Ultramarathoners push their bodies to the limit, but can a giant pre-race dose of vitamin D really keep their bones from breaking down? In this episode, we dig into a trial that tested this claim – and found a statistical endurance event of its own: six highly interchangeable papers sliced from one small study. Expect missing runners, recycled figures, and a peer-review that reads like stand-up comedy, plus a quick lesson in using degrees of freedom as your statistical breadcrumbs.
Statistical topics
- Data cleaning and validation
- Degrees of freedom
- Exploratory vs confirmatory analysis
- False positives and Type I error
- Intention-to-treat principle
- Multiple testing
- Open data and transparency
- P-hacking
- Salami slicing
- Parametric vs non-parametric tests
- Peer review quality
- Randomized controlled trials
- Research reproducibility
- Statistical sleuthing
Methodological morals
- “Degrees of freedom are the breadcrumbs in statistical sleuthing. They reveal the sample size even when the authors do not.”
- “Publishing the same study again and again with only the outcomes swapped is Mad Libs Science, better known as salami slicing.”
References
- Boswell, Rachel. Pre-race vitamin D could do wonders for ultrarunners’ bone health, according to science. Runner’s World. September 25, 2025.
- Mieszkowski J, Stankiewicz B, Kochanowicz A, et al. Ultra-Marathon-Induced Increase in Serum Levels of Vitamin D Metabolites: A Double-Blind Randomized Controlled Trial. Nutrients. 2020;12(12):3629. Published 2020 Nov 25. doi:10.3390/nu12123629
- Mieszkowski J, Borkowska A, Stankiewicz B, et al. Single High-Dose Vitamin D Supplementation as an Approach for Reducing Ultramarathon-Induced Inflammation: A Double-Blind Randomized Controlled Trial. Nutrients. 2021;13(4):1280. Published 2021 Apr 13. doi:10.3390/nu13041280
- Mieszkowski J, Brzezińska P, Stankiewicz B, et al. Direct Effects of Vitamin D Supplementation on Ultramarathon-Induced Changes in Kynurenine Metabolism. Nutrients. 2022;14(21):4485. Published 2022 Oct 25. doi:10.3390/nu14214485
- Mieszkowski J, Brzezińska P, Stankiewicz B, et al. Vitamin D Supplementation Influences Ultramarathon-Induced Changes in Serum Amino Acid Levels, Tryptophan/Branched-Chain Amino Acid Ratio, and Arginine/Asymmetric Dimethylarginine Ratio. Nutrients. 2023;15(16):3536. Published 2023 Aug 11. doi:10.3390/nu15163536
- Stankiewicz B, Mieszkowski J, Kochanowicz A, et al. Effect of Single High-Dose Vitamin D3 Supplementation on Post-Ultra Mountain Running Heart Damage and Iron Metabolism Changes: A Double-Blind Randomized Controlled Trial. Nutrients. 2024;16(15):2479. Published 2024 Jul 31. doi:10.3390/nu16152479
- Stankiewicz B, Kochanowicz A, et al. Single high-dose vitamin D supplementation impacts ultramarathon-induced changes in serum levels of bone turnover markers: a double-blind randomized controlled trial. J Int Soc Sports Nutr. 2025 Dec;22(1):2561661. doi: 10.1080/15502783.2025.2561661.
Kristin and Regina’s online courses:
Demystifying Data: A Modern Approach to Statistical Understanding
Clinical Trials: Design, Strategy, and Analysis
Medical Statistics Certificate Program
Epidemiology and Clinical Research Graduate Certificate Program
Programs that we teach in:
Epidemiology and Clinical Research Graduate Certificate Program
Find us on:
Kristin - LinkedIn & Twitter/X
Regina - LinkedIn & ReginaNuzzo.com
Intro & claim of the episode
Runner’s World headline: Vitamin D for ultramarathoners
Kristin’s connection to running and vitamin D skepticism
Ultramarathon world—Regina’s stories and Death Valley race
What ultramarathons do to your bones
Boy story: four stress fractures in one race
Study design—40 male runners in Poland
Missing flow diagram and violated intention-to-treat
The intervention: 150,000 IU megadose
Blinding details and missing randomization info
Measuring bone biomarkers—no primary outcome specified
The wrong clinicaltrials.gov registration
Discovery of six papers from one dataset (salami slicing)
Why salami slicing misleads readers
Inconsistent reporting across papers
Changing inclusion criteria and sloppy methods
Typos, Polish notes, and misnumbered references
Peer review comedy gold—“Please define vitamin D”
Reviewer laziness and p-hacking admission
Results: implausible bone growth mid-race
Degrees of freedom sleuthing reveals hidden sample sizes
Open data? Kristin emails the authors
Lessons from Kristin’s own ultramarathon dataset
Fishing expeditions and misuse of parametric tests
Strength of evidence: one smooch each
Methodologic morals—Mad Libs Science & degrees of freedom breadcrumbs
Anyone can spot red flags—trust your eyes
Outro: skip the vitamin D shot before your next run
