How A Runner’s Blood Test Could Improve Both Health & Performance

The athlete blood-testing company, InsideTracker, recently published results of its massive runner database. The findings could help you.

Last Updated:

A company that specializes in blood testing endurance athletes recently published the first medical article describing how a runner’s training, diet, and weight can affect important blood markers. 

InsideTracker’s 1InsideTracker. (n.d.). Www.insidetracker.com. https://www.insidetracker.com/ research with 23,000 runners revealed potentially important links to cholesterol levels, chronic inflammation, the need for critical nutrients, and much more.

We runners are always looking for signs of improvement. We track our paces, distances, heart rate, and other body signals. We even appreciate indications that things are headed in the wrong direction. They tell us we need to reassess and re-boot.

However, few runners have ever tracked their blood markers over time. That seems like a missed opportunity. After all, physicians consider blood testing the most basic and informative measure of body wellness, or body dysfunction.

This new study from InsideTracker (IT) investigated 30 different blood markers among more than 23,000 runners. The paper, which appears in the free, full-text form at PLoS One, is titled “Dose response of running on blood biomarkers of wellness in generally healthy individuals.”2Nogal, B., Vinogradova, S., Jorge, M., Torkamani, A., Fabian, P., & Blander, G. (2023). Dose response of running on blood biomarkers of wellness in generally healthy individuals. PLOS ONE18(11), e0293631–e0293631. https://doi.org/10.1371/journal.pone.0293631

‌The sheer volume of information in the paper is so great that it helps to look first at an individual case study.

A blood sample in a test tube.

Shalane Flanagan Wants To Do 6 Marathons In 7 Weeks

In the fall of 2021, Shalane Flanagan spotted a rare opportunity. In that Covid year, three traditional springtime marathons moved to a fall date–Tokyo, Boston, and London.

Flanagan, a former elite runner with an Olympic medal and a New York City Marathon win on her resume, realized she could enter and run all six World Marathon Majors in seven weeks from mid-September to early November: Berlin, London, Chicago, Boston, Tokyo, and New York City.

She decided to go for it.

There was only one problem: Flanagan wasn’t training seriously and wasn’t sure her body was ready for the big challenge. She decided to have InsideTracker test her blood before and several times during the six-marathon quest.

It proved a wise decision. The first test revealed that her Vitamin B-12 level was “sub-optimal.” Flanagan consulted with an endurance-savvy nutritionist who recommended that she start taking a B-12 supplement while also eating more B-12 foods like organ meats, fish, dairy, and eggs. 

How A Runner’s Blood Test Could Improve Both Health & Performance 1
Shalane Flanagan, Photo Credit: Wikimedia

A Strong Start But Then …

Flanagan ran strong in Berlin, the first of her six marathons. She was pleased by how she felt and her 2:38 finishing time.

London was a different story.

She found the local food unappealing, didn’t eat well, and felt terrible for the entire 26.2 miles. She “bonked” in the second half of the marathon. She had to stop and walk several times for the first time in her running career. 

After London, a distressed Flanagan took another InsideTracker blood test. This one revealed dramatically high liver and muscle-breakdown markers, a high level of systemic inflammation, and an increase in white blood cells.

“I felt really fatigued,” said Flanagan, “and the blood results told me it wasn’t just in my head. I knew I had to make a big reassessment.”

She made significant changes to her training, travel, and fueling. She also began to concentrate on consuming more carbs in the 30 to 60-minute “window” after workouts. “It was hard,” she admitted. “You’re not necessarily hungry then.”

But it also made a difference. She felt better and ran much stronger in marathons three and four: Chicago and Boston. “I think I might have been under-fueled for much of my career,” she concluded.

How A Runner’s Blood Test Could Improve Both Health & Performance 2
Shalane Flanagan, Photo Credit: Flickr

Better Blood = Faster Marathons

One more blood test after Boston proved Flanagan was back on track. The elevated muscle and inflammation markers had dropped down nicely, which boosted her mental state.

This gave her the confidence to race more aggressively in her sixth and last marathon in New York City. “I wanted to show that I could not just survive but actually thrive,” she said.

And she did, finishing in 2:33:32 with a negative-splits effort. She covered the second half of the tough New York City course a minute faster than the first.

Digging Into That New Paper About Runners’ Blood

The InsideTracker paper presents an almost mind-boggling array of results, as you would expect with so many runners and measured markers. There’s no single conclusion and no simple take-home message.

Still, the findings are reassuring for runners and, in some cases, instructive.

Here are the study details:

The runner subjects fell into three groups:

  • Low-volume runners (less than 3 hours a week)
  • Middle-volume runners (3 to 10 hours a week)
  • High-volume runners (more than 10 hours a week)

The three levels of training volume allowed InsideTracker to correlate a runner’s total training with his or her key blood biomarkers.

IT then compared these results with those from 4,000 “generally healthy” but sedentary non-runners. To add a little pizazz to its finding, IT also included 82 professional runners—those at the highest end of training volume.

