Are you actually absorbing your iron? |
“It’s the little details that are vital. Little things make big things happen.”
~John Wooden
For endurance athletes, fatigue is often worn like a badge of honor. In many ways, it’s part of the process, especially as we near the end of a training cycle. But there’s a difference between expected training fatigue and something more concerning. When heavy legs from months of work turn into consistently flat workouts, a stubborn plateau, or unexplained performance decline, it’s worth a closer look.
As always, I’d encourage anyone experiencing these patterns to first examine their training load, recovery strategy, and overall life stress. Those are the usual, and often correct, culprits. But occasionally, the limiter isn’t programming or sleep. It’s physiology—specifically, iron deficiency.
Iron is essential for metabolic, respiratory, and immune function. It plays a central role in transporting oxygen in the blood and muscle tissue—a foundational requirement for anyone asking their body to go long and go hard. Iron also plays a critical role in healthy bone turnover, an area gaining well-deserved attention in the running community.
And yet iron deficiency remains one of the most common and overlooked performance limiters in endurance sport, affecting athletes at rates far higher than the general population. Complicating matters further, iron status is not always easy to improve. Dietary changes alone often move the needle slowly, and even supplementation produces variable results.
In this newsletter, we’ll explore why iron is so frequently under-absorbed, especially in endurance athletes, and how you can better position yourself to maximize the iron you consume, whether from food or supplements.
Why do endurance athletes frequently encounter iron deficiency?
The higher incidence of iron deficiency in endurance athletes appears to result from increased iron losses, insufficient dietary intake, and reduced absorption—factors that are further compounded by the effects of training itself. In endurance athletes, higher iron loss is thought to come from multiple sources: heavy sweating, blood in the urine (hematuria), potential gastrointestinal bleeding, and foot-strike hemolysis—the small breakdown of red blood cells caused by repetitive impact on the feet while running.
However, in recent years, exercise-induced inflammation has received increasing attention for its role in suppressing iron absorption from both food and supplements. Scientists have focused on the post-exercise response of hepcidin, a hormone produced by the liver, and its impact on iron metabolism in athletes. Exercise-induced increases in hepcidin have been shown to reduce iron stores by as much as 25-40% in as little as six weeks during heavy bouts of training! The idea isn’t to try to suppress hepcidin, but to ensure that iron absorption from supplements and iron-rich foods isn’t disrupted by this hormone.
What causes hepcidin levels to increase?
Hepcidin levels in the body rise through several mechanisms. One of the most consistent is a natural diurnal rhythm, with levels increasing throughout the day. Sleep deprivation and fatigue seem to exacerbate this pattern. Food intake also causes hepcidin levels to increase, especially grains, soy, legumes, eggs, colorful fruits and veggies, coffee, and tea. Paradoxically, iron supplements themselves can elevate hepcidin for up to 48 hours. We will touch on this point again a bit later.
Exercise, particularly prolonged or high-intensity sessions, triggers a transient increase in inflammation as the body responds to the stress of training, further increasing hepcidin levels. These levels typically peak approximately three hours post-exercise and can remain elevated for up to six hours, significantly reducing iron absorption during this window. For endurance athletes, timing iron-rich meals and supplement intake is critical.
How do we address iron deficiency?
Food
Aside from receiving an iron infusion, a topic beyond the scope of this discussion, athletes have two primary ways to increase their iron levels: through food or supplements. Even under ideal conditions, iron is relatively poorly absorbed. Iron from animal sources is absorbed at a rate of roughly 15–35%, while iron from plant-based foods is absorbed much less efficiently, at about 2–20%. For those eating a mixed diet, overall absorption typically falls in the 14–18% range, whereas vegetarians absorb only 5–12% of the iron they consume.
Given that the RDA for iron is 18 mg/day for women (33 mg for vegetarian women) and 8 mg/day for men (14 mg for vegetarian men), and that only a fraction is absorbed, it’s easy to see how athletes might develop a false sense of security about their intake. While the research is still limited, endurance athletes likely require higher iron intake than the general population to meet their physiological needs. Click HERE for a list of foods high in iron.
Supplements
In addition to increasing dietary iron intake, oral iron supplementation is typically part of the first-line treatment for iron deficiency. While there is no shortage of supplemental iron on the market, some forms are absorbed more efficiently than others. And in case you haven’t noticed, absorption is an issue worth paying attention to. Ferrous fumarate appears to have a superior absorption rate compared to other forms of supplemental iron.
As previously mentioned, taking iron supplements can cause hepcidin levels to rise and remain elevated for up to 48 hours. For this reason, alternate-day supplementation is generally recommended, with some studies suggesting doses of 100–200 mg per session. Alternate-day dosing also helps minimize the common gastrointestinal side effects often associated with taking supplemental iron. To further improve absorption, moderate evidence suggests pairing iron with vitamin C, ideally at a 2:1 ratio of vitamin C to elemental iron. So if you consume 200mg of iron, you would take this with approximately 400mg of vitamin C.
Putting it all together
Now that we've covered the why, let's turn to what this looks like in practice.
- Take supplemental iron in 100-200mg doses on alternate days
- Aim to take iron in the morning
- Take before or immediately after a workout
- Consider including vitamin C with your iron (2:1 ratio of vitamin C to iron)
- Consider ferrous fumarate, which is generally well absorbed
*Labs - Labs to test for iron deficiency are recommended to be collected in the morning in a rested and hydrated state following an overnight fast. Early morning blood draws following a rest day are ideal. Ferritin levels are the best proxy for full-body iron status. Research suggests that levels under 50 ng/ml suggest inadequate iron levels for athletes.
Keep moving forward!
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Creamy Vanilla Millet Porridge
In the spirit of iron-rich foods, a single serving of millet has nearly 8mg of iron! When you include other iron-rich toppings you might include, this can pack quite a punch!
