“The worthlessness of vitamin D is mildly exaggerated” by dynomight

For a while there, many people thought vitamin D was magical—that it could improve bones, the heart, infections, cancer, heart disease, longevity, even mental health. But among people I respect, opinion is now overwhelmingly that taking vitamin D does nothing unless you're severely deficient. The central argument is that while vitamin D levels are correlated with ~all positive health outcomes, when you actually test vitamin D supplements against placebo in randomized trials, nothing ever happens.

That's what I used to think, too. But I've come to think the skeptics have over-corrected. Yes, randomized trials have shown the magical correlations are not causal. But if you start with non-insane expectations, the trials look like weak but positive evidence. And if you consider what we know about biology and evolution, I think the balance of evidence tips pretty clearly in the direction that people with low-ish levels would be wise to supplement.

Am I certain that vitamin D is beneficial for people with low-ish levels? Absolutely not! But I claim that's the best bet given the limits of our knowledge.

The classical view: Boring bone vitamin

Most vitamins are "ingredients" that the body uses to do stuff. Vitamin D is more [...]

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Outline:

(01:19) The classical view: Boring bone vitamin

(04:28) The correlation view: Magical mystery cure

(07:58) Meanwhile in biology

(11:10) Then came the RCTs

(15:12) I made some tables

(16:32) Squinting at the data

(22:24) Where are we?

(23:15) The case for supplementing anyway

(23:19) It's biologically plausible that vitamin D is good

(24:40) Humans evolved to have a lot of vitamin D

(27:14) What do you expect from vitamin D?

(29:22) What do you expect from vitamin D trials?

(31:11) The trials do find slightly helpful numbers

(32:50) You're probably already taking vitamin D

(34:34) So that's my story

The original text contained 35 footnotes which were omitted from this narration.

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First published:
June 23rd, 2026

Source:
https://www.lesswrong.com/posts/sF5gAxnmifQe2TBNt/the-worthlessness-of-vitamin-d-is-mildly-exaggerated

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Narrated by TYPE III AUDIO.

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