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  Last updated: 11/1/2015

(including iron metabolism)

A few definitions:


There are multiple potential reasons for the development of anemia (a low blood count i.e. hct or hgb) in athletes, including acute blood loss from an accident as well as nutritional deficiency states. We will review three of the more common, acquired anemias found in athletes. Specifically:

A) Dilutional Anemia

This is not a true anemia in that total body hemoglobin (the total amount of hemoglobin being circulated by the cardiovascular system) is normal. The anemia reflects the fact that the amount of serum (liquid portion of the blood volume) has increased in this condition and as a result "diluted" their red blood cell concentration. In essence, the total blood volume may have increased slightly (to 10.5 pints for example), but the number of red blood cells circulating is the same as in the normal (original) 10 pints - thus the concentration of hemoglobin per pint (or cc) and hematocrit decrease slightly.

It is speculated that with work outs you become slightly dehydrated (which means your total blood volume decreases below normal - now 9.5 pints instead of the "normal" 10), the body senses this deficiency in the kidney (where the renin/angiotensin system kicks in) and in the pituitary (which increases ADH hormone release). Both of these lead to fluid retention (overcompensation) when you are able to drink adequately again.

The degree of dilutional anemia correlates with the intensity of exercise and is in the range of 5 - 20% below your hgb concentration (not a 20% drop in your absolute hgb or hct.) With moderate exercise, the hgb may decrease 0.5 gms, and in elite athletes the hgb may be as much as 1 gram lower than the athlete's "normal" range. Dilutional anemia may be seen within a few days of starting or intensifying exercise, and will correct back to normal within a few weeks of stopping exercise. It is high on the list of potential reasons for anemia when the ferritin is normal in the face of newly noted anemia, and can be confirmed if the hct or hgb returns to normal when the athlete stops exercising for one to two weeks.

Teleologically, are there potential benefits of a dilutional anemia? In the short term (before the body can produce additional red blood cells in response to the demands of exercise) it is a rapid way for the body to increase the blood volume and thus the cardiac stroke volume resulting in more blood being pumped through the lungs and muscle circulation per beat. It also decreases blood viscosity, which may be important to blood flow at the capillary level during exercise.

b) Exertional Hemolysis

This is also referred to as "march anemia" or "foot strike" anemia. Although it was at one time felt to be related to direct trauma to the red blood cells as they were traumatized by the foot hitting the ground with running or long marches, it can also seen in non impact sports such as swimming, rowing, and weight lifting. Interestingly it has never been reported in basketball or baseball players. As it is seen most frequently in runners, local trauma from the "foot strike" remains the primary suspect. It is most likely a combination of several factors: The triad that helps make the diagnosis is: This is not a problem seen in cyclists but it is included here for completeness. How might one go about treating it?

C) Iron Deficiency Anemia

A low blood count (anemia) from iron deficiency is the most common true (versus dilutional - see above) anemia in athletes. It is most often due to a poor oral intake of iron with a component of increased iron loss (menstrual loss in women, and rarely bleeding from the GI tract see A and B for more detail). Iron deficiency anemia is more common in female athletes who have a higher incidence of eating disorders (poor intake) with the added component of iron loss associated with monthly menses.

Iron is a necessary for the production of hemoglobin in red blood cells - and is also a component of many metabolic enzymes throughout the body. It is absorbed from the food we eat and is regularly lost through the kidneys and intestinal tract (as colon lining cells are shed). When absorption and loss of iron are in balance, you have enough iron stored in your body to support red blood cell production for about one year.

The diagnosis of iron deficiency can be suspected when an exam of the blood cells reveals a small rbc size (when there is not enough iron, new blood cells, lacking adequate hemoglobin, are smaller than normal), and confirmed by finding a low ferritin.

The recommended daily dietary iron intake for an adult male is ~ 8 mg per day, whereas for a menstruating, non-pregnant females it is higher (~ 18 mg) to account for increased basal iron losses via menstruation. The bioavailability of iron in food depends on the source. Absorption of iron is greatest for iron in the hemoglobin and other iron containing enzymes in red meats (15% to 35%), as compared with 2% to 20% of the non heme iron present in vegetables and grains (vegetarians are at increased risk of becoming iron deficient.)

