This paper, "Insufficient Sun Exposure Has Become a Real Public Health Problem" [1] states "thus, serum 25(OH)D as an indicator of vitamin D status may be a proxy for and not a mediator of beneficial effects of sun exposure".
That is, while vitamin D can be used to deduce someone's amount of sun exposure, the benefits of sun exposure are not coming from the vitamin D.
For one, your body absorbs an abysmal fraction of supplements, nearly all of it comes back out the other end. This isn't unique to vitamin D, naturally, but for practically all food supplements. Your body is pretty good at taking care of itself so long as you give it the tools it needs. If you live in a northern climate, vitamin D supplements may be a part of it, but for most part, spending some time outside and getting your movement in is a reasonable baseline.
In his TED talk, Weller gives a potential alternate benefit source in nitric oxide production which leads to potential cardiovascular benefits. So, the short answer is it may not be (just) vitamin-d. The bigger point though is that there are a lot of very complex things that happen when you go outside and benefits could come from many of them. We want an easy 'this one thing happens so we can replicate just that' but maybe it is actually a combination of many things so there may be no easy one pill replacement.
You can, and it’s quite possible that provides most of the benefit. But very few systems in the body operate in total isolation, and it’s equally possible something else depends on vitamin D synthesis or on UV absorption, and that thing might matter for health.
I know almost nothing about this apart from being vitamin D deficient and taking supplements. My doc says that vitamin D absorption through the gut is much less efficient than vitamin D created by the body.
Because studies into vitamin D supplementation haven't shown that it reduces mortality, and because there are other benefits of sunlight, such as nitric oxide.
Did this study factor in supplementing for other things sunlight provides, i.e. the nitric oxide? If not, then it's a poor study in relation to supplementation as a means of obtaining the benefits of sun exposure.
Did these studies that show that these supplements don't reduce mortality, were they done in such a way where they are measuring the multiple supplements that are received through sun exposure at the same time? No? Didn't think so.
because the exact mechanism whereby uv light might lengthen lives is unknown
when your skin synthesizes vitamin d using sunlight, it synthesizes it from cholesterol. i wonder, does that lower serum cholesterol? if so, which serum cholesterol?
Light that is brighter than is possible inside most homes makes you more alert (almost immediately), and if done during the first 3 hours of wakefulness has benefits that last throughout the day according to Andrew Huberman.
Specific qualities present in sunlight when the sun is very low in the sky (or under the horizon, namely yellow-blue contrasts detected by the intrinsically-photosensitive retinal ganglion cells) entrain the body's clock, making it easier to get to sleep at a consistent and suitably early time of day.
UVB exposure triggers a skin-brain-gonadal axis through skin p53 activation (nothing to do with vitamin D). I.e., 20 or 30 min every other day of strong sunlight on bare skin and into your eyes increases your levels of the sex hormones.
Certain red and infrared wavelengths help cytochrome c oxidase turn serotonin into melatonin, which is a potent antioxidant right where antioxidant activity is most needed (in the mitochondria).
What's the fifth one?
ADDED. It just occurs to me that with "I know 5" the conversation might have jumped from the object level to a comment on the social and emotional effects of my comment. D'oh!
They seem a bit remote for someone at high risk of skin damage due to skin type and location. I am surprise the immune system does not get more attention in discussions like this. I would put it high on the list.
There is an interesting wtf correlation between incidence and severity of multiple sclerosis and latitude. Sunlight (or alternatively narrowband UVB in a controlled clinical setting) can be an effective treatment for some skin conditions.
