Direct sun exposure.

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.

Fish

anything you want that is source of vitamin D3

https://www.healthline.com/nutrition/vitamin-d2-vs-d3#TOC_TI...

Below are "a few ideas" listed from the article (direct quote):

1- mushrooms that have been exposed to ultraviolet light (11Trusted Source)

2- Take fish oil supplements such as cod liver oil

3- Eat fatty fish twice a week (12Trusted Source)

4- Choose milk or orange juice that’s been fortified with vitamin D

5- Eat some eggs and butter (13Trusted Source)

6- Spend at least half an hour in the sun daily, if possible

My thoughts on their recco:

Option 6 is the typical US doctor prescription before vitamin supplements and seems the most reasonable, though depends on your location.

Options 3 and 5 are legit options and worth noting. Not sure if the portion sizes would be a financial detriment to many though.

Option 2 and 4 essentially seem like taking Vitamin D supplements.

Option 1 is huh? Who has a UV gun for mushrooms? Do stores do this automatically?

You simply let your mushrooms get 30 minutes of sun exposure daily.

"choose milk or orange juice that has been fortified with vitamin D" You mean like supplements?

Yeah, I've seen "Vitamin D enriched" mushrooms at the Coop supermarket in the UK.