Iron plays an important role in several exercise-relevant processes such as red blood cell production, and energy metabolism, whilst also contributing to our cognitive function, and the body’s immune response. Despite being important, the human body has no innate way to generate its own iron supply, and therefore, we are reliant on dietary iron intake to obtain our daily needs. Interestingly, exercise results in several outcomes that cause us to utilise our iron stores (i.e., red blood cell genesis; adaptation), or to lose iron from the system (i.e., sweating, occult gastrointestinal blood loss). Accordingly, having an adequate iron supply in the diet (or from supplements if needed) is an important consideration for athletes.
Absorption of iron is inefficient
Although an adequate supply of dietary iron is essential, you might be surprised to learn that the absorption of iron from the food we eat is relatively inefficient. For instance, meat eating athletes may only absorb 5-35% of the heme iron found in any given serving of meat, whereas vegetarian athletes may only absorb 2-20% of the non-heme iron found in leafy green vegetables. You can learn more about this here.
Although an adequate supply of dietary iron is essential, the absorption of iron from the food we eat is relatively inefficient
Not only does this tell us that iron absorption is inefficient at the level of the gut, but also that vegetarian athletes must make extra effort to ensure adequate sources of non-heme iron are consumed to make up for the lower relative absorption rate. Notwithstanding, the other foods we consume with our high iron containing meals are also important for us to consider, since we know that the co-ingestion of calcium, tannins (in tea and coffee), and phytates, for instance, can further inhibit the absorption of iron, whereas the addition of Vitamin C (ascorbic acid) to our meal can actually enhance our ability to absorb iron from our food.
Timing of iron consumption
In addition to the type of iron we eat and the foods we consume it with, it is important to note that we must also consider the timing of when we consume our iron in relation to our exercise schedule. The reason why this becomes an important factor is due to the action of the lever-produced hormone known as hepcidin, which is our body’s primary iron regulator. One of the roles of this hormone is to control the amount of iron that passes through the gut and into our system. Generally, increases in hepcidin levels results in a decrease in iron absorption, whereas decreases in hepcidin result in an increase to iron absorption.
We must also consider the timing of when we consume our iron in relation to our exercise schedule
So… if iron is really important, why would the body do this you ask? Good question! It turns out that, although important, too much iron is actually toxic to our body, and so, hepcidin acts to regulate the absorption of iron to ensure we don’t have too much in our system (i.e., more isn’t necessarily better when it comes to iron). To put this in perspective, some of you may have heard of the medical condition hemochromatosis, where patients have extremely high iron levels – well, this condition is characterised by an inability to produce the hepcidin hormone, effectively meaning iron uptake is unregulated and iron toxicity becomes a concern. Given this, it is clearly an evolutionary benefit for us to produce hepcidin as an iron regulator, but why is this relevant to exercise you ask?
Regulation of hepcidin
To answer this question, we must first ask what regulates this hormone? Our current understanding is that factors such as an increase in inflammation or increases in iron act to elevate the hepcidin response, whereas an increase in hypoxia (i.e., altitude) can decrease the hepcidin response. Of course, this latter response is handy when we go to altitude and need more iron in the system to help produce those new red blood cells. However, it is the inflammatory response to training that becomes of interest to iron regulation relevant to exercise.
Influence of exercise
Importantly, we know that exercise results in a transient (temporary) inflammatory response, and when we measure this, we see an increase in the levels of the inflammatory cytokine known as Interleukin-6 (IL-6), which just so happens to be one of the signals to increase hepcidin activity. The research that showed this cytokine/hepcidin link  noted that when IL-6 levels were artificially increased, hepcidin levels were elevated 3 hours later.
Exercise results in a transient (temporary) inflammatory response... which increases IL-6... which signals to increase hepcidin activity
Given this relationship, a number of research teams have since shown a transient increase in IL-6 post-exercise, and a subsequent increase in hepcidin levels in the subsequent 3-6 hour period , which appears to subside over the hours that follow. Importantly, we also know that hepcidin seems to fluctuate in a diurnal pattern, such that higher levels can be expected in the afternoon as compared to the morning (even in a rested condition).
So, what does all this mean? Well, what it potentially means is that there is a more appropriate time around exercise for us to be consuming our iron, in order to avoid the peak periods of hepcidin activity. Currently, we  think that consuming your higher iron containing foods or supplements in the morning may be more affective than in the afternoon (due to the diurnal effect), and that consuming this iron either before your morning training session, or within 30 min post-exercise, might allow you the best absorption opportunity (due to the exercise effect). Possibly, this also means that our ability to absorb more iron exists on rest days (removing the exercise effect), however, more research is needed to explore the impact of iron periodisation – stay tuned for that!
Kemna, E., et al., (2005). Time-course analysis of hepcidin, serum iron, and plasma cytokine levels in humans injected with LPS. Blood, 106(5): 1864-1866.
McCormick, R., et al., (2020). Refining Treatment Strategies for Iron Deficient Athletes. Sports Medicine, 50(12): 2111-2123.
Peeling, P., et al., (2009). Effects of exercise on hepcidin response and iron metabolism during recovery. International Journal of Sport Nutrition and Exercise Metabolism, 19(6): 583-597.