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What Causes What
MECHANISM+ INCREASES

Time outdoors → 25-hydroxyvitamin D

Higher Time outdoors increases 25-hydroxyvitamin D.

Skin makes 25(OH)D from UV-B; Samefors 2021 then showed that encouraging 40 Swedish nursing-home residents outdoors for two summer months raised median serum 25(OH)D from 42.5 to 53.5 nmol/L.

Observational · High confidence · N=40

Supported by an experiment or a tightly identified design. Read the caveats.

Mechanism

UV-B on uncovered skin turns 7-dehydrocholesterol into previtamin D3. The liver then makes 25(OH)D. That is the main natural source. Autier, Boniol, Pizot, and Mullie reviewed the observational catalog in 2014. Low 25(OH)D tracks almost every chronic disease. Pill trials do not reproduce those links. Their reading is that low D marks ill health, indoor life, and body fat. The sun pathway is why the marker exists. Samefors, Tengblad, and Östgren tested that outdoor step. Three nursing homes in Jönköping, 57°N. Two homes were told to get residents outside 20 to 30 minutes between 11 a.m. and 3 p.m. every day for two months in the summer of 2018. The third home lived as usual. Staff reminded the outdoor homes. Everyone kept a diary. Eighteen people in the outdoor arm and 22 in the control arm gave blood before summer. Median 25(OH)D in the outdoor arm rose from 42.5 to 53.5 nmol/L (p=0.011). The control arm went from 52.0 to 65.0 nmol/L without a significant change. After summer the two arms did not differ from each other. About one in six people in each arm was already taking a vitamin D tablet. Fish was on the menu about as often in all three homes. Latitude, season, melanin, clothing, and hours outside still set most of the number. That is why 25(OH)D tracks lifestyle and class, and why it is a poor instrument for success.

Caveats

Samefors is small and open-label. Homes were the units, not people. Staff knew the assignment. The significant 25(OH)D rise is a within-arm test. After summer the outdoor and control medians were not different. The control arm also lived through a bright Swedish summer. Median age was about 86. Many residents were chairbound. People with severe dementia were not asked. About 17 to 18 percent were already on vitamin D tablets. The trial also scored SF-36 mental health. The authors say they cannot tell whether that change came from 25(OH)D, company, or movement. It is not this arrow. Autier is a review of observational D-and-disease papers and of supplementation trials. It is not a sunlight experiment. Winter at high latitude, indoor work, and covered skin can make this arrow too weak. That is when a clinician has a job. This page is not a sun prescription and not an argument against treating rickets.

Effect

qualitative. Samefors 2021: outdoor arm 42.5 to 53.5 nmol/L (p=0.011), N=40 analyzed. Control 52.0 to 65.0, not significant. Autier 2014: UV-B cutaneous synthesis is the main natural source.

Nodes

Lifestyle & environmentTime outdoorsBiology & endocrinology25-hydroxyvitamin D

Named confounders

Cite this page

What Causes What. “Time outdoors → 25-hydroxyvitamin D.” https://whatcauseswhat.org/edges/e-outdoors-vitd (atlas updated 2026-09-02).

https://whatcauseswhat.org/edges/e-outdoors-vitd

Papers