Clinical Context
Extreme endurance exercise places remarkable stress on the skeletal system. Ultramarathons—typically defined as races exceeding 42.2 km—impose hours of repetitive loading on bones, triggering acute changes in bone turnover markers. Studies consistently show that prolonged endurance exercise acutely increases bone resorption markers while bone formation markers show variable responses. Over time, this pattern could contribute to stress fractures and bone loss if not adequately counterbalanced during recovery.
Vitamin D plays critical roles in bone metabolism: it enhances intestinal calcium absorption, regulates parathyroid hormone (PTH), and influences osteoblast and osteoclast function. Vitamin D deficiency is common in athletes, particularly those training indoors or in northern latitudes. Low vitamin D status may impair bone’s ability to respond optimally to mechanical loading and recover from exercise-induced microdamage.
The concept of acute, high-dose vitamin D supplementation (“stoss” therapy) before extreme exercise is novel. The hypothesis: rapidly elevating vitamin D levels might optimize the skeletal response to the upcoming stress—enhancing bone formation and attenuating resorption. This double-blind RCT tested whether a single mega-dose of vitamin D before a mountain ultramarathon could favorably modify bone turnover marker responses.
Study Summary (PICO Framework)
Summary:
In semiprofessional male ultramarathon runners, a single 150,000 IU vitamin D₃ dose 24 hours pre-race significantly increased bone formation markers and reduced bone resorption markers post-race compared to placebo, with no reported adverse effects.
| PICO | Description |
|---|---|
| Population | 35 semiprofessional male ultramarathon runners in a mountain ultramarathon. |
| Intervention | Single oral dose of 150,000 IU vitamin D₃ (cholecalciferol) in oil, given 24 hours pre-race. |
| Comparison | Placebo solution administered 24 hours before the race (n=19). |
| Outcome | Greater 25(OH)D increase (147% vs 85%), higher PINP (formation marker), lower CTX, PTH, sclerostin, procalcitonin (resorption/stress markers) 24h post-race. No adverse effects. |
Clinical Pearls
1. The bone turnover marker changes have clear directionality. Higher PINP (procollagen type I N-terminal propeptide) indicates enhanced bone formation—osteoblasts actively synthesizing new bone matrix. Lower CTX (C-terminal telopeptide of type I collagen) indicates reduced bone resorption—less osteoclast-mediated bone breakdown. This favorable shift in the formation/resorption balance suggests better skeletal adaptation to the extreme mechanical stress.
2. PTH suppression is a key mechanism. The vitamin D group showed lower post-race PTH levels. PTH rises during prolonged exercise (possibly due to calcium flux, acidosis, or stress response) and stimulates bone resorption to maintain serum calcium. By ensuring optimal calcium absorption via vitamin D, the need for PTH-mediated bone resorption to maintain calcium homeostasis is reduced. This is the likely mechanism for the attenuated resorption response.
3. Sclerostin reduction is intriguing. Sclerostin, produced by osteocytes, inhibits bone formation by blocking Wnt signaling in osteoblasts. Lower sclerostin permits greater osteoblast activity. That vitamin D reduced post-exercise sclerostin suggests it may enhance the anabolic bone response to mechanical loading—not just limit catabolism.
4. The timing and dose are unconventional. A single 150,000 IU dose is far above typical daily supplementation (1,000-4,000 IU). This “stoss” approach is sometimes used to rapidly correct severe deficiency but is novel for pre-exercise optimization in presumably replete athletes. The 24-hour lead time may not be sufficient for full 25(OH)D elevation, yet effects were observed. This suggests mechanisms beyond simple repletion.
Practical Application
For ultramarathon and extreme endurance athletes: Consider assessing vitamin D status as part of pre-competition preparation, especially for athletes in northern latitudes or with limited sun exposure. Ensuring adequacy (25(OH)D >30 ng/mL, ideally 40-60 ng/mL) may support optimal bone adaptation to training and racing stress. Regular supplementation (2,000-4,000 IU daily) is more conventional than single mega-doses.
Pre-race mega-dosing requires caution: While this study showed no adverse effects, single doses of 150,000 IU are not routinely recommended. Vitamin D is fat-soluble and can accumulate; repeated mega-doses could cause hypercalcemia. This strategy should be considered experimental and not adopted without medical supervision. For most athletes, maintaining adequate baseline vitamin D through regular supplementation is safer and likely sufficient.
Don’t extrapolate beyond this population: These were semiprofessional male ultramarathon runners—a highly specialized population. Results may not apply to recreational runners, shorter distances, or athletes with different training backgrounds. Women, older athletes, or those with baseline vitamin D deficiency might respond differently.
Bone health requires comprehensive approach: Vitamin D is one factor in bone health. Adequate calcium intake (1,000-1,300 mg daily), appropriate training load progression (avoiding abrupt volume increases), adequate energy availability (avoiding RED-S/relative energy deficiency in sport), and sufficient recovery are all essential for maintaining skeletal integrity in endurance athletes.
How This Study Fits Into the Broader Evidence
Endurance exercise acutely increases bone resorption markers—a consistent finding across studies of marathons, triathlons, and ultramarathons. The long-term skeletal consequences of repeated acute resorption spikes remain debated; some endurance athletes develop low bone mineral density while others maintain or improve bone health, suggesting individual variability and the importance of recovery factors.
Vitamin D supplementation in athletes has been studied for various outcomes including muscle function, immunity, and injury prevention. Effects on bone are less studied. Some evidence suggests vitamin D-replete status supports stress fracture prevention, but data are inconsistent. This study adds a novel acute intervention approach to the literature.
The use of high-dose vitamin D for rapid repletion (stoss therapy) is established in clinical medicine for severe deficiency but hasn’t been specifically studied for pre-exercise bone protection. This represents a new application that requires replication and longer-term outcome studies before clinical adoption.
Limitations to Consider
Small sample size (35 total, split between groups) limits statistical power. Only male athletes were studied. Bone turnover markers are surrogates; the study doesn’t demonstrate prevention of stress fractures or preservation of BMD. The single mega-dose approach is unconventional and safety with repeated use isn’t established. Whether effects translate to clinically meaningful outcomes (fewer injuries, better bone health) is unknown.
Bottom Line
A single 150,000 IU vitamin D dose 24 hours before a mountain ultramarathon favorably modified bone turnover markers in semiprofessional male runners—enhancing formation markers and reducing resorption markers compared to placebo, with no adverse effects. While intriguing, this mega-dose approach remains experimental. For endurance athletes, ensuring adequate baseline vitamin D status through regular supplementation is a more conventional and safer strategy to support bone health during extreme training and competition.
Source: Błażej Stankiewicz, 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.” Read article here.
