I've recently run into and explored research explaining that natural sunlight stimulates energy production in mitochondria, resulting in an energizing and healing effect completely separate from vitamin D production.
I feel bad for posting this without references so here's one reference I could find right now:
> This study shows that longer wavelengths in sunlight are transmitted through the body. When these are presented via NIR LEDs in a laboratory-controlled environment at much lower energies, they again have the ability to be transmitted through the body and also are associated with improved visual function independent of ocular exposure. Hence, body penetration by longer wavelengths impacts systemically. Longer wavelengths improve mitochondrial membrane potential and ATP production and improve function in a species conserved pattern that can impact on mobility, visual function and cognition, particularly in ageing. In short lived animals they can extend average life span.
> Taken together, these data provide evidence of the importance of the full spectrum of sunlight for human health. They also highlight the potential dangers of the restricted spectra found in white LED lighting in the modern built environment that lacks longer wavelengths and whose output is generally restricted to around 400–650 nm. The absence of longer wavelengths from LED light sources may have implications for public health that should be addressed.
One interesting aspect of Vitamin D that is not regularly discussed is the photo-isomerization of Vitamin D into inactive isomers in the skin.
I think of it similarly to Beta-Carotene, in that consuming the "natural source" has no known toxic amount, while consuming processed vitamin A can be toxic in large doses.
Increasing sun exposure will not raise blood vitamin D metabolite concentration indefinitely, and we don't have good studies that accurately measure the amounts of Vitamin D and its photo-isomers and their concentration in relation to large doses of UV exposure.
Therefore, through sun exposure, people may be storing large doses of Vitamin D isomers in their skin, keeping their levels much higher than typically prescribed oral supplementation levels.
I feel bad for posting this without references so here's one reference I could find right now:
> This study shows that longer wavelengths in sunlight are transmitted through the body. When these are presented via NIR LEDs in a laboratory-controlled environment at much lower energies, they again have the ability to be transmitted through the body and also are associated with improved visual function independent of ocular exposure. Hence, body penetration by longer wavelengths impacts systemically. Longer wavelengths improve mitochondrial membrane potential and ATP production and improve function in a species conserved pattern that can impact on mobility, visual function and cognition, particularly in ageing. In short lived animals they can extend average life span.
> Taken together, these data provide evidence of the importance of the full spectrum of sunlight for human health. They also highlight the potential dangers of the restricted spectra found in white LED lighting in the modern built environment that lacks longer wavelengths and whose output is generally restricted to around 400–650 nm. The absence of longer wavelengths from LED light sources may have implications for public health that should be addressed.
Source: https://www.nature.com/articles/s41598-025-09785-3#Sec5