Epistemological status: I studied bioinformatics but I'm no domain expert and layout my current view on the issue based on the facts I found in the literature. This is not health advice.

tl,dr:

There’s a strong evolutionary selection for producing enough Vitamin D. Vitamin D works together with Vitamin K2, and as a result, high Vitamin D supplementation should come with Vitamin K2 supplementation.

Personally, I decided to consume every morning 10 000 IU of Vitamin D3 and 200 µg of K2 (all-trans MK7) and believe that it’s likely beneficial for many fellow rationalists who don’t spend a lot of time exposing a lot of skin to the sun to take the same supplements.

The importance of Vitamin D

Gwern's meta-analysis on Vitamin D suggests that the existing academic publications suggest that this costs 0.33 years of life. Later in this article, I will make the case why I believe that the literature is likely biased to underrate the effect for systematic reasons.

Which Vitamin D should be supplemented?

Humans produce Vitamin D3. While Vitamin D2 that gets produced by mushrooms can also be used by humans, it makes sense to supplement Vitamin D3 as it’s the form of Vitamin D around which evolution optimized us given that it’s what’s available in the natural environment. Besides the evolutionary argument, The case against ergocalciferol (vitamin D2) as a vitamin supplement lists a few other reasons as well.

How much Vitamin D3 should be taken?

According to the Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline:

For clinical care, it appears that all current methodologies are adequate if one targets a 25(OH)D value higher than current cut points; for example, a value of 40 ng/ml (100 nmol/L) is without toxicity and virtually ensures that the individual's “true” value is greater than 30 ng/ml (75 nmol/L). A clinical approach of targeting a higher 25(OH)D value seems prudent in that improving vitamin D status should reduce multiple adverse consequences of vitamin D deficiency at an extremely low cost with minimal toxicity risk.

In a German study they found median 25(OH)D levels of 19.8 ng/ml. While this isn't a meta-review it illustrates that plenty of people is strongly under the recommendation of the Endocrine Society. I'm German and I used to take 5000 IU vitamin D3 per day and got tested by my doctor and got a value of ...

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