German reporting in. According to Weatherspark [1] starting November to March around 70% of the days are cloudy and it's cold so you would barely expose 15% of your body to the remaining sun. In feburary, around midday, on a cloudy day you don't make enough vitamin d. And even on a cloudless day it takes more than 4 hours around midday to get the daily recommended dose. [2]. Being a dog owner I spend a lot of time outside, also during midday and 3/4 of the year I go for a run around midday every other day. Last year in january I was diagnosed with severe vitamin D deficiency at around 6ng/ml. Took a few months to get that back to normal using supplements.

[1] https://weatherspark.com/y/68301/Average-Weather-in-Hamburg-... [2] https://fastrt.nilu.no/VitD_quartMEDandMED_v2.html

For the UK: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-...

> During the autumn and winter, you need to get vitamin D from your diet because the sun is not strong enough for the body to make vitamin D.

Likely applies to many other countries.

You literally cannot absorb significant amounts of vitamin D over winter in vast swathes of the world, including most of Europe.

At higher latitudes you can only get vitamin D from the sun for a few months of the year. In winter, the sun is too low in the sky, and the UV wavelengths needed to produce the vitamin D is entirely blocked by the atmosphere(though unfortunately the UV for sunburns is not).

This is a method of "storing" summer vitamin D for winter consumption.

I live in Minnesota and the sun just came out in the beginning of May. I feel like I haven't felt real sun exposure since maybe October of last year. Let me tell you, I definitely take vitamin D supplements in the cold season!

As an aside, I also drink Mushroom Hero tea all winter. Love that stuff.

I wear long sleeves or a light jacket for most of the summer, so it doesn't always matter. And it is pointless in winter - 4.5 hours between sunrise and sunset, with the sun at a low angle. I'm fairly far north.

It's dark out for most of the day in some climates. In others, it's dark out for much of the year.

You would be surprised how seldom you can get Vitamin D from the sun for many places! https://whencanigetvitamind.com

That's always a great option, but weather and seasonal differences can keep large portions of the world from achieving healthy levels of Vitamin D.

In much of the northern hemisphere for most of the year this is insufficient. Even a really long walk.

In NC USA, even with regular walks, running and biking my Vitamin D levels were too low.