I have made this comment on HN many times [0] now but: it's the goddamn sunlight.
Sunlight has many pathways in the body, and increased vitamin D levels are just one of those pathways. Swallowing one element of a many-element pathway cannot simulate the human synthesis of vitamin D! This should be obvious! Your car might not run because of bad needle bearings, but the solution is not to add them to the gas tank! First, you must always understand.
Sunlight / vitamin D is an especially aggravating issue because we have increasing amounts of research on how sun exposure improves general health. Sleep quality and regularity, mood, cancer incidence (as the article notes), eyesight, skin conditions, overall mortality, virtually every part of human health is improved by going outside. And when I say "improved" I mean improved to such a degree that it would blow 95% of prescribed drugs out of the water. See, for example, the Melanoma in Southern Sweden study where the highest sun exposure cohort had _half_ the all-cause mortality of the lowest group. Unfortunately the Western medical establishment is thoroughly captured by industry and you can't sell a pill that makes you go outside.
I think science is just too granular to be a reliable input into wellness decision-making. It is too easy for interested parties to enable everyday people to confirmation bias their way into defying common sense.
Of what utility is the truth in decision-making if it is such a narrow truth that it might as well be a falsehood in practice?
Alcoholics talking endlessly about flavonoids, obese people insisting in polyunsaturated fat mayo, sedentary people that don't resistance train drinking soda with protein in it.
Here in northern Europe, there just isn't enough sunlight to not become deficient, so a vitamin D supplement is a must.
Vitamin D is fat soluble, so it would be feasible to build up a supply in summer to last through winter. I've seen it theorized that a cyclical weight gain and loss tied to preindustrial seasonal food availability also plays a role.
People tend to fatten up on winter and peaking at around winter solstice
Not sure how your plan should work?!
Because of industrial food chains making that possible. Before we could refridgerate and ship food all over, most of your calories came from seasonal foods, and summer is usually more bountiful than winter.
I looked up data from a calorie tracking app I used a while ago and I got just enough vitamin D. Main sources were fish, eggs and dairy products. However, I ate a lot and I'm quite sure in winter there will have been periods where I didn't get the recommend intake.
So if you live in a northern country and you don't like fish, I suppose it's quite a good idea to take a supplement.
Though as OP said, even if the sun isn't strong enough, there are plenty of benefits to going outside beside vitamin D.
If you gain weight during summer (so that excess vitamin D is stored in fat cells) and use body's stored fat in the winter (eat at a deficit, so that your stored fat, rich in vitamin D, is used), you need no supplementation. This is what all animals living in the region do.
Unfortunately, sun exposure (specifically UV exposure) is heavily tied to skin cancers and aging. I haven't researched it myself, but my feeling is that, ultimately, there is a calculation where sun exposure / skin risk needs be balanced on your specific genetic makeup.
We really need to drill down into ethnic and racial boundaries to really make sense of how much sun exposure helps us.
Which could be one of the reason people near the Mediterranean sea (for example in Italy) and tennis players do tend to live longer than average. Now of course there s probably more than one reason but those sure do enjoy time outside.