The Iron Story

The most important determinant of performance in an endurance athlete is VO2 max. It is a direct measurement of how much oxygen an athlete uses (i.e., burns) in a set period of time. Following an oxygen molecule in its journey through the body it is clear that VO2 max is determined by several factors; macroscopic (e.g., size and function of the heart and lungs), microscopic (e.g., capillaries, muscle tissue type), and molecular (e.g., hemoglobin). Iron is involved at the molecular level in oxygen utilization and is thus a critical element in athletes.

Iron is a component of hemoglobin, found in red blood cells and used to transport oxygen to the tissues. Myoglobin, used to store and transport oxygen in muscle, also contains iron. Another use for iron is in cytochromes, which are catalysts in energy transfer within the cell. In total the body contains about 3 - 5 grams of iron, and 80% of this is used in the molecules mentioned above. The rest is complexed with compounds which are used for iron transport and storage (e.g., ferritin).

Iron balance is determined by losses and dietary intake. The body can lose iron via blood, urine, or sweat -- although these last two don’t appear to be very important. Certain circumstance may make athletes more likely to experience iron loss. It has been shown that a significant percentage of runners experience small amounts of gastrointestinal bleeding after runs longer than 10 km. Another possible source is referred to as “foot-strike hemolysis”. In this situation a small number of red blood cells burst in the vessels of the feet, due to the pounding of running on a hard surface. Most of the iron in free hemoglobin can be reclaimed, but some is filtered out into the urine. If the exercise is unusually severe, damaged muscle cells will spill myoglobin into the blood where the kidneys will excrete it in the urine. (Note: This is a dangerous condition referred to as rhabdomyolysis. If your urine ever looks like Coca-cola following severe exercise stress, seek medical attention immediately). In addition to these routes, female athletes have the additional monthly menstrual loss of iron.

There are a variety of sources for dietary iron and several factors which influence how well dietary iron is absorbed. In general dietary iron is absorbed poorly. Animal sources of iron are about 10 - 25% absorbed. Plant sources are only 2 - 5 % absorbed. Ascorbic acid (vitamin C) increases iron absorption, whereas tannic acid, found in tea and coffee, decreases it. Women have a dietary iron requirement of approximately 15 mg/day, and men, 10 mg/day.

Iron deficiency can occur in almost anyone, but the greatest risk occurs in women who are vegetarians. Iron deficiency may begin to affect athletic performance based upon the degree of severity. If it is severe enough to affect the production of hemoglobin (i.e., anemia) then there is no question that performance will suffer. Controversy begins to creep in when one tries to define the earliest onset of iron deficiency. Since ferritin levels are the best measure of total body iron stores many doctors and coaches will frequently measure this, and the complete blood count, in individuals at risk for iron deficiency. The goal is to try to identify athletes early on before deficiency becomes a problem.

What to do?

One practical suggestion would be to give supplemental iron to everyone. The problem with this shotgun approach is that iron overload can cause severe heart and liver damage. Another problem is that there is no evidence that giving iron to athletes who are mildly iron deficient, but not anemic, improves performance. A final consideration is that one cannot assume that an anemic athlete is simply deficient in dietary iron, and that you need to give them more. There may be an underlying serious medical condition, such as colon cancer, which is responsible for the blood loss. Despite the controversy one thing is certain, athletes who know and monitor their body, in conjunction with a skilled practitioner, are more likely to identify any sort of problem early.