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Comment by bcebulla | Hacker News Reader

That's not true. In the controlled setting, sunscreen affects ability of skin to make vitamin D, by about half: http://www.ncbi.nlm.nih.gov/pubmed/22512875

Sunbathers who use sunscreen tend to not have lower vitamin D levels than non-sunbathers who don't use sunscreen, because, well, people who don't use sunscreen don't get sun exposure anyhow.

Also, when the human genome was evolving, sun exposure accounted for 90%+ of all vitamin D intake, diet accounted for less than 10%. We know this by looking at the diets and vitamin D levels of hunter gatherers.

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You're referring to a study that used 37 volunteers that were exposed for 20 min.

When I apply sunblock to me or my daughter, it's because we're going to be outside for well over 20 minutes. For that length of time, depending on conditions, I probably wouldn't even bother.

My point stands, if you apply sunblock intending to spend 1+ hour(s) in the sun, then you'll likely benefit from the vitamin D exposure.

In a controlled setting, sunblock will always affect vitamin D production, whether you expose for 20 minutes or 60 minutes or 120 minutes, or whether you test 37 subjects or 370 subjects.

The reason is that 7-dehydrocholesterol is photolyzed by UV light in the 280-320nm range (mostly UVB) to previtamin D. Sunblock works to absorb UVB (and usually UVA) to prevent it from penetrating your skin. Thus sunblock limits the amount of UVB penetrating your skin, and limits your ability to make vitamin D.

That being said, yes, you will still make some vitamin D with sunblock on because sunblock is not perfect at absorbing UVB. Just not as much as you would without.

None of that changes the fact that increased exposure to UV will have net-positive benefits in regards to vitamin D intake. If you apply sunblock you just have to spend more time outside in order to absorb more vitamin D via UV.

You can't uniformly apply sunscreen on your entire body. Depending on type of skin, vitamin D will hit maximal daily production from sunlight in range of very fast to very slow.

So, yes, sunscreen might block some vitamin D production for some people, or might not do it at all.

Its perhaps prudent to apply it to some skin parts that are most affected by sunlight (face, shoulders) and leave other body parts without it. There you have benefits of both worlds.

Furthermore, vitamin D is not the only substance produced by the skin via sunlight (although we don't know much about other ones), so vitamin D from diet is not a substitute for it. Furthermore, vitamin D from diet can easily cause calcification depending on other factors (vitamin K and A in the diet and genetics) while there is internal protection