Vitamin D3 was associated with lower marathon biomarkers

Vitamin D gets plenty of attention from runners, especially during winter training blocks. This study matters because it tested whether correcting low vitamin D before a marathon changed the body’s post-race stress response.

Reference: Weng et al. Effects of vitamin D3 supplementation on cardiac, muscle, and immune responses after a marathon race: a single-blind, placebo-controlled trial. Journal of the International Society of Sports Nutrition (2026) DOI: 10.1080/15502783.2026.2657316. PubMed link: https://pubmed.ncbi.nlm.nih.gov/41966073/.

Study snapshot

A quick, practical summary for runners and coaches.

Quick answer

This 8-week single-blindBlinding is when people in a study don’t know which treatment they’re getting. It stops expectations or beliefs (from patients or researchers) from skewing the results. “Single-blind” means participants don’t know; “double-blind” means participants and researchers don’t know; “triple-blind” means that the participants, researchers, and data analysts are kept in the dark. The goal is simple: fair tests and trustworthy findings., placeboA dummy treatment that looks like the real one but has no active ingredient, or no active effect for the outcome being studied, and is used for fair comparison.-controlled trial studied 21 amateur marathon runners with low vitamin D. Vitamin D3 supplementation was associated with lower post-marathon blood markers of muscle stress, cardiac stress, and protein oxidation. The main caution is that the study was small and measured biomarkers, not real-world recovery.

Key takeaways

  • Vitamin D3 was associated with lower 24-hour post-marathon creatine kinase and creatine kinase-MB.
  • The study did not show better soreness, illness risk, training return, injury risk, or performance.
  • Runners with low vitamin D may benefit from testing and correcting it, but that is practical interpretation, not proof of faster recovery.

How confident should we be?

Evidence confidence: Low

The placebo-controlled design and clear methods help. The small sample, unclear randomisationRandomization means assigning people to different parts of a study (e.g., groups in a randomised controlled trial) by chance, not by choice. This helps make the groups similar at the start and reduces bias, so any differences you see are more likely due to the treatment, not background differences. In a crossover study, randomization usually decides the order in which each person gets the treatments (for example, Treatment A first then B, or B first then A). This way, order effects—like learning, fatigue, or simple time passing—are less likely to skew the results., and biomarker-only outcomes limit how much runners should act on it.

Bottom line

Vitamin D3 may be worth thinking about if you are a runner with low vitamin D. But this study does not show that vitamin D3 helps marathon runners recover faster, race better, or stay healthier after a race. Test first. Supplement sensibly. But consult your doctor before turning your cupboard into a supplement store.

Read the deep dive below for a practical interpretation, actionable decisions, my thoughts, my Rating of Perceived scientific Enjoyment, the study details, full results, strengths, and limitations.

Running science research reviews for endurance runners

The deep dive

The details behind the headline result, including the practical meaning, full findings, limitations, and my interpretation.

Practical meaning

What does this research mean for runners and coaches?

For runners

This study is most relevant to marathon runners with low vitamin D. It suggests vitamin D3 supplementation may be associated with a smaller post-race rise in some blood markers that reflect muscle stress, cardiac stress, and protein oxidation. That is interesting, but it is not the same as showing better recovery.

If you have low vitamin D, correcting it may be useful for general health, and this study suggests it could also influence some post-marathon biomarker responses. That is interpretation, not a direct study finding.

The study does not show that vitamin D3 reduces soreness, prevents illness, protects the heart, speeds up your return to training, or improves marathon performance. It also does not tell us whether runners with already adequate vitamin D would benefit.

For trail runners and ultra runners, the idea is relevant but indirect. The study tested road marathon runners. Trail races and ultras add extra chaos: more downhill muscle damage, longer race duration, different fuelling, sleep loss, heat, cold, altitude, and the occasional poor life choice involving a headtorch at 3 am.

