A Single Dose of Vitamin D Before an Ultramarathon Protects Runners From Dangerous Bone Loss
New clinical trial reveals why vitamin D supplementation is becoming essential for serious endurance competitors.
Ever wonder if that massive ultramarathon you’re training for could actually be harming your bones? Recent research reveals a troubling paradox: while moderate exercise strengthens bones, extreme endurance events like ultramarathons may trigger bone breakdown processes that could compromise long-term skeletal health.
This double-blind randomised controlled trial examined whether a single high-dose vitamin D supplement could protect runners’ bones during one of the most demanding physical challenges imaginable: a 240-kilometre mountain ultramarathon.
Key Points
Overview
Researchers investigated the bone health impact of extreme endurance exercise and the potential protective effects of vitamin D supplementation in 35 semiprofessional male ultramarathon runners. The participants, who averaged 42 years old, completed a gruelling 240-kilometre mountain race with 7,670 meters of elevation gain across temperatures ranging from 18°C to 4°C.
Study Design and Methods
This rigorously designed clinical trial (NCT03417700) randomly assigned participants into two groups: 16 runners received a single high dose of vitamin D3 (150,000 IU) dissolved in vegetable oil 24 hours before the race. In contrast, 19 control group runners received an identical-looking placebo solution. Blood samples were collected at three critical time points: 24 hours before the race, immediately after completion, and 24 hours post-race to assess various bone turnover markers.
The researchers measured several key indicators of bone health, including CTX (a bone breakdown marker), PINP (a bone formation marker), parathyroid hormone (PTH), sclerostin, and procalcitonin - all crucial factors that regulate bone remodelling processes.
Findings
Vitamin D Supplementation Effectiveness
The single high-dose vitamin D supplementation proved remarkably effective, increasing serum vitamin D levels by 147% in the supplemented group compared to only 85% in the control group 24 hours after the race. This demonstrates that the intervention successfully elevated vitamin D status when the body needed it most during recovery.
Bone Formation Enhancement
The supplemented group showed significantly enhanced bone formation activity, with PINP levels (indicating new bone building) increasing immediately after the race and remaining elevated 24 hours later. This contrasted sharply with the control group, which showed no significant changes in bone formation markers.
Bone Breakdown Protection
Perhaps most importantly, vitamin D supplementation appeared to protect against excessive bone breakdown. CTX levels (indicating bone resorption) decreased significantly in the supplemented group both immediately and 24 hours after the race, while the control group showed no such protective effect.
Reduced Inflammatory Response
The supplemented group demonstrated substantially lower levels of procalcitonin, an inflammatory marker that increased by 357% in the control group 24 hours post-race but showed no significant increase in the vitamin D group. This suggests vitamin D’s anti-inflammatory properties may help protect against exercise-induced tissue damage.
Hormonal Regulation Benefits
Parathyroid hormone (PTH) levels, which promote bone breakdown when elevated, increased by 93% in the control group but only 48% in the supplemented group. Similarly, sclerostin levels (which inhibit bone formation) were significantly higher in the control group 24 hours after the race compared to the supplemented group.
Practical Applications for Athletes
The findings offer several actionable insights for endurance athletes:
Pre-Competition Supplementation: A single high dose of vitamin D3 (150,000 IU) taken 24 hours before extreme endurance events may provide significant bone protection. However, athletes should consult healthcare providers before implementing such high-dose protocols.
Bone Health Monitoring: Ultramarathon and extreme endurance athletes should prioritise regular bone health assessments, as prolonged intense exercise may paradoxically increase bone breakdown despite the general benefits of physical activity.
Recovery Optimisation: The anti-inflammatory effects observed suggest that adequate vitamin D status may enhance recovery processes following extreme physical exertion.
Baseline Vitamin D Status: The study participants had relatively low baseline vitamin D levels (around 27 ng/mL), suggesting many endurance athletes may benefit from regular vitamin D monitoring and optimisation.
Study Limitations and Considerations
Several important limitations should be noted when interpreting these results. The study included only male ultramarathon runners, limiting generalisability to female athletes who have different bone metabolism patterns due to hormonal differences. Additionally, the researchers did not measure calcium levels or dietary calcium intake, which could have influenced the observed effects since vitamin D primarily works through calcium regulation.
The extreme nature of the 240-kilometre mountain ultramarathon means these findings may not apply to shorter endurance events or different types of exercise. Furthermore, only immediate post-exercise effects were studied, leaving questions about long-term bone health impacts unanswered.
Key Takeaways
This research provides compelling evidence that extreme endurance exercise can disrupt bone metabolism, potentially leading to increased bone breakdown and compromised skeletal health. However, strategic vitamin D supplementation appears to offer significant protection by enhancing bone formation processes while reducing bone resorption and inflammatory responses.
The study demonstrates that a single high dose of vitamin D3 can effectively modulate bone turnover markers during extreme physical stress, potentially through effects on calcium-parathyroid hormone homeostasis. These findings suggest that vitamin D supplementation protocols could become an important component of bone health management strategies for athletes engaging in prolonged, high-intensity endurance activities.
For recreational and competitive endurance athletes, maintaining optimal vitamin D status may be crucial not just for performance, but for protecting long-term bone health against the potentially detrimental effects of extreme physical exertion.
Reference
Stankiewicz, B., Kochanowicz, A., Brzezińska, P., Niespodziński, B., Reczkowicz, J., Waldziński, T., Kowalik, T., Piskorska, E., Wędrowska, E., Antosiewicz, J., & Mieszkowski, J. (2025). Single high-dose vitamin D supplementation impacts ultramarathon-induced changes in serum levels of bone turnover markers: a double-blind randomized controlled trial. Journal of the International Society of Sports Nutrition, 22(1), 2561661. https://doi.org/10.1080/15502783.2025.2561661
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