We Run Ultras

(8) Outdoor Mileage Does Not Guarantee Enough Vitamin D

By Francesco//7 min read/Training science & durability

Winter daylight is not a dose

A runner can spend ten hours outside each week and still make little vitamin D from UK winter sunlight. The relevant input is ultraviolet B radiation reaching exposed skin, not daylight, distance or time on feet. From October to late March, outdoor mileage is therefore a poor substitute for the UK recommendation to consider 10 micrograms (400 IU) of vitamin D daily. That is a population baseline for musculoskeletal health, not evidence that a larger dose will improve running performance.

Webb, Kline and Holick established the seasonal mechanism in 1988: winter sunlight at 52°N did not produce previtamin D3 in their model. UK guidance now treats skin synthesis as minimal through autumn and winter. The distinction matters because a bright January long run may feel like substantial sun exposure while delivering the wrong wavelengths for meaningful synthesis.

Exposure is more than time outdoors

In Greater Manchester, Webb's team followed 125 white adults for a year. Personal ultraviolet exposure was high in spring and summer, low in autumn and negligible in winter; average 25-hydroxyvitamin D peaked in September and reached its trough in February. A controlled Manchester experiment produced a useful summer result: 13 minutes of simulated midday exposure, three times weekly for six weeks, with 35% of skin uncovered, brought most white participants above the study's sufficiency threshold. That protocol describes timing, UVB and skin area. It does not translate into a weekly-mileage target.

Clothing changes the calculation. Datta and colleagues exposed different skin areas in 21 adults and found that face and hands produced much less cholecalciferol than upper- or whole-body exposure. Pigmentation and behaviour add further variation. In Kift's year-long Manchester cohort, 125 South Asian adults recorded similar time outdoors to white adults but lower measured UV exposure, less exposed skin and much lower vitamin D concentrations. A controlled study of 17 women found similar synthesis after an identical UVR dose, however, so baseline status and delivered exposure complicate simple rules based on pigmentation.

Outdoor runners have an advantage, not immunity

The strongest counterargument is that running outside does help. Bârsan's 2023 meta-analysis pooled 5,150 athletes and found outdoor athletes averaged 3.73 ng/mL more 25-hydroxyvitamin D than indoor athletes, although the unadjusted difference narrowly missed statistical significance and within-season comparisons were generally not significant. Season and latitude were doing much of the work.

Runner-specific evidence points the same way. Pegreffi's small autumn study found higher concentrations in 23 runners than in non-runners, but inadequate status remained common. During a UK winter, Close and colleagues found concentrations below 50 nmol/L in 38 of 61 professional athletes. Harju's later review estimated insufficiency in 30% of elite adults, while warning that winter was under-represented. In He and colleagues' 225 endurance athletes, inadequate or deficient status increased from 38% to 55% across winter training. Outdoor training shifts probability; it does not certify adequacy.

Raising the blood value is not the same as running faster

Athletes may reasonably ask whether a larger supplement dose offers performance insurance. The trials do not give a consistent yes. Carswell's UK experiment brought 97% of participants above 50 nmol/L using simulated sunlight or oral vitamin D3, yet neither intervention improved a 1.5-mile run, strength or jump performance. Farrokhyar's meta-analysis likewise found that supplementation raised 25-hydroxyvitamin D without improving measured performance across seven relevant trials.

A 2024 elite-athlete review was more favourable, but it contained only three studies of aerobic outcomes and called for better research. The most runner-specific update is Gervasi's 2026 trial: eight weeks of 2,000 IU daily raised concentrations in recreational runners through autumn and winter but did not improve VO2max. Recent marathon and ultramarathon trials report changes in oxidative, muscle or vitamin D biomarkers; those outcomes do not establish faster racing or better functional recovery.

Use the calendar, then individual risk

For a UK-based runner, the practical default is mundane: consider 10 micrograms daily from October through March, keep normal sun protection, and do not use winter mileage as a reason to skip it. People with dark skin or consistently little exposed-skin sunlight should consider that intake year-round under NHS guidance. Food contributes, but national survey data show average adult intake from food remains well below the 10-microgram reference.

Do not manufacture exposure by courting sunburn or using sunbeds. Sunscreen evidence is evolving: a 2019 review found little real-world effect from moderate-SPF products, while the 2025 Sun-D trial found daily SPF50+ use modestly lowered concentrations. Neither result overturns NHS sun-safety advice; supplementation is the controllable variable.

Testing belongs to a clinical question, not a mileage threshold. NICE advises it for symptoms, very high risk or a medical reason, and treatment doses should be clinician-led. Avoid testing immediately after an ultra: Mieszkowski measured an 83% post-race rise in 25-hydroxyvitamin D3 even across both trial groups. The durable decision inside training science and durability is to follow the seasonal baseline, preserve skin safety and escalate only when the athlete's history provides a reason.

Sources

UK guidance and safety

  1. Scientific Advisory Committee on Nutrition, "Vitamin D and Health" https://www.gov.uk/government/publications/sacn-vitamin-d-and-health-report

  2. Scientific Advisory Committee on Nutrition, "Fortifying foods and drinks with vitamin D: main report" https://www.gov.uk/government/publications/fortifying-food-and-drink-with-vitamin-d-a-sacn-rapid-review/fortifying-foods-and-drinks-with-vitamin-d-main-report

  3. NHS, "Vitamin D" https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/

  4. NHS, "Sunscreen and sun safety" https://www.nhs.uk/live-well/seasonal-health/sunscreen-and-sun-safety/

  5. National Institute for Health and Care Excellence, "Vitamin D: supplement use in specific population groups — Recommendations" https://www.nice.org.uk/guidance/ph56/chapter/Recommendations

  6. Public Health England and Food Standards Agency, "NDNS: results from years 9 to 11 (combined) — statistical summary" https://www.gov.uk/government/statistics/ndns-results-from-years-9-to-11-2016-to-2017-and-2018-to-2019/ndns-results-from-years-9-to-11-combined-statistical-summary

Sunlight, skin and season

  1. Webb, A.R., Kline, L. & Holick, M.F., "Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin" https://pubmed.ncbi.nlm.nih.gov/2839537/

  2. Webb, A.R. et al., "The role of sunlight exposure in determining the vitamin D status of the U.K. white adult population" https://pubmed.ncbi.nlm.nih.gov/20716215/

  3. Rhodes, L.E. et al., "Recommended Summer Sunlight Exposure Levels Can Produce Sufficient (≥20 ng ml−1) but Not the Proposed Optimal (≥32 ng ml−1) 25(OH)D Levels at UK Latitudes" https://pubmed.ncbi.nlm.nih.gov/20072137/

  4. Kift, R. et al., "Lifestyle factors including less cutaneous sun exposure contribute to starkly lower vitamin D levels in U.K. South Asians compared with the white population" https://pubmed.ncbi.nlm.nih.gov/23855783/

  5. Hakim, O.A. et al., "Vitamin D production in UK Caucasian and South Asian women following UVR exposure" https://pubmed.ncbi.nlm.nih.gov/27016471/

  6. Datta, P. et al., "Size of the exposed body surface area, skin erythema and body mass index predict skin production of vitamin D" https://pubmed.ncbi.nlm.nih.gov/26093795/

  7. Neale, R.E. et al., "The effect of sunscreen on vitamin D: a review" https://pubmed.ncbi.nlm.nih.gov/30945275/

  8. Tran, V. et al., "Effect of daily sunscreen application on vitamin D: findings from the open-label randomized controlled Sun-D Trial" https://pubmed.ncbi.nlm.nih.gov/40927943/

Athletes and runners

  1. Harju, T. et al., "Prevalence and novel risk factors for vitamin D insufficiency in elite athletes: systematic review and meta-analysis" https://pmc.ncbi.nlm.nih.gov/articles/PMC9596536/

  2. Bârsan, M. et al., "Difference in Levels of Vitamin D between Indoor and Outdoor Athletes: A Systematic Review and Meta-Analysis" https://pmc.ncbi.nlm.nih.gov/articles/PMC10147028/

  3. Close, G.L. et al., "Assessment of vitamin D concentration in non-supplemented professional athletes and healthy adults during the winter months in the UK: implications for skeletal muscle function" https://pubmed.ncbi.nlm.nih.gov/23083379/

  4. Carswell, A.T. et al., "Influence of Vitamin D Supplementation by Sunlight or Oral D3 on Exercise Performance" https://pubmed.ncbi.nlm.nih.gov/30048414/

  5. Farrokhyar, F. et al., "Effects of Vitamin D Supplementation on Serum 25-Hydroxyvitamin D Concentrations and Physical Performance in Athletes: A Systematic Review and Meta-analysis of Randomized Controlled Trials" https://pubmed.ncbi.nlm.nih.gov/28577257/

  6. Wyatt, P.B. et al., "Effects of Vitamin D Supplementation in Elite Athletes: A Systematic Review" https://pubmed.ncbi.nlm.nih.gov/38188620/

  7. Pegreffi, F. et al., "A Snapshot of Vitamin D Status, Performance, Blood Markers, and Dietary Habits in Runners and Non-Runners" https://pmc.ncbi.nlm.nih.gov/articles/PMC11597173/

  8. Gervasi, M. et al., "Effects of vitamin D supplementation during autumn and winter on blood biomarkers and physical performance in runners and non runners" https://pubmed.ncbi.nlm.nih.gov/41720868/

  9. He, C.-S. et al., "Influence of vitamin D status on respiratory infection incidence and immune function during 4 months of winter training in endurance sport athletes" https://pubmed.ncbi.nlm.nih.gov/23977722/

  10. Mieszkowski, J. et al., "Ultra-Marathon-Induced Increase in Serum Levels of Vitamin D Metabolites: A Double-Blind Randomized Controlled Trial" https://pubmed.ncbi.nlm.nih.gov/33255807/

  11. Weng, P.-W. et al., "Effects of vitamin D3 supplementation on cardiac, muscle, and immune responses after a marathon race: a single-blind, placebo-controlled trial" https://pubmed.ncbi.nlm.nih.gov/41966073/

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