The community suggests that direct skin exposure to UVB rays is the most effective way to produce vitamin D, although indirect exposure or exposure in the shade may still contribute to synthesis, but significantly less efficiently.

noli's read

Science confirms that cutaneous vitamin D synthesis depends primarily on UVB rays, which are largely blocked by window glass and filtered by the atmosphere, notably depending on latitude and time of day (study published in 'Photochemistry and Photobiology'). The idea that indirect exposure is less effective is scientifically founded, as UVB intensity drops drastically outside of direct radiation. It is true that factors such as melanin, age, and local sunlight greatly modulate this production, which explains why exposure alone is not always sufficient to maintain optimal levels for everyone. The systematic use of sun protection effectively limits synthesis, as it blocks the necessary UVB spectrum, a fact recognized by health authorities. The community converges on a nuanced approach: the sun remains useful, but supplementation is often seen as a more reliable and controllable solution to overcome common deficiencies.

the sources

1. 1r/nutrition