Two runners cross the same marathon finish line, hit the same wall at mile 20, and post nearly identical times. Their blood tells different stories: one shows a sharp spike in the enzymes that leak out when heart tissue is stressed, the other barely registers a change.
A new trial suggests vitamin D status before race day may explain part of that gap. Researchers from Taipei Medical University, National Taiwan Sport University, and Universitas Gadjah Mada gave vitamin D3 to marathoners who started out deficient, then tracked their cardiac, muscle, and immune markers from 24 hours before the gun to 24 hours after the finish.
The setup: only deficient runners qualified
The trial, published in the Journal of the International Society of Sports Nutrition, screened amateur runners for blood 25-hydroxyvitamin D and enrolled only those below 30 ng/mL, the threshold widely used to define insufficiency. Twenty-four runners started the protocol and 21 finished it: 11 in the vitamin D3 group, 10 on placebo.
The supplemented group took 5,000 IU of vitamin D3 daily for eight weeks before their official marathon, with doses bumped by 2,000 IU if their blood levels weren’t climbing fast enough toward a target of 48 ng/mL. By race week, their vitamin D status had moved significantly above the placebo group’s, which stayed flat in the low-20s.
Everyone then ran the same 42.195-kilometer race under normal field conditions, with blood drawn a day before, then at 30 minutes, 2 hours, and 24 hours after crossing the line.
Cardiac markers moved less in the supplemented group
Marathon running reliably elevates creatine kinase and creatine kinase-MB, enzymes that spill into the bloodstream when muscle and heart tissue take on mechanical stress. Both groups saw these markers climb after the race, but the interaction between group and time was statistically significant for CK and CK-MB alike, and by 24 hours post-race the vitamin D3 group’s levels were significantly lower than placebo’s.
Cardiac troponin I, a marker doctors use to flag actual heart muscle injury, told a similar descriptive story. At 30 minutes post-race, 45% of the vitamin D3 group tested positive for elevated troponin compared with 70% of the placebo group. At 2 hours, it was 64% versus 100%, meaning every placebo runner had detectable troponin elevation. By 24 hours, positivity had fallen back to 18% versus 30%.
Exercise-induced troponin release after endurance events is common and, in healthy hearts, generally considered a benign, transient phenomenon rather than a sign of damage. Even so, a supplementation strategy that reduces how many runners show a positive troponin signal at all is a meaningful finding for a marker cardiologists watch closely in endurance populations.
Oxidative stress told a more selective story
The researchers also tracked oxidative stress through two separate markers: thiobarbituric acid reactive substances (TBARS) and protein carbonyls. Only one of them responded to vitamin D3.
Protein carbonyls, a marker of oxidative damage to proteins, rose significantly in the placebo group between the day before the race and 24 hours after it. In the vitamin D3 group, that rise never reached statistical significance, and the group-by-time interaction for protein carbonyls was itself significant. TBARS, by contrast, rose in both groups by a similar amount regardless of supplementation, as did the antioxidant enzymes catalase and superoxide dismutase.
That split matters because it argues against a simple “vitamin D is a blanket antioxidant” story. Instead, the supplementation effect looks targeted: it showed up in protein oxidation and in the cardiac and muscle injury markers, but not in lipid peroxidation or the body’s own enzymatic antioxidant response.
Immune cells shifted the same way in both groups
Marathon running is well known to trigger a temporary redistribution of immune cells, sometimes called an “open window” that researchers have linked to short-term susceptibility to upper respiratory infection in the days after a race. This trial saw the expected pattern: white blood cells and neutrophils rose sharply at 30 minutes and 2 hours post-race, while lymphocytes dropped over the same window, in both groups.
Critically, there was no significant group-by-time interaction for these immune counts. Whatever vitamin D3 was doing to muscle and cardiac enzymes, it was not detectably blunting the acute immune cell shifts that follow marathon-distance exercise, at least not in a sample this size. Flow cytometry on specific immune cell subsets, including natural killer cells and activated T cell populations, showed only modest, non-significant differences between groups.
Why deficiency status was the entry ticket
The trial’s design choice to enroll only runners below 30 ng/mL is worth dwelling on. Vitamin D deficiency has been tied in prior research to impaired muscle repair, altered inflammatory signaling, and blunted immune cell function, mechanisms that could plausibly make deficient athletes more vulnerable to exercise-induced tissue stress in the first place.
By restricting enrollment to runners who were already low, the researchers were effectively testing whether correcting a deficiency, rather than pushing an already-sufficient athlete into a “more is better” range, produces a measurable recovery benefit. The CK, CK-MB, and protein carbonyl findings suggest correcting deficiency did something real. Whether the same protocol would move the needle for a runner who starts the race already replete in vitamin D remains an open question this trial wasn’t built to answer.
The dose and timeline that produced these results
The practical takeaway isn’t a single pill the morning of the race. It’s eight weeks of daily supplementation, starting at 5,000 IU and adjusted upward if blood levels weren’t tracking toward the 48 ng/mL target, with the goal of correcting a documented deficiency well before race day.
That timeline lines up with how vitamin D metabolism actually works. Serum 25-hydroxyvitamin D rises gradually over weeks in response to supplementation, not overnight, which is part of why the researchers built in an eight-week runway rather than a pre-race loading dose.
For runners managing broader recovery strategies, the creatine guide covers a separate but complementary angle on muscle repair, while endurance athletes weighing supplement stacks may also want to look at how protein timing factors into post-race recovery.
What this trial does and doesn’t establish
With 21 total finishers split across two arms, this is a small trial, and the authors’ own framing reflects that: the immune findings were described as “time-dependent, with descriptive differences” rather than a proven effect, and several markers showed no group difference at all. It’s a signal worth following up, not a settled verdict.
What it does add is a controlled, blood-marker-based data point to a growing body of research on vitamin D and endurance recovery, in a population, deficient recreational marathoners, that has been underrepresented in prior studies focused mostly on elite or already-sufficient athletes. The next test is whether a larger trial reproduces the CK-MB and protein carbonyl findings, and whether they translate into anything a runner would actually feel, like faster return to normal training load.
For now, the most defensible reading is narrow but real: runners who show up to marathon training already low in vitamin D may be running at a measurable disadvantage in how their heart and muscle tissue handle the stress of 26.2 miles, and eight weeks of correcting that deficiency changed what their blood looked like afterward.