The IT analyses included more than 30 blood biomarkers. These ranged from the well-known (like cholesterol, glucose, and hemoglobin) to the moderately known (cortisol and C-reactive protein) to the little-known (sex hormone binding globulin and red blood cell distribution width).

The goal?

To determine if regular running, from modest to very high weekly mileage, affected key blood biomarkers. And, if so, was that effect good or bad?

A strong runner.

The 3 Big Take-Home Messages

Three key findings stood out most prominently from the results. We’re simplifying them here for brevity and clarity. As noted, you can read the full text on the PLoS One web page.

First, “Recreational running appears to be an effective intervention toward modifying several biomarkers of improved metabolic health.”

These included better cholesterol readings among runners and lower chronic inflammation levels. 

Sure, we all want to run faster. But “improved metabolic health” is important on its own and undoubtedly related to improved performance.

Second, some of these positive outcomes occurred because of “an apparent dose-response relationship between running volume and BMI.” That is, running is good for you in part because it helps you maintain a healthy body mass index or weight.

IT used a genetic analysis tool called Mendelian Randomization to verify this relationship. Mendelian Randomization showed that lower BMI likely played a causative role in achieving improved metabolic health.

So, stick to your running program, but also keep an eye on your weight.

Third, high-level runners should pay attention to their magnesium intake and blood magnesium levels. “Serum and red blood cell magnesium were both significantly lower in professional runners relative to all other groups.” 

You can get more magnesium with supplements like magnesium citrate or magnesium malate. You can also eat more pumpkin seeds and other seeds, dark leafy greens, nuts, beans, and dark chocolate. All are naturally rich in magnesium.

InsideTracker also discovered a case of “sexual dimorphism” when it came to the question of low ferritin in runners. Translation: Sometimes, males and females are different.

Past ferritin research with female runners has shown that a low ferritin level is often linked to low iron status and anemia.

However, in the IT database, low ferritin in males did not indicate poor iron status. It appeared to be a positive signal. It showed that the runners had a low level of chronic inflammation, meaning they had adapted well to their training.

Elite runners running fast on a track.

An Interview With The Top InsideTracker Scientist

Here, we asked Gil Blander to respond to several key questions regarding the new research paper, of which he was the lead author. Blander is the founder and chief scientific officer of InsideTracker.

What do you consider the most important finding of the analysis? Why?

“Most exercise intervention studies looking at serum marker effects are performed in either metabolically unwell people (e.g., they are obese, or have type II diabetes) or well-trained athletes. Very few look at how exercise affects the already healthy. 

“That’s what we were able to do. We showed that increasing amounts of running were associated with improved biomarkers of metabolic wellness like lower blood lipids, glycemia, and inflammation even in those who were healthy to begin with.”

Did any of your results come as a surprise?

“As scientists, when we approach data analysis, we anticipate and hope to be surprised! A couple of things that stood out was the suboptimal magnesium status in our elite runners, which, after reviewing the literature, maybe was not unexpected. 

“Also, we didn’t expect to see low ferritin levels in higher-mileage runners. This probably didn’t indicate low iron stores, but low levels of inflammation, a good thing.”

Runners' legs.

More About BMI, Running, And Metabolic Health

It’s almost verboten these days to talk about lower body weights among runners. Yet your data seems to indicate quite strongly that lower BMI is healthier. Could you explain what you found and what it means?

“I think our data suggests that lower BMI within the normal healthy range does associate with an improved biomarker signatures, but one can’t extrapolate that to lower is always healthier. In general, our data supports what’s already been published in epidemiological literature (i.e., maintaining a healthy weight in addition to regular exercise is consistently associated with better health outcomes).”

How might an individual runner use the IT service to improve his/her running and health?

“By monitoring their blood, runners can receive recommendations on how to optimize their markers for top performance that could include personal records. We quite often see blood results indicating low ferritin (low iron) among runners, particularly pre-menopausal runners. This can lead to anemia and decreased performance. 

“Additionally, we have diagnosed runners with low testosterone, high cortisol, and very high CK (physical and emotional stress caused by overtraining), among other issues.

“Runners can use IT to gain a clearer understanding of their body’s overall health, and to develop a personalized action plan that includes food, supplements, training, and lifestyle changes to optimize their performance.”

For a closer look into runner’s nutrition, check out the following guide:


Photo of author
Amby Burfoot stands as a titan in the running world. Crowned the Boston Marathon champion in 1968, he became the first collegian to win this prestigious event and the first American to claim the title since John Kelley in 1957. As well as a stellar racing career, Amby channeled his passion for running into journalism. He joined Runner’s World magazine in 1978, rising to the position of Editor-in-Chief and then serving as its Editor-at-Large. As well as being the author of several books on running, he regularly contributes articles to the major publications, and curates his weekly Run Long, Run Healthy Newsletter.

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.