Think about that feeling of finishing a race, whether it’s a 5k or 10k, even a half marathon. For me, there’s that immediate feeling of, “it’s done, I’ve finished,” followed by a sense of accomplishment and a deep curiosity about my time. While I’m pondering how I did, the fatigue is starting to creep in. But honestly, it’s nothing that a short recovery walk with some water and a quick snack won’t fix.
But those are shorter races. How about that feeling that comes after a marathon, or the even more demanding ultra? Now we’re talking about the kind of fatigue that a short walk and some electrolytes simply can’t fix. For marathons, full recovery can take more than a few days and, for some ultramarathons, full recovery can take weeks.
Let’s talk about that.
Why These Races Take Such a Toll
We’ve always known that high endurance events such as the marathon and ultra are hard on the body. And even as early as the late 1800s, we knew that our bodies paid a price for running marathons. Until recently though, we’ve lacked the scientific ability to figure out exactly what that price was and why and how it was being paid.
In February of 2026, a study was released in the journal Blood: Red Cells & Iron, led by Travis Nemkov, PhD, an Associate Professor at the University of Colorado Anschutz Medical Campus. This study evaluated red blood cells in runners who participated in both marathon and ultramarathon distances, and the results were pretty interesting (at least I think so). And, before continuing, no, I’m not going to get all scientific here. In fact, we’ll be talking a lot about UPS trucks!
Again, this isn’t the first time that we’ve looked at red blood cells in endurance events. We’ve known for some time that red blood cells experience damage and get replaced more often post-race. But, for the first time, we now have the ability to look inside of those cells, to see what’s happening on a molecular level, evaluating things like proteins, lipids and metals to understand exactly why and how this damage is occurring.
What Red Blood Cells Do
To start off, let’s cover the basics: what red blood cells actually do.
Simply put, they carry oxygen to every part of our body and transport things like carbon dioxide out of it. That’s it. And, keep in mind that every single organ and muscle in our bodies relies on red blood cells doing their job at a near flawless level in order to keep operating at a high level.
Imagine your body was a company who relied heavily on deliveries going in and out of your doors. Your chosen fulfillment partner is UPS. As long as those deliveries and pickups happen on schedule, everything’s going to go well. But, once there’s a hiccup in the chain, things begin to break down. That’s essentially what happens in marathons and ultras, somewhere in UPS’s system, things go awry and your business pays the price.
What the Study Looked At
Researchers tested blood before and after a marathon and an ultra — the particular events being the 40k (roughly 25 miles) Martigny-Combes à Chamonix and the 171k (or 106 miles) Ultra Trail de Mont Blanc races. To analyze the samples they used testing called multiomics, which is like a 360 degree molecular health scan. For example, if a dashboard warning light on one of the UPS trucks comes on, multiomics can take the entire truck apart and evaluate every single component to see what caused the light to go on, and figure out what else may have contributed to the warning light.
Multiomics can evaluate any number of things. For this study, they specifically looked at metabolomics (chemical byproducts), lipidomics (fats), proteomics (proteins), and metallomics (metals) that exist within our red blood cells. And with that, all of the scientific terms are officially out of the way…for the most part.
What the study found
Increased Inflammation
Inflammation is one of the body’s primary defense mechanisms, the “first responders”, if you will. When damage in the body is sensed, white blood cells are sent to localized sites to fix what’s broken. It works really well as a short-term fix. However, when inflammation persists, we run the risk of healthy tissue being compromised. It’s important to note that the inflammation that we see as a result of ultramarathons, though intense, is typically short lived and highly regulated.
In both the marathon and ultra distances, increases in inflammation were present (which makes sense). One way we measure this is by looking at two markers called IL-6 and kynurenine. Simply put: when elevated levels of both are present, it means the body is under significant stress and is switching its available resources into survival mode. Think of IL-6 as the person waving a red flag saying, “Hey, we need help here!”, while kynurenine is evidence that the body has started diverting its normal “business-as-usual” fuel into emergency repairs.
Once that IL-6 starts waving its flag, other things also start happening inside of our red blood cells.
Oxidative Stress and Molecular Damage
Let’s talk about our UPS trucks again.
When delivery vehicles are used as they’re designed to be used, they’ll last a long time and remain that reliable key component of what’s needed to run our business. Let’s say though that the drivers of these vehicles go off book and use them in ways that they’re not intended to be used. Imagine them taking these trucks through “short cuts” that involve rough dirt roads, ice, snow and a few “Here, hold my beer!”, moments. Those trucks aren’t going to last very long. Before long, we’ll see them dented with paint scratched and rusting, with what’s underneath the hood experiencing similar damage.
