>Denying that vitamin D is important seems silly when people who are indoors so often feel remarkably better after spending even just a few hours outside in the sun.

The two things are not exactly the same, however.

If you feel better after a mid-winter break in a sunny place, maybe it is because the mid-winter break in a sunny place or a few hours outside do good, and you should do that.

Staying in a non-sunny environment indoors and taking vitamin D might (or might not) provide the same wellness.

I guess it is the same thing with meal substitutes, maybe (though I doubt it) we have arrived to a point where we could take a single pill or gulp a dose of some (possibly terribly tasting) high-tech mush containing all that is needed, but maybe sitting at a table, making a pause for half an hour, eating some good tasting food, sniffing its aroma, etc. are the things that actually do the good.

I've had first-hand experience of something that's been discussed elsewhere: a lack of omega 3 fatty acids and vitamin D3 itself may be associated with heavy sunburns and sun allergy (itchy skin, runny nose etc right as you're out in the sun, and not caused by fragrances or cold air).

I'm not able to look up studies for this right now so I'm just going by my own experience and other anecdotal reports, but it's a kind of circular dependency that intrigues me.

I'm at 52 deg N and an enthusiastic convert to D3 supplementation.

Yet there are other factors which make going out into the sunshine pleasant. UV light causes nitric oxide to be released from the skin, relaxing the blood vessels. Also I think just seeing brightly lit outside objects and moving about feel good. It's a different world and 'a change is as good as a rest'.

https://www.youtube.com/watch?v=jURTHzLakoo

I'm not convinced. Living in an oppressively sunny place, you'll notice the same salutary effect of a run of overcast weather.