If an athlete (especially a vegetarian or woman) is worried about becoming iron deficient, options for prevention include:

But there are risks of unneeded supplementation, especially if larger doses are used. Too much absorbed iron can lead to iron overload disease (hemochromatosis) with significant irreversible and negative impacts on the heart, liver, and pancreas.

Treatment of iron deficiency when there is a low blood count (anemia) is not controversial.


a) Low Ferritin Without Anemia

You can become deficient in iron (your total body stores of iron are below normal) before the blood count begins to fall. This has led to speculation that a low total body iron state without anemia may contribute to poor performance as iron is found in many enzymes (besides red blood cells) that are involved in aerobic metabolism (mitochondrial oxidative enzymes, cytochromes of the electron transport chain). In this scenario, the serum ferritin would be low, but the blood count may not yet have started to drop. It is speculated that these might be athletes with a ferritin at the low end of the "normal" range. Women are a special case. Although the normal ferritin for women is 15, many of the women whose blood studies were used to set the normals for women may have had low iron stores from chronic menstrual blood loss. Thus 15 may indicate mild iron deficiency and the true normal for women may be nearer to 25.

Nonetheless, all the controlled studies to date fail to show a benefit of iron supplements UNLESS the iron deficiency is associated with anemia (references - 1, 2, 3).

This conservative recommendation to treat only when anemic is not only based on the lack of evidence as to benefit, but also as there are risks of over treatment. Higher is not better, when it comes to serum ferritin levels. If one tries to push iron supplements in the face of a normal ferritin - remember 25 is normal for some athletes - the risk of iron overload disease is significantly increased. And animal models have shown that iron overload can lead to a decrease in exercise performance.

b) Oral iron as a performance enhancer

The final question - does increasing your iron intake - without a low blood iron/ferritin or anemia - provide a performance benefit. To quote from a review of the literature, "Iron supplementation can raise serum ferritin levels, but increases in ferritin concentration, unaccompanied by increases in hemoglobin concentration, have not been shown to increase endurance performance.

This series of articles is a good example of how these "myths" start - and then are perpetuated. First, the original article which, I feel, misrepresents the conclusions of the referenced study: Iron Levels and Altitude. Taking supplements may increase the benefits of thin-air training. Which it was then reprinted for cyclists in Bicycling: This Common Supplement Could Help You Ride Better at Altitude"

This was a pretty strong claim so I thought I'd look for more details in the original article in PLOS. Although the PLOS article was quoted as supporting improved athletic performance at altitude with iron supplements, as far as I can determine it only found that IRON DEFICIENT athletes incorporate more substrate (iron) into their blood cells if they were given supplements - which makes sense. They need iron (even at sea level as they are iron deficient), so given iron plus the stimulus of altitude, it is only reasonable to assume that they will absorb more of it. And as iron is in many enzymes (along with iron in blood cells), one might also expect some performance improvement (non heme level related) in the iron deficient athletes as well. And of course there is no evidence that the iron deficient group actually improved performance - only that the blood cell mass increased and thus it was assumed they would perform better as well.

So in my reading, this is a great example of an unfounded interpretation in the lay press that will lead to iron supplementation abuses by athletes who have adequate iron stores, and expose those looking for that performance edge to the toxicity of iron supplements.


When you donate blood, you will be giving 1 pint (about 10% of your total blood volume). In doing so you will temporarily decrease the oxygen carrying capacity of your blood by 10%. Your body will compensate by replacing the fluid volume within 48 hours, and the hemoglobin will be synthesized (replaced) in a week to 10 days. Until your hemoglobin has been replaced, your cardiovascular system will adapt by increasing your heart rate for any level of exercise - thus increasing the amount of oxygen delivered to the cells during any period of time.

You won't notice much of a change in performance on moderate rides. However if you did intervals, you would notice a decrease in performance until the hemoglobin is replaced. It is a good idea to give blood on a rest day and keep the next few workouts light. And be sure to wait at least 2 weeks before entering a performance event. Over time, repeated blood donation might lead to iron deficiency. Use the prevention measures mentioned above to decrease your risk.