The study suggests that the same thing happens to our red blood cells when we run in ultramarathons.
The key thing to come out of this study was that, as a result of running that 106 mile ultramarathon, the red blood cells in the runners had appeared to “age”. Red blood cells hang around in our system for somewhere around 120 days before they’re cycled out and replaced. Because of the damage that occurred to the cells as a result of the race, a red blood cell that is, say, 20 days old, may appear as if it was 100 days old.
This happened for a number of reasons.
Firstly, the cell walls themselves became more rigid, losing their flexibility. This occurs because the cell’s skin, the “lipid layer” hardens and loses its ability to flex and bend. This matters because red blood cells need to flex in order to fit through a myriad of narrow capillaries. When those cells can’t bend, they can’t bring that much needed oxygen to organs and muscles and, just as importantly, they aren’t able to carry all of those waste products, like carbon dioxide and hydrogen ions.
This lack of flexibility happens because of oxidation, and we see oxidation everywhere in everyday life. On our delivery trucks, it shows up as rust on the body panels or hardened rubber on tires and weather stripping around windows. That exact same thing happens to cell walls and just as we perceive a rusty car as being old, those red blood cells also appear to have aged before their time.
And, while oxidation was having its way on the outside of the cell, the insides suffer as well. Imagine the brakes and suspension creaking and squealing, the welds that keep the vehicle stable, and engines eventually seizing, giving up. The proteins that live inside of these red blood cells, because of this oxidation, simply can no longer do their jobs; the ability to carry badly needed oxygen to the organs and muscles is significantly compromised, not to mention carrying out waste products like carbon dioxide and hydrogen.
So, we get this double-whammy of damage that happens. The cell walls are stiffening, making it difficult to navigate those tight turns inside of smaller blood vessels and capillaries, and they lose the ability to carry the cargo from one place to another.
When The System Gets Overwhelmed
The body is an amazing thing though, and it has a process in place to deal with these issues. Think of this process as the repair facility that keeps all of those UPS trucks running in peak condition. In the body, it’s called the Lands cycle and its essentially a mobile repair shop that fixes the normal wear and tear that occurs in the body. During ultramarathons though, this system becomes overwhelmed and it cannot keep up with the demands that we’re asking of it.
It’s important to keep in mind that the body is always getting rid of older, lower performing red blood cells as part of its business-as-usual routine. What this study also showed is that the process of clearing out underperforming cells speeds up, citing that “there is enhanced removal of damaged cells from circulation”. The downside is that it takes time to produce these new red blood cells. And, as is the case with the Lands cycle, this system isn’t able to keep up with the demand.
We’ve spent a lot of time talking about ultramarathons, but what about marathons?
Marathon vs Ultramarathon
We see this same type of cellular damage happening in marathons, but the distance is less severe and therefore more recoverable — the body repair mechanisms appear to have the ability to keep up with the pace of what’s being asked of it. Lead researcher Travis Nemkov said this, “At some point between marathon and ultra-marathon distances, the damage really starts to take hold,”. So, once you push past those 26.2 miles, the math changes.
In both the marathon and the ultramarathon groups, damage to red blood cells was evident. But it was the ultramarathon group showed that red blood cell oxidation happening — the cell walls stiffening and the internal components starting to fail.
Another Interesting Observation
Probably one of the most intriguing things to come out of this study, again for me at least, was the comparisons to red blood cells after ultramarathons and what’s seen in stored blood used for transfusions. And people like Nemkov are actively looking at ways to prevent this type of oxidation from happening so that blood can last longer in bags as it’s waiting to be used.
At the end of the day, there were no recommendations or guidance telling people that they shouldn’t run ultramarathons; that wasn’t the aim of this study. It was merely something that was tested because it was interesting — the extreme nature of ultramarathons makes the body a very unique and effective laboratory for evaluating things like this. And, it helps explain why that fatigue lasts so long after we push our bodies past that marathon distance. But that’s how science works — as we do a bunch of studies that are interesting, our knowledge base grows. And as that knowledge base grows, we gain a better understanding of what’s happening in our world. We’re then able to take all of the accumulated knowledge and come up with treatments and technologies that eventually go on to make meaningful differences in people’s lives.
For something like this, perhaps it can make the difference between life and death after someone’s been in an automobile accident. Perhaps it will mean that recovery happens just that much quicker for Olympic runners so they can train more effectively. We don’t know.
It’s been said that the best reaction that a researcher can have after a study isn’t, “That’s exactly what I expected!”, but rather, “Hmmm…that’s interesting…” And that’s what this study was, a point of interest, a thing that expands our body of knowledge about a thing that may lead to future discoveries. That’s how science works, and that’s why science is cool.