Iron Supplementation – Endurance Athlete Specific Studies

Is iron the missing link in our performance?

To start, iron supplementation can be dangerous. Go ahead and google iron overdose and all the search results contain an overwhelming list of symptoms. For this reason, it’s important to remind you that I am not a doctor and this is not medical advice. BUT, for this article, we will look at a few studies on endurance athletes and a few published expert opinions to help inform the correct protocol for athletes to supplement iron.


Endurance athletes mainly rely on their aerobic energy system to produce energy for locomotion. The world ‘aerobic’ can be roughly broken down to ‘life with air’ or as used in the cycling world, ‘with oxygen’. Even at very high intensity short efforts, anything over sixty to ninety seconds, the aerobic engine is the main contributor to total energy utilization for that effort. The oxygen used in the aerobic pathway passes from the mouth, to the lungs, into the blood stream, and lastly absorbed into the muscles.

When an athlete trains consistently, one of the adaptations is an increase in red blood cell production. This increase in production replaces any red blood cells that may have degraded during exercise, but it also attempts to increase total red blood cell counts as a training adaptation. This process of creating new red blood cells is erythropoiesis. The more red blood cells in an athlete’s body, the more oxygen they can deliver to the working muscles for use. EPO (erythropoietin) is a naturally occurring hormone produced in the kidney as a signal to increase erythropoiesis. Famously, EPO was used by cyclists in the 90s and 2000s to dramatically increase performance (think 20% improvements), but it also caused a few deaths because the rider’s blood would be too thick from red blood cells to flow properly within the body. This phenomenon is very unlikely in a normal health adult who doesn’t inject exogenous EPO.

Why is iron important?

Red blood cells are iron rich. The oxygen in the lungs strongly sticks to the iron in the red blood cells and allows it to be easily transported. If an athlete trains and elicits an increase in natural EPO in their body, erythropoiesis begins. Now imagine the body as a baker without flour. There ain’t much bread being made without flour. It is the same thing with our body. Even if an athlete elicits an increase in EPO from training, if the substrates are not substantial enough then there will be no increase in red blood cells and the response to the training stimulus will be blunted.

Three nutrients are essential to red blood cell production: iron, vitamin B12 and vitamin B9 (folate). Folate is added to many consumer foods and it is rare to be deficient. Vitamin B12 can be found in meat and dairy and is also generally not a concern unless the athlete is on a vegetarian diet. Iron, on the other hand, may be inadequate for endurance athlete demands. The following studies attempt to explore the importance of iron supplementation for the endurance athlete.


The first study “Iron supplementation in athletes…” by Zoller and Vogel, recommends caution when supplementing with iron for athletes. Zoller states that iron supplementation does not generally improve performance UNLESS the athlete is anemic or close to anemic. Further, he recommends careful monitoring if taken as a preemptive measure. Specific concerns from Zoller include genetic issues in a subset of the population and, as stated at the beginning of this article, issues with overdosing on iron.

Another study by a similar name “Iron Supplementation in Athletes” by Nielsen and Nachtigall claims some of the studies on athletes without anemia were too short and did not use a high enough dose of iron to sufficiently suggest that iron supplementation for non-anemic athletes was ineffective. The most compelling statement from Nielsen comes from the end of the abstract, specifically, “We believe that there are sufficient arguments to support controlled iron supplementation in all athletes with low serum ferritin levels.”

A third study “Iron as an ergogenic aid…” by Rodenberg and Gustafson suggests a fairly robust method of determining if iron supplementation is the correct protocol. They suggest “athletes who are found to be anemic secondary to iron deficiency do benefit and show improve performance with appropriate iron supplementation”. When it comes to non-anemic iron depleted athletes, the evidence is more murky, but based on the literature Rodenberg suggests “supplementation for the iron-depleted nonanemic athlete does not appear to be justified”.

Lastly, an interesting study on iron absorption “Effect of tea and other dietary factors on iron absorption” by Zijp et al. discusses foods that may affect absorption of dietary iron. Specifically, tea, coffee, vegetables, and dietary calcium (dairy, etc) can inhibit iron absorption when consumed alongside iron rich foods. Ascorbic acid (vitamin C) is the main ‘absorption enhancing factor’ and Zijp recommends adjusting one’s diet to increase vitamin D and iron rich food consumption and limit inhibitory foods during means. One specific suggestion is to drink tea between meals rather than with meals.


What do these studies mean? It seems intuitive that the first step is to determine the athlete in question’s iron status. This is most commonly done with a blood test from a doctor. If they are anemic, then iron supplementation appears to be recommended by the medical population. If they have low seerum ferritin levels, then there is a dispute among researchers about the value of iron supplementation. They do seem to agree that careful monitoring of iron status is important when supplementing, thus blood tests should be done consistently during the protocol.

Although it seems intuitive from learning about erythropoiesis that “Hey! More iron, more red blood cells, more FAST.” But the research and the medical community does not appear to support this evidence. For a normal athlete with a good diet, it seems that there is no benefit and actually there may be a risk associated with additional iron supplementation.

What do I recommend? Get a good diet. Dietary iron is the most important source and is heavily affected by the foods you choose. In addition to the high carbohydrate demands of endurance athletics, we need to find space for iron rich foods that will be properly absorbed. I won’t list off iron rich foods here, but including a few iron rich foods everyday and sufficient vitamin C intake from (hopefully fresh) fruit is a good way to ensure you meet your needs.

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