Just stand facing the sun and open your mouth.

If you live in the north, the sunlight you might get is inadequate.

Also it’s hard to expose skin to sun zero temperature.

I’m guessing northern ancestors ate organ meat for supplements in the past.

I read an interesting theory that people near rivers ate fish and were able to be healthier than the people inland. And also tended to retain the darker complextion of their southern ancestors.

I read that years ago it's in my brain but no idea where it comes from.

That's not possible during the winter in countries in northern climates.

But preferred when it is possible.