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June 23, 2020 3 min read 5 Comments

Foot Strike Hemolysis - An Insidious Cause of Anemia in Athletes

Iron deficiency anemia is a common cause of fatigue in athletes. While diagnosing iron deficiency anemia itself is relatively easy, the more challenging aspect is discovering why a person suffers from runner’s anemia in the first place.

Low iron diets (vegan, vegetarian) are the most common cause for iron deficiency. Women with heavy menstrual bleeding can sometimes become anemic from this as well (and this is made worse with low iron diets!) Otherwise, the body really has no direct avenue for iron loss, other than negligible amounts lost in the course of cell shedding from the GI (gut) and genitourinary (urinary) tracts.

However, athletes are subject to a more insidious form of iron loss known as ‘foot strike hemolysis’ - sometimes known as runner’s anemia. This occurs when red blood cells are subsequently destroyed as they pass through the feet from the repetitive pounding of running. Of course, it takes a lot of running for this to happen, and a while to manifest.

Runners aren’t the only athletes susceptible to exercise induced hemolysis (destruction of red blood cells). This form of anemia has also been reported in rowers, swimmers and weight lifters, too.

However, runners typically wind up with about four times the amount of red blood cell destruction than other athletes do.

Footstrike anemia occurs most often in high mileage runners - or, in athletes whose training volume increases to a point where red cell production mechanisms are overwhelmed by training - think ‘two a day’ runs, marathoners and ultra runners - this is the group at greatest risk for foot strike hemolysis, sometimes referred to as hell strike hemolysis induced anemia.

So, how do we prevent foot strike hemolysis from happening?

One of the first, most obvious places is to look at your footwear - are your shoes worn out and compressed (more than 250-300 miles on them, typically)? Be sure to monitor your shoe's wear and tear and monitor the mileage on them - one of the best things you can do to decrease risk of injury (including foot strike hemolysis) is to maintain a good pair of running shoes.

foot strike anemia in runnersThe next most important place to look is the surface on which you run. Please, please tell me you don’t run on concrete! It’s absolutely the worst surface to run on and will absolutely increase your risk of developing runner’s anemia!. In fact, concrete is roughly 50 times harder than asphalt (the stuff most streets are made of). 

I would tell you to run in the road but we’d get in trouble for giving you dangerous advice! - but in all seriousness, running on asphalt is much, much better for your body and will greatly decrease the amount of pounding on your body, plus of course your red blood cells.

The next best place to run is dirt, of course as it’s a lot softer than asphalt. I realize not everyone has these options, so do what you can. And if you can’t run anywhere else than on concrete, it’s important to monitor and maintain your iron and red blood cell (RBC) numbers. 

If you’re a high mileage runner, or someone who has a history of anemia, you need to check iron and RBC numbers every 6 months. It’s a simple, inexpensive test to do – let your doctor know your history, and that you’re a high mileage runner and make sure you get this test done. Don’t get behind with this!

Once you’ve become anemic, it’s a deep hole to dig yourself out of – plan on at least 3 months until you feel ‘right’ again. That’s how long it takes your body to manufacture enough red cells to normal levels. It’s far easier to prevent foot strike anemia than to treat it - it’s often an insidious condition that a runner won’t notice until they’re feeling really poorly.

If you trend toward low iron, it’s ok to use an iron supplement judiciously. Don’t wait until your lab numbers show actual anemia before you start treating it – remember the idea here is to have adequate iron stores in your body, so it can offset any extra iron loss from foot striking, your diet, or menstrual status. 

You do need to be careful with iron – it should only be supplemented under a physician’s supervision and with lab testing to monitor whether you need it or not. More is definitely not better here! Excess iron can be stored, leading to toxicity and widespread inflammation. 

And as always, keep iron supplements in a safe place - it’s toxic and a common cause of death by overdose in little kids. 

5 Responses

April R
April R

June 26, 2022

I came across this article and was curious if there would be an increased risk in a female teen competitive Fencer. With Fencing drills and running regularly to increase endurance. In particular when a Fencer lunges the force where their feet contact the ground is several times their body weight.

Dr. Jason Barker
Dr. Jason Barker

February 01, 2021

Hi Jennifer – I take it your doctor has ruled out other causes of your anemia – GI issues, celiac disease, etc? Treadmill running is a lot softer than asphalt but I can only make an assumption that it would cause less red cell damage – everyone’s case is different.

Jennifer Summers
Jennifer Summers

February 01, 2021

I have needed iron infusions every 4 months the past 2 yrs. due to anemia…foot strike hemolysis. Do you know if running on a treadmill preserves red blood cell destruction in comparison to road running??

Dr. Jason Barker
Dr. Jason Barker

September 20, 2020

Hi Aaron – thanks reading and comments. While I respect Toker’s writings on Slowtwitch, his expertise is in analytical chemistry and not materials science. I drew my statement that concrete is 50 times harder than asphalt from this source:

Concrete is harder than asphalt – there is no doubt about that – and while I’m familiar with what many are saying in that the difference in hardness of these two surfaces is negligible when it comes to the elastic deformity provided by shoes and our connective tissues, every body has a different tolerance on what it can withstand when it comes to training on these surfaces (and I have seen differences clinically in people who’ve stopped running on concrete and switched to softer surfaces), hence my recommendation to run on asphalt or dirt, which I stick by.

Again, thanks for reading and commenting!

Aaron stryker
Aaron stryker

September 20, 2020

concrete is 50 times harder than asphalt? I had to read that 3 times to assure myself that it wasn’t a typo. Jonathan Toker, PhD, presents the scientific evidence behind surface hardness – stating that “the hardness difference between concrete and asphalt is insignificant when running in shoes, because the cushioning afforded by shoes far exceeds any cushioning provided by those surfaces.” 
Please. Telling people that its safer to run in the road as opposed to the sidewalk is irresponsible and not thought through, as well as making absolutely no scientific sense.

Both surfaces are effectively completely incompressible compared to our legs and have almost no effect on the expression above for the total spring constant. The difference in total stiffness when running on the two road surfaces is the negligible difference between 1+1/10,000 and 1+1/500,000."

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