Q. I am a 27 year old road-racer. I had a seizure in my first race of the season. In the end I was diagnosed with brain cancer, had a tumor removed, went through radiation, and am now on chemo for the next 12 months. I am going to race this year even through it, as a swift finger toward the cancer, and a sign of hope and strength for myself. The big effects of the chemo is low hemoglobin and hematocrit levels. Here is the big question. I've been vegetarian since I was ten, and now I am wondering if maybe I should start eating red meat. Other thoughts? - MN


Q. I was recently queried by a reader about their laboratory results and the possible need for iron supplements. Values of interest (and I'll leave off units of measurement):

A. In answering this question I am going to use direct quotes from a very nice summary article I recently ran across.

1. Why should an athlete care about iron deficiency? Because low body iron stores will impact performance.

The oxygen carrying capability of blood is directly related to the hemoglobin molecule concentration which is in turn is directly related to iron availability (a key component of hemoglobin production). And there is a speculative component that iron deficiency may impact muscle enzyme levels directly (iron is part of many enzyme systems). In addition "...impairment of exercise capacity in anemic individuals may reflect more than reduced oxygen transport. Subjects with low hemoglobin concentrations also manifest a decrease in blood viscosity and reduction in systemic vascular resistance, effects that increase cardiac work and strain circulatory function."

2. Is this athlete anemic? Questionable

Although suggestive, these numbers (hct 47) do not support a clear diagnosis of anemia. Blood counts do vary a few points from blood draw to blood draw, so a hct of 45 on repeat might be 47. And individuals have their own "normal" that may always be a bit lower or higher than the population averages. In addition, to quote "......Borderline low hemoglobin concentrations with normal ferritin concentrations are sometimes observed in athletes...."

3. Does this athlete have iron deficiency (early, without anemia)?

This is where it gets murky. Blood test are indirect measures of body iron stores. The gold standard is a bone marrow exam that directly assesses body iron levels (iron is stored in the bone marrow). The blood iron level and transferrin are not as useful as the ferritin levels, so if they give conflicting result, the ferritin is generally considered the better test for diagnosis of body iron levels.

"Serum iron levels demonstrate a great deal of variability over time (both circadian and rate of release from the reticuloendothelial system), bear no reliable relationship to ferritin concentration, and cannot be expected to accurately reflect body iron status."

"Ferritin circulating in the blood stream accurately reflects body iron stores and serving as a reliable marker of iron deficiency. A value of 1 ng/mL of ferritin corresponds to about 5 to 9 mg of stored iron."

But a qualifier here - "...It is important to recognize that plasma ferritin is an acute phase reactant, and levels will be consequently elevated following vigorous physical activity or in an individual with an inflammatory illness. Thus, determination of ferritin levels should be postponed in athletes in an inflammatory state (ie, those who have recently completed an acute bout of exercise training or in those with acute febrile illness)." This means that one could be iron deficient with a low serum ferritin and yet see it falsely elevated into the normal range after a bout of strenuous activity.

So to increase its predictive value, a serum ferritin should be checked after a few days of rest (or at best only mild or rest day activity.

The answer to the original question? It is unclear with the information available.

Next steps? Take a few days off and repeat the blood count and the serum ferritin. If the athlete cannot do that, then a bone marrow exam would give the only clear answer.

Should they just take iron "just in case"? There are definite risks to this approach as iron can be toxic to the liver, heart, and pancreas if taken at high doses for long periods of time. Again, to quote: "Expert opinion is in agreement that, considering these possible risks, athletes with normal ferritin levels should not take iron supplements."

There was a spirited discussion of iron deficiency in cyclists on recently. I, along with a number of other health care people responded. I'll reprint portions that summarize nicely the key issues in considering iron supplements for anemia and iron deficiency in athletes.

The article: "Supplement Worth Re-considering"

"Last week's piece about the potential benefit of iron supplements ("A Supplement Worth Considering") sparked lots of feedback, including e-mails from six medical professionals. In general, the docs expressed great concern about men taking extra iron. Although we included a warning about that and the wisdom of consulting a physician first, they either overlooked the caveat or wanted to emphasize it. Important comments from four doctors are below."

The bottom line, perhaps, was provided by this e-mail from a roadie who asked that his name not be used: I am not an expert on this but do have an experience regarding the blood issue. My cardiologist, also a bike racer, found out that I donated blood regularly. His immediate response was that sport cyclists should not donate blood without a serious review of the pros and cons. "Riders may be better off reducing their blood donations than taking iron supplements."

A portion of this material was excerpted from a talk given at the 10th annual Sports Medicine Conference held at Sun Mountain Lodge 1/2004 by Kimberly Harmon, MD, Sports Medicine Clinic, University of Washington.

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