for the last few years i looked in to sound frequency therapy, which is very effective and now available in small reasonably priced units from a company called 'spooky.' the units basically operate with a computer and a set of known frequencies to match a list of issues, ranging from broken bones and covid, to parasites.  all you need is a  frequency and to allow the field generator to act. the company supply every frequency you need, and the community of users often push the boundaries into uncharted waters. however now there's a new therapy which is photo-bio-modulation, where light is used. i'm looking into this as it ties in with the light eater people who subsist upon light alone. now this is an extreme practice but i know how they do it, and why it works. the issue is being committed enough to stop eating food the way we have been conditioned to, although light people do eat occasionally, they just eat very close to the source and always a small amount.

as far as healing goes we already know various frequencies of light, infra red for example is very good for cell repair, cardio vascular repair, enhancing the immune system and inflammation in short bursts. it's currently being looked at to cure cancer. 

same goes with ultra violet, therefore one can extrapolate that other light frequencies assist with other conditions. 

that's why sunlight is essential, i'm doubtful weather the supplement of vitamin d has the same kind of quality as sunlight. health is very much dependant on environment. get some sun, early morning or late evening is the best for longer exposure.