Essential Biomarkers Every Runner Should Track for Better Performance

If you run, you probably track something. Maybe it’s your mileage, your heart rate variability, or your estimated VO2 max. But if you only pay attention to the numbers on your watch, you’re likely leaving time on the table.

I know from experience. After an international trail race this year, I couldn’t seem to bounce back. As the fatigue lingered, forcing me to slow down my workouts and even skip some runs, I realized it was time to look inward, so I asked my doctor for a basic biomarker test.

Biomarkers are measurable indicators of biological processes—including hormones, vitamins, inflammation, and more—that act as your body’s report card. They show how well your systems are holding up under the stress of life and training.

When my panel came back, it showed I was deficient in vitamin D, iron, and magnesium—all nutrients that play a role in energy and endurance.

As a 17-time marathon runner and UESCA- and RRCA-certified coach, here’s one thing I know: Internal red flags will make themselves known in your training.

Low iron or ferritin? Your muscles won’t get the oxygen they need, which means slower splits and a lot more struggle. Blood sugar control issues? You might bonk halfway through your long run. Off-the-charts cortisol? Forget hitting a PR—your nervous system’s stuck in survival mode. Chronic inflammation? Your muscles may not be able to repair and rebuild, reducing recovery and increasing your risk of injury when running.

Biomarker testing can give you a clear blueprint for adjusting your nutrition, recovery, sleep, and stress management to maximize your potential. Staying on top of these runner-specific biomarkers can be the difference between crushing your next race or crashing halfway through training.

Essential Biomarkers for Runners

Nutrient Biomarkers

You can only run as well as you fuel—a lesson it took a good seven marathons for me to really embrace. I regularly struggled with the final 10K until I hired a nutritionist before the Chicago Marathon in 2021 and learned what nutrients I was lacking and how to more easily incorporate them into my diet. The result? An eight-minute personal best time.

Vitamin D

More than 90 percent of endurance athletes aren’t getting enough vitamin D, according to a 2023 study published in the International Journal of Environmental Research and Public Health. ¹

Vitamin D plays an important role in bone health, and a deficiency not only increases the risk of stress fracture, research ² has found, but it also ups the risk of muscle and other soft tissue injuries, per a 2018 study on athletes. ³

You probably already know the importance of eating vitamin D-rich foods, but you also need adequate exposure to sunlight for your body to synthesize vitamin D. If you run before the crack of dawn because of your job or to beat the heat, try to get outside for 5 to 30 minutes as close to noon as possible to get enough sun exposure.

Magnesium and Potassium

Runners focus so much on sodium, we sometimes forget we’re also losing magnesium and potassium as we sweat. A full 57 percent of us aren’t getting enough of those two essential electrolytes, which help with muscle function and fluid balance, according to the 2023 study above.

The good news: A 2020 study published in the journal Nutrition suggests that runners who hit their daily quota have better race-day performances and cardiovascular health. ⁴

Iron and Ferritin

Your body needs iron to make hemoglobin and myoglobin, proteins that aid oxygen transport to the muscles so those cells can produce enough energy to keep you on pace. Ferritin is a protein in which your body stores extra iron.

Between 15-35 percent of female athletes and 3-11 percent of male athletes are iron deficient, according to a paper published in 2019 in the European Journal of Applied Physiology. ⁵ Common signs you might be among them include fatigue and impaired performance, a 2023 review published in the journal Life determined. ⁶

If you’re not getting enough iron through your diet or via supplements, you may want to talk to your doctor about iron infusions. Older research shows these can increase performance metrics like running economy, lactate threshold, and VO2 max, as well as increasing iron stores. ⁷

Cardiovascular Biomarkers

Inflammation gets a bad rap, but it’s actually a necessary part of the recovery process. When I did a genetic test a few years ago, I learned that I’m more likely to experience increased inflammation after training. This helped me better structure my weekly programming so I was giving myself enough time to recover.

C‑Reactive Protein and High‑Sensitivity C‑Reactive Protein (hs‑CRP)

Running is a high-impact sport. It causes microscopic muscle damage that triggers an inflammatory response to initiate the repair process.

CRP is a general marker of systemic inflammation, and the amount and intensity of exercise can influence how much your CRP levels increase. An acute increase post-workout or race is normal, but chronic inflammation can impair recovery and performance, a 2024 review published in Cureus found. ⁸

You can feel excessive inflammation in the form of delayed onset muscle soreness, AKA DOMS. Inflammation isn’t the cause of DOMS; it’s the result of too much microscopic muscle damage. That stiff, sore sensation can last for 24 to 72 hours after a really tough workout, but if you’re feeling it after your normal workouts, that’s a sign that your body isn’t recovering properly.

Hormone Biomarkers

Running can help improve your hormone levels, but it can also disrupt them, especially if you’re overtraining and underfueling.

Because hormones regulate a whole bunch of bodily functions (and play a major role in overall energy levels and emotional wellbeing), one of the first things my doctor recommended when I mentioned my recent fatigue was a hormone panel. My issues ended up not being hormone-related, but the exercise showed me how important it is for runners to stay on top of a few key hormones.

Testosterone

This hormone, present in both men and women, helps build muscle and boost red blood cell count. More red blood cells mean more oxygen to your working muscles, allowing you to work harder.

There’s a correlation in long-distance runners between low-energy availability and reduced testosterone levels, according to a 2017 study in the European Journal of Applied Physiology. Researchers found that bone injuries are 4.5 times more prevalent in male athletes with low testosterone. ⁹

One potentially easy way to remedy low testosterone levels? A 2015 study published in The FASEB Journal suggested that athletes specifically can benefit from optimizing their vitamin D levels. ¹⁰

Cortisol

Known as “the stress hormone,” cortisol is activated during exercise to prioritize certain functions, like upping the amount of glucose in your blood for easily accessible energy, while downgrading others, like digestion.

A cortisol surge during exercise is normal and dissipates quickly afterwards. But if your cortisol stays elevated, it’s a potential red flag for overtraining syndrome—when you do too much without enough recovery—according to a 2019 study in the BMJ Open Sport & Exercise Medicine. ¹¹ And endurance athletes are more likely to see cortisol imbalances, according to an older study published in the journal Psychoneuroendocrinology. ¹²

Stress is stress; your body can’t tell the difference between the stress of exercise and the stress of a looming deadline at work. That’s why proper recovery—whether that’s an extra day between workouts or even skipping a workout entirely—is so important.

Practicing meditation has long been shown to lower cortisol levels, plus this kind of mindfulness training can give your endurance performance a boost, a 2020 study published in Neural Plasticity found. ^{13 14}

Performance Metrics

We’re living in an era of data-driven self-optimization, but not all of the data we get from the supercomputers we wear on our wrists is accurate. Plus, obsessing over all those watch-based metrics can be damaging to your mental health (mine went off the rails after my Garmin deemed my training “unproductive” for an entire marathon training cycle). For the most accurate performance metrics, it’s best to work with an expert. Still, there are some on your watch that can offer potential red flags.

VO2 Max

Your VO2 max measures how well your body can deliver oxygen to the working muscles; the more oxygen you can efficiently consume and use, the easier exercise will feel at a given intensity level.

A higher VO2 max isn’t just a sign of increased fitness, it’s a positive sign for health and longevity. While many fitness watches estimate VO2 max, they aren’t currently as accurate as professional testing; your best bet is to go to a performance lab, physical therapy facility, or medical center for testing.

Lactate

Lactate is a metabolic byproduct in the blood that’s produced when your cells don’t have enough oxygen to break down glucose. It’s used to determine lactate threshold—the intensity you can sustain for about 60 minutes. ¹⁵

Lactate threshold helps you define training zones better than heart rate or pace, so you can train more efficiently. A lab test, complete with a blood test, will provide the most accurate results.

Resting heart rate

Your resting heart rate (or RHR) is the number of times your heart beats per minute when you’re sitting or lying down; you (or your wearable) can measure it right when you wake up. Generally, the lower your RHR, the fitter you are—and this is an easy metric to track as long as you consider lifestyle factors that may be in play.

Stress, illness and medications, activity levels, alcohol, and poor sleep can all cause your RHR to rise. One day isn’t a cause for alarm, but longstanding research confirms a continuously increasing RHR could be a sign of overtraining or illness. ¹⁶

How Often Should Runners Test Their Biomarkers?

Runners should start with a full panel of biomarker testing to establish a baseline and identify any potential problem areas. Make sure you work with a qualified healthcare provider to interpret your results. Testing once a year is enough for most athletes; if you have a deficiency, getting tested again at the beginning of each training cycle—probably twice a year—or even quarterly can help you stay on top of things.

The coolest thing about tracking biomarkers over time? You start to see patterns or trends based on your life, and instead of following blanket advice or copying what worked for your running buddy, you can fine-tune your training based on what’s working for you (and what isn’t).

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About the reviewer

Jason Machowsky is a board certified sports dietitian and registered clinical exercise physiologist who specializes in working with people post-rehab to return to sports, training, or daily activities.