Time outdoors → 25-hydroxyvitamin D
Sun makes 25(OH)D. This is the boring, real arrow that makes D a terrible instrument for success.
Observational · High confidence
Supported by an experiment or a tightly identified design. Still read the caveats.
Mechanism
Cutaneous synthesis from UV-B is first-year physiology. Latitude, season, melanin, clothing, and time outside dominate pills for most ambulatory people. That is why D tracks lifestyle and class.
Caveats
Winter at high latitude plus indoor work can make this arrow too weak, which is when supplementation has a medical job.
Effect
qualitative. UV-B cutaneous synthesis is the dominant natural source at most latitudes.
Nodes
Named confounders
Cite this page
What Causes What. “Time outdoors → 25-hydroxyvitamin D.” https://whatcauseswhat.org/edges/e-outdoors-vitd (atlas updated 2026-08-31).
Papers
- Vitamin D status and ill health: a systematic review
P. Autier, M. Boniol, C. Pizot, P. Mullie · 2014 · The Lancet Diabetes & Endocrinology