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Comment by ericmcer | Hacker News Reader

Yeah… doctors recommend supplementing vitamin D if you don’t get much sun. This almost feels comedic.

Doctors recommend supplementing vitamin D if you have a measured deficiency. Most people naturally settle to 20 ng/mL or higher, and just incidental exposure, or eating certain foods can help you either absorb or synthesize it.

There are also people, like me, who no matter what, we can't make as much vitamin D for whatever cluster of genetic factors causes that. Some of us are always tired unless we take 50,000 IU of D3 a week.

That’s over double the dose considered “safe”. Obviously, I’m not suggesting you do otherwise; it’s just way outside the bounds of what most adults would do unless directed to by their physician.

For context, this study found that 30 minutes of midday summer sun exposure in Oslo, Norway was equivalent to consuming 10,000–20,000 IU of vitamin D.

https://www.ncbi.nlm.nih.gov/pubmed/19667143

Must be much higher in sunnier regions.

The neat thing about vitamin D synthesis in the skin is that it stops once there's enough of it - it is literally impossible to overdose on vitamin D through sun exposure. So, there's a ceiling that makes "must be higher in sunny regions" far from obviously true!

What happens to unused vitamin D consumed from supplementation?

That gets stored in the fat. Overdose through ingestion is a thing, our bodies aren't so smart about that; it's skin synthesis specifically that limits itself.

How much causes overdose?

I’m prescribed 50,000 3x a week. The RDA was possibly miscalculated, but the only real way to know what will work for you is test - dose - test, repeat.

Did the study not control for confounding factors?

It did, they state that right in the summary.

But they don't necessarily say what those confounding factors are in the abstract (I'm assuming they do in the full paper but I don't have access).

It's easy for me to imagine tons of confounding factors that are associated with vitamin supplementation, e.g. wealth (wealth is associated with better health outcomes nearly everywhere you look), diet in general, exercise levels, etc. I highly doubt they accounted for all of these.

Then the confidence levels would blow up.

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