What I like best about iron is that I actually understand why it’s important for me as an endurance athlete. I certainly believe that other micronutrients—riboflavin, say—are important, but I’d have a tough time articulating why. For iron, though, it’s crystal clear: it’s a key component of hemoglobin, the protein in red blood cells that ferries oxygen from your lungs to your muscles. If you’re short on iron, you risk starving your muscles of oxygen. It doesn’t get any more important than that.
Pretty much every endurance athlete I’ve ever met has at one point or another been convinced that they’re low on iron. And surprisingly often, they’re right. The standard estimate is that 3 to 11 percent of male athletes and 15 to 35 percent of female athletes have some form of iron deficiency, but that’s across many sports. Zero in on higher-risk groups like female endurance athletes and you sometimes get prevalences of over 50 percent—and that’s a pretty clear sign that getting enough iron is trickier than it seems.
A new review in the European Journal of Applied Physiology, from a team of mostly Australian researchers led by Peter Peeling of the University of Western Australia, sifts through the existing evidence to give some advice to athletes on how to get their iron levels right. There’s no simple panacea that solves everything instantly. Having too much iron can be as serious as having too little, potentially leading to problems in the liver and heart, so you can’t just pump yourself full and hope for the best. But if you’re aware of some of the most common pitfalls, you’ll have a better chance of getting it right.
The first question to consider is why athletes are so susceptible to low iron. There are a bunch of theories, such as the idea that runners crush red blood cells with each footstrike. But it’s unclear that this really makes a big difference, and doesn’t explain why athletes in non-impact sports like cycling are also at risk. Gastrointestinal bleeding and blood in the urine are also possible sources of iron loss, but neither is particularly common among endurance athletes. Certainly menstruation is a significant factor, and is probably the main reason female athletes are at higher risk than men. You also lose some iron in sweat. And athletes may be more prone than most to have low levels of iron in their diets, either because they’re avoiding iron-rich foods like red meat or because they’re not getting enough calories overall.
But most of the recent research has focused on an inflammation-linked hormone called hepcidin, whose levels spike after hard workouts and inhibit iron absorption, with the worst effects occurring between about three and six hours afterwards. If you’re training hard and eating your main iron-rich meals during that relatively long post-workout window, you may simply not be absorbing it. Add in the fact that tea, coffee, whole grains, legumes, and dairy products contain components that interfere with iron absorption, and the fact that hepcidin levels gradually increase throughout the day independent of exercise, and you’ve suddenly got a pretty complicated jigsaw puzzle to piece together.
One solution is to focus on the period immediately after a morning workout, within about one hour of finishing. Hepcidin levels are low in the morning, and they take time to rise after exercise, so that’s a good time to eat iron-rich foods (fish, poultry, meat, and—somewhat ironically—whole grains) or, if needed, take a supplement. How do you know if you need a supplement? That, too, is complicated.
It’s surprisingly difficult to nail down how much iron is enough. For non-athletes, doctors tend to look at how much hemoglobin you have in your blood. But that’s not a reliable marker for endurance athletes, because training increases your blood volume which in turn dilutes the concentration of hemoglobin. The more common marker for athletes is ferritin, which is the form in which iron is stored in the bone marrow, liver, and spleen for later use. There’s reasonable (but by no means unambiguous) evidence that if you’ve got normal hemoglobin but low ferritin, your athletic performance will be compromised.
The review paper offers some athlete-specific threshold values for three stages of iron shortage. In addition to hemoglobin and ferritin, they include another marker of iron status called transferrin saturation, which is essentially what fraction of your blood’s iron capacity is being used.
- Stage 1 is iron deficiency. You’ve got normal hemoglobin (above 115 g/L) and transferrin saturation (above 16 percent), but your ferritin stores are a bit low (below 35 micrograms/L). There’s little evidence that your performance will suffer at this point, but it’s a red flag that problems could be in your future if you don’t get your iron levels up.
- Stage 2 is iron-deficient non-anemia. The “non-anemia” part indicates that your hemoglobin levels are still fine (above 115 g/L), but now your transferrin saturation is below 16 percent and your ferritin is below 20 micrograms/L. This is where the interesting debates are. Your family doctor may think you’re fine, but most sports scientists would say that your performance will improve if you get your iron levels up.
- Stage 3 is iron-deficient anemia. Now, in addition to low transferrin saturation and ferritin below 12 micrograms/L, your hemoglobin is below 115 g/L. There’s no ambiguity here: you’re in trouble and need to correct it.
So how do you correct it? Changing your diet is the first option, adding iron-rich foods and thinking about the timing to avoid eating during periods of high hepcidin or alongside other foods that interfere with absorption.
The second option is oral iron supplements, which are effective but cause GI upset in many people. There are lots of different types of oral iron; the standard recommendation, according the review paper, is slow-release ferrous sulphate at a dose of ~100 mg per day, taken with some vitamin C to enhance absorption. Some studies see ferritin increases of 40 to 80 percent after two to three months on this type of regimen.
And finally, there’s the option of taking iron intravenously or through intramuscular injections. This undoubtedly works: studies show ferritin increases of 200 to 400 percent, and by bypassing the gut you avoid GI upset. But it’s also a lot more hassle, and you probably only want to consider it if the first two options haven’t worked.
There are lots of other nuances to the iron picture—for example, it’s particularly crucial to have sufficient ferritin stores before heading to altitude, in order to support the production of new red blood cells. I saw a talk a few weeks ago by Lindsay Golich, a sports physiologist with the U.S. Olympic Committee in Colorado Springs, and she mentioned that they have athletes get a blood test before coming to train at altitude, and if ferritin levels are too low (the precise threshold varies depending on the demands of each sport), the athletes are told to stay home.
In the end, of course, not every bad race or blah feeling means you’re low on iron. Before you start mainlining iron supplements, you should get a blood test—the review article suggests getting tested somewhere between quarterly and yearly if you’re in serious training, depending on your risk profile. But it’s something to keep in mind, because the evidence suggests low-iron is pretty common—and your muscles definitely need that oxygen.
My new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, with a foreword by Malcolm Gladwell, is now available. For more, join me on Twitter and Facebook, and sign up for the Sweat Science email newsletter.
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