Vitamin D supplements may work after all—with caveats, says LessWrong analysis

A recent LessWrong article titled "The worthlessness of vitamin D is mildly exaggerated" challenges the growing consensus that vitamin D supplements are useless for anyone without severe deficiency. The author, dynomight, acknowledges that early hype—claims that vitamin D cures cancer, heart disease, and depression—has been debunked by large randomized controlled trials. However, they argue that the skeptics have over-corrected. By examining the biological mechanism (vitamin D as a signaling molecule, not just a calcium regulator) and evolutionary history (humans evolved with high UV exposure), the post suggests that levels below ~50 nmol/L may still carry subtle, hard-to-measure benefits.

The piece dissects the classical endocrine view: the kidneys convert storage vitamin D into active form only when calcium is low, and as long as levels stay above ~25 nmol/L, the body can maintain calcium balance. But dynomight points out that this narrow view ignores the fact that almost every cell in the body produces provitamin D and may use it for local signaling. The correlation data—showing higher vitamin D levels linked to better health outcomes—is not purely confounded, they argue, because the biological plausibility is strong. The author concludes that the safest bet is to supplement to moderate levels (e.g., 50-80 nmol/L), especially for those with limited sun exposure, though they admit uncertainty remains.