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The only form of Vitamin D that matters is D3. If you are not getting that, you're just ingesting a toxin that is useless at best.

You say this with authority (but no sources), but it does not appear to be true. D3 seems to be preferable, and more efficacious, but that does not mean that D2 does nothing.

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

https://pubmed.ncbi.nlm.nih.gov/22552031/

https://pubmed.ncbi.nlm.nih.gov/23792937/

https://www.health.harvard.edu/newsletter_article/vitamin-d2...

Just eat what people have been eating for hundreds of thousands of years.

And spend 8 hours a day hunting, foraging, or working the fields, I suppose.

Vitamin D2 is produced in plants because they use that. Similarly, D3 is produced in animals because they use that. Supementation of D2 has no effect on how much D3 animals end up with.

From your 3rd link:

Total serum 1,25(OH)2D did not change significantly with either treatment (p>0.05, post-treatment vs baseline).

From your second link:

only 3 of 10 possible studies included in the meta-analysis were randomized, controlled, double-blind, parallel intervention studies, which is the gold-standard design in nutrition research. Therefore, because of the lack of specific information surrounding the randomization and allocation of intervention to participants in all studies, there was an unknown degree of selection, performance, and detection bias.

A meta-analysis of RCTs indicated that supplementation with vitamin D3 had a significant and positive effect in the raising of serum 25(OH)D concentrations compared with the effect of vitamin D2 (P = 0.001).

Yes. Like I said. More efficacious. That doesn't make D2 "useless at best". Both D2 and D3 have metabolites that bind to the (human) Vitamin D receptor[1]. BTW both need to go through liver/kidney[2], D3 by itself is similarly "useless" otherwise.

[1] https://en.wikipedia.org/wiki/Ergocalciferol#Mechanism

[2] https://en.wikipedia.org/wiki/Cholecalciferol#Mechanism_of_a...

> The D3 produced in your body because of sun exposure doesn't have to go through the liver.

Most people don’t have access to that much sunlight, whether it’s because they have to spend most of the day working indoors, or, like me, they live in a polar region where you literally can’t get sun most of the year.

While it may be better for your liver not to have more to process, I highly doubt taking vitamin supplements is worse than living with a deficiency.

Also, in polar cultures, eating large amounts of vitamin D rich foods is exactly what people have been doing for thousands of years, without it causing health problems. And I can tell you from experience, that failing to get that vitamin D WILL cause health problems.

>they live in a polar region

People have been living in the polar regions for at least 10,000 years and they never took supplements and never had vitamin deficiency.

I'm not following your explanation.

> Let's say you're taking D3+K2 as oral supplementation [...] You'd have to eat a kilo (2.2 lbs) of Salmon

So, what's so bad about D3+K2 if you do not want to eat a kilo of Salmon a day?

The point is our bodies haven't evolved to process large amounts of ingested D3.

Ok, I see what you mean now. Interesting. Do you know how much time one would have to stay outside to get, say, 5,000 IU?

You can use this https://fastrt.nilu.no/VitD_quartMEDandMED_v2.html to calculate how much to get the recommended amount from the Sun which I think is 1000 IU

I think 4000 IU is sun burn territory for a single sitting outside. But I only just scanned the website above.

I don't know how long it lasts, or if its cumulative, but I heard Vit D is stored in your fat so theres a bit of storage going on.

Perhaps therefore, sitting outside in Summer each day during lunch would be enough to get 5000 IU / week.

Depends. Sunlight just converts a precursor molecule (7-dehydrocholesterol) to D3. The precursor is a product of the cholesterol synthesis system.

I personally believe you don't need any exposure to direct sunlight to get required D3 as long as you eat food natural to humans, specially if the diet is close to that of indigenous groups living in similar environment for thousands of years. We store D3 in our fat and you can also get it when you eat animal fat.

However, if you really want to use sun exposure, I recommend an app called dminder which tells you, based on your location, if and when sun exposure will give you Vitamin-D.