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Comment by paulsutter | Hacker News Reader

Could you provide some references? I'm really interested to learn more

EDIT: I'm looking for backup to the claim that sunlight provides something that you don't get in a vitamin D3 supplement. It seems reasonable but has a big impact on the solution.

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[5] forms (vitamers) of vitamin D exist (see table). The two major forms are vitamin D2 or ergocalciferol, and vitamin D3 or cholecalciferol, vitamin D without a subscript refers to either D2 or D3 or both. These are known collectively as calciferol....Vitamin D3 (cholecalciferol) is produced by ultraviolet irradiation (UV) of its precursor 7-dehydrocholesterol. This molecule occurs naturally in the skin of animals and in milk. Vitamin D3 can be made by exposure of the skin to UV, or by exposing milk directly to UV (one commercial method).

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Vitamin D3 (cholecalciferol) is produced industrially by exposing 7-dehydrocholesterol to UVB light, followed by purification.[143] The 7-dehydrocholesterol is a natural substance in wool grease (lanolin) from sheep or other woolly animals. Vitamin D2 (ergocalciferol) is produced in a similar way using ergosterol from yeast or mushrooms as a starting material.[143]

Tl;dr of 5 types & only D2 and D3 are commercial avail

--Wikipedia: http://en.wikipedia.org/wiki/Vitamin_D

It looks to me like the supplement is D3 and the one you get from UV exposure is D3, so no, spending time outdoors doesn't give you any better Vitamin D than the supplement. Of course, exercise has other benefits.

I would amend that to no proven benefits. The body is a complex system. D3 generated by the body via natural pathways may be more effective than ingested D3 in some way we don't understand yet.

Leaving aside any other ill-understood benefits of sun exposure.

That simply lists precursors to calcitriol, which you can get but isn't recommended because it is hard to dose.

There are other reasons to sunbathe (e.g. NO release) but there is only one active form of D.

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