been elucidated through human and animal experiments, and several classic reviews
have been published.6,7 It is not surprising that hemoglobin iron,
when lacking, can profoundly alter physical work performance via a decrease in
oxygen transport to exercising muscle. What is intriguing, however, is that
although nonheme iron associated with enzyme systems comprises only 1 percent
of total body iron, profound deficits of these components per se may have
detrimental effects on athletic performance. Studies illustrate that maximal
oxygen consumption is determined primarily by the oxygen-carrying capacity of
the blood and is thus correlated to the degree of anemia. Endurance performance
at reduced exercise intensities, however, is more closely related to tissue
iron levels, since a strong association is seen between the ability to
maintain prolonged submaximal exercise and the activity of the oxidative
enzyme pyruvate oxidase. There is disagreement in results of human studies
regarding the concept of a Hb threshold phenomenon. Research by Edgerton et
al.8 suggests that the decrement in work performance in
iron-deficient anemic subjects was a reflection of the level of anemia
rather than other non-Hb related biochemical changes. Unlike the data
of Perkkio et al.,9 these studies suggest a more linear relationship between Hb and work performance and, thus, do not necessarily support the presence of a Hb threshold phenomenon.