Do we produce Vitamin D in the shade?
Do our bodies produce Vitamin D in the shade?
If so, what is the percentage decrease in the amount of Vitamin D produced compared to full exposure to sunlight?
For example, sitting outside in the sun will result in the production of x units of Vitamin D. If the same person then sits under an umbrella, what percentage of x (if any) will they produce?
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The exact value depends on a lot of things - skin tone, the time of day, latitude, how deep the shade is,...
The study Vitamin D effective ultraviolet wavelengths due to scattering in shade found that the UVB radiation at 280–320 nanometers that is needed for vitamin D production dropped to levels at around 50% in a tree shade and under an umbrella. It was much less on a covered veranda - around 11% - and completely absent in a car with closed windows.
Latitudinal Variations over Australia of the Solar UV-Radiation Exposures for Vitamin D3 in Shade Compared to Full Sun came to similar conclusions (emphasis mine):
Using shade for UVD3 exposures can reduce total UV-radiation exposure by 37% to 58% compared to full sun UVD3 exposures. This research indicates that an improved approach to optimize UV-radiation exposures for the production of vitamin D3 is to use diffuse UV radiation under shade in and around the middle of the day.
Since exposure to these wavelengths is what drives vitamin D production, an approximate answer to your question is probably "around 35 to 55%".
About 1/4 - it varies with the reflectivity of the surface, and angle of the sun. Detailed study is at www.ncbi.nlm.nih.gov/pubmed/20199222
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