https://astralcodexten.substack.com/p/contra-hoffman-on-vitamin-d-dosing

[epistemic status: pretty uncertain about each individual fact, moderate confidence in general overview]

I. Hoffman Contra Me

I've said many times that (to a first approximation) Vitamin D is a boring bone-related chemical. Most claims that it does exciting things outside of bones - cure COVID! prevent cancer! decrease cardiovascular risk! - are hype, and have failed to stand up to replication.

Ben Hoffman disagrees, and writes How To Interpret Vitamin D Dosage Using Numbers. I'm compressing his argument for space reasons; read the link to check if I'm still being fair:

I am sick of people rejecting good evidence about vitamin D because they are confused about the bad evidence and can't be bothered to investigate, so I am going to explain it […]

Hunter-gatherers in the environment where most of our evolution happened might have been outside all day shirtless. On average the sun's halfway from peak, so that might be equivalent to 8 hours of peak sunlight at the equator. [A study shows people in these conditions synthesize 400 IU of Vitamin D/5 minutes, which comes out to] 8000 IU per hour is 32,000 IU (800 micrograms) per day by this estimate.

When deciding how much is actually appropriate to supplement, we need to take into account diminishing returns; eventually the sunlight starts producing other secondary metabolites which are also good for us, so a 16,000 IU supplement is lower-quality than sunlight but similar in the effective dosage of the most important chemical our evolutionary ancestors' bodies would have made from sunlight; in practice I wouldn't take more than that.

Now let's look at the object-level studies that Scott Alexander says show that vitamin D doesn't work. I'm just going to look at the randomized controlled trials because observational studies for or against vitamin D are trash for anything except hypothesis generation unless they have a very carefully selected instrumental variable.

The colon cancer link is broken but the breast cancer study reports a dosage of 400 IU/day. On the exercise scale that's FIVE MINUTES of brisk walking. FIVE MINUTES is not very long at all compared with FOUR HOURS. [An all-cause mortality study used a thrice-yearly dosing] that amounts to about 800 IU/day, or ten minutes of brisk walking on the exercise scale. [Other studies that found no effect of Vitamin D also used doses around this range].