For coaches

The practical message is simple: vitamin D status is worth checking in athletes at higher risk of low levels. That includes runners who train indoors, avoid sun exposure, live at higher latitudes, have darker skin, cover most skin outdoors, or have a history of low vitamin D.

But vitamin D is not a substitute for the big rocks: sensible training load, enough food, enough carbohydrate, sleep, recovery time, and not racing every weekend like a caffeinated golden retriever.

Practical decision

Should runners change anything?

Maybe. Vitamin D testing may be useful if you are at risk of low vitamin D, but this study does not justify blind high-dose supplementation for every runner.

Consider this if

  • You have known or suspected low vitamin D.
  • You train through winter or get little sun exposure.
  • You are preparing for a marathon and want to tidy up basic health foundations.

Do not overreact if

  • Your vitamin D status is already adequate.
  • You are looking for a proven recovery hack.
  • You want a supplement that makes marathon legs feel less like wet concrete.

A sensible next step

Ask a qualified health professional about a blood vitamin D test. If your level is low, correct it with an appropriate dose and follow-up testing. Do not blindly take high-dose vitamin D for months. More is not automatically better; it is just more.

TIP: Never make any major changes to your training or lifestyle habits based on the findings of one study, especially if the study is small or provides low-quality evidenceA low quality of evidence means that, in general, studies in this field have several limitations. This could be due to inconsistency in effects between studies, a large range of effect sizes between studies, and/or a high risk of bias (caused by inappropriate controls, a small number of studies, small numbers of participants, poor/absent randomisation processes, missing data, inappropriate methods/statistics). When the quality of evidence is low, there is more doubt and less confidence in the overall effect of an intervention, and future studies could easily change overall conclusions. The best way to improve the quality of evidence is for scientists to conduct large, well-controlled, high-quality randomised controlled trials.. Check whether other trials confirm the findings. If there is a meta-analysisA meta-analysis quantifies the overall effect size of a treatment by compiling effect sizes from all known studies of that treatment. on the topic, look at the effect sizeA standardised measure of the magnitude of an effect of an intervention. Unlike p-values, effect sizes show the size of the effect and how meaningful it might be. Common effect size measures include standardised mean difference (SMD), Cohen’s d, Hedges’ g, eta-squared, and correlation coefficients., the variability between studies, and the quality of evidenceCertainty of evidence tells us how confident we are that the published results accurately reflect the true effect. It’s based on factors like study design, risk of bias, consistency, directness, precision, and publication bias. High certainty means that the current evidence is so strong and consistent that future studies are unlikely to change conclusions. Whereas, low certainty means more doubt and less confidence, and that future studies could easily change current conclusions..

Expert interpretation

My thoughts

Running science from Thomas Solomon at Veohtu

This study is useful, but it sits firmly in the “interesting signal” pile rather than the “practice-changing thunderbolt” pile.

I like that the researchers studied real runners doing a real marathon. That matters. Lab tests are tidy, but marathons are not tidy. Marathons involve pacing errors, weather, gels, nerves, toilet strategy, and the quiet existential crisis at kilometer 35. So, a real-world race gives the study some practical bite.

The study was also fairly transparent. The authors described the participants, dose-titration plan, blood sampling schedule, outcome measures, and statistical approach. They also stated that laboratory personnel and statisticians were blindedBlinding is when people in a study don’t know which treatment they’re getting. It stops expectations or beliefs (from patients or researchers) from skewing the results. “Single-blind” means participants don’t know; “double-blind” means participants and researchers don’t know; “triple-blind” means that the participants, researchers, and data analysts are kept in the dark. The goal is simple: fair tests and trustworthy findings., even though the study was described as single-blind.

But the sample was tiny. Only 21 runners were included in the final analysis. The allocation method was matched by baselineThe measurements taken before the intervention begins, used as the “before” or “pre-study” point for comparison with “after” or “post-study” measurements. vitamin D status, not clearly randomized. The outcomes were blood markers, not how the runners felt or performed. So, the findings are neat, but they do not yet answer the question runners actually care about: will I recover better and train normally again?

My take is boring but useful: consult your doctor to test vitamin D if you are at risk of low levels, correct it if needed, and do not treat this as a guaranteed marathon recovery tool. The small triumph is that a basic health marker may matter. The mild worry is that runners will see “vitamin D” and immediately start supplementing without checking whether they need it.

My Rating of Perceived scientific Enjoyment

owlRPsE: 6/10

I experienced moderate scientific enjoyment because the study was transparent, placebo-controlled, and relevant to marathon runners, but the small sample, unclear randomisation, and biomarker-heavy outcomes kept the practical usefulness fairly modest.

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Read on for further details about the paper.

Research question

What did the researchers ask?

The authors asked whether 8 weeks of vitamin D3 supplementation changed the body’s response to a marathon.

More specifically, the researchers wanted to know whether vitamin D3 affected post-race markers of muscle damage, cardiac stress, oxidative stress, antioxidant enzyme activity, and immune cell changes in amateur runners with low vitamin D.

Study design

What type of study was this?

This study was a single-blind, placebo-controlled, dose-titrated experimental trial.

That means the participants did not know whether they were taking vitamin D3 or placebo. The study also used a comparison group, which helps separate the supplement effect from normal post-marathon changes.

However, the article does not clearly state that the participants were randomly assigned. The researchers used matched allocation based on baseline vitamin D levels. That improves group balance, but it weakens confidence that the groups were fully comparable in all other ways.

So, the study can support cautious intervention-based interpretation. It cannot give the same level of confidence as a larger, clearly randomized, double-blind trial.

Participants

Who took part?

The researchers recruited 24 healthy amateur runners aged 20 to 60 years old. All participants had blood vitamin D below 30 ng/mL at baseline.

The final analysis included 21 runners because 3 withdrew during the marathon. The placebo group included 10 runners: 8 males and 2 females. The vitamin D3 group included 11 runners: 8 males and 3 females.

The participants had been running at least 40 km per week for the previous 6 months. The researchers excluded people with cardiovascular, liver, kidney, immune, or diabetic disease, and those with short-term exercise-related injuries.

Methods

What did the researchers do?

Who? 21 amateur marathon runners with low vitamin D
What? Vitamin D3 or placebo before a marathon
How long? 8 weeks

The runners were allocated to either vitamin D3 or placebo for 8 weeks before a full marathon.

The vitamin D3 group took 5,000 IU per day for the first 4 weeks. The researchers checked blood vitamin D after 4 and 6 weeks, then adjusted the dose by 2,000 IU if needed. The goal was to raise blood vitamin D to at least 48 ng/mL by week 8.

The placebo group took medium-chain triglycerides designed to look, taste, and smell like the vitamin D3 supplement. The placebo group also went through a mock dose-adjustment process, which helped keep the participant experience similar.

All participants completed an official marathon of 42.195 km. The runners completed 1 of 2 official marathon events. The study took place in real-world race conditions, so the researchers did not standardise or formally record in-race hydration, fuelling, caffeine intake, or non-steroidal anti-inflammatory drug use.

The researchers collected blood before supplementation, during supplementation, 24 hours before the marathon, 30 minutes after the race, 2 hours after the race, and 24 hours after the race. They measured vitamin D status, muscle damage markers, cardiac stress markers, oxidative stress markers, antioxidant enzymes, white blood cells, and selected immune cell types.

Main findings

What did the study find?

Vitamin D status Increased with vitamin D3
Creatine kinase and creatine kinase-MB Lower at 24 hours with vitamin D3
Immune responses No clear vitamin D3 effect

The vitamin D3 protocol raised blood vitamin D. By the end of the 8-week intervention, the vitamin D3 group had higher vitamin D levels than the placebo group.

The marathon increased markers of muscle damage and cardiac stress in both groups. That included creatine kinase, creatine kinase-MB, lactate dehydrogenase, and cardiac troponin I. In plain English, the race shook the system. No surprise there; it was a marathon, not a gentle stroll to buy a pint of milk.

For muscle and cardiac stress markers, creatine kinase and creatine kinase-MB changed differently between the groups over time. Both markers rose after the race in both groups, but they were significantly lower in the vitamin D3 group than in the placebo group 24 hours after the race. This suggests a smaller delayed biomarker response in the vitamin D3 group.

Cardiac troponin I also rose after the race, but vitamin D3 did not produce a statistically clear group difference. The authors reported that fewer vitamin D3 runners crossed the cardiac troponin I threshold after the race, but this was a descriptive observation. It should not be treated as strong evidence of cardiac protection.

For oxidative stressOxidative stress happens when the body makes more reactive molecules than it can safely control. This can damage cells, but small bursts also help the body adapt to exercise and other challenges., protein carbonyls changed differently between groups over time. Protein carbonyls rose 24 hours after the race in the placebo group, but not in the vitamin D3 group. However, the researchers did not find statistically significant between-group differences at individual time points, the effect on oxidative stress was uncertain, especially because the other oxidative stress marker, thiobarbituric acid reactive substances (TBARS), did not show a clear vitamin D3 effect. And, the antioxidant enzymes superoxide dismutase and catalase also did not show clear vitamin D3 effects.

For immune responses, the marathon changed white blood cells and some immune cell types over time. White blood cells and neutrophils rose shortly after the race, while lymphocytes fell 30 minutes and 2 hours after the race. Vitamin D3 did not clearly change these broad immune responses.

The immune-cell correlation analyses were exploratory. The authors did not adjust those analyses for multiple comparisonsMultiple comparisons happen when you test many hypotheses or look at many outcomes in the same data set. Each test has some chance of finding a difference just by luck (this is a “false alarm”, i.e., a false positive). When you do lots of tests, those chances add up. That means you’re more likely to find something that looks “significant” but is actually just noise., so they are best viewed as hypothesis-generating.

The authors concluded that vitamin D3 supplementation may attenuate protein carbonyl responses and was associated with lower creatine kinase and creatine kinase-MB levels 24 hours after marathon running. They also concluded that immune changes were mainly time-dependent, with descriptive differences in recovery patterns between groups.

What helps my confidence in the findings?

The strengths

  • The study used a placebo-controlled design.
  • The researchers studied real runners completing real official marathons.
  • The study focused on runners with low vitamin D, the group most likely to respond.
  • The methods, supplementation plan, blood sampling schedule, and statistical approach were clearly described.
  • The researchers blinded laboratory personnel and statisticians to group allocation.

What limits my confidence in the findings?

The limitations

  • The final sample was small, with only 21 runners.
  • The study was single-blind, and the article did not clearly state that group allocation was randomized.
  • The outcomes were biomarkers, not runner-centred outcomes such as soreness, fatigue, illness, injury, return to training, or performance.
  • In-race hydration, fuelling, caffeine use, and non-steroidal anti-inflammatory drug use were not standardised or formally recorded. These are all things that could affect the outcome of the study.
  • The follow-up only lasted 24 hours after the marathon.
  • The findings may not apply to runners with healthy vitamin D levels.
  • The researchers did not measure post-race hemoglobin or hematocrit, so they could not correct biomarker changes for changes in plasma volume that occur during exercise and which are typically large after long-duration exercise, such as a marathon.
  • The exploratory immune correlation analyses were not adjusted for multiple comparisons.

Funding and conflicts

Who funded the study?

The study was funded by the Ministry of Science and Technology, Executive Yuan, Taiwan, and the National Science and Technology Council, Taiwan.

The authors declared no conflicts of interestA conflict of interest happens when a person or group has a personal, financial, or professional interest that could influence their judgment. It does not always mean they did something wrong. But it can create bias or make others question whether the decision or result is fully fair and trustworthy.. That lowers concern about obvious commercial bias. The paper named the vitamin D3 product used in the study, but the source did not report industry funding from the manufacturer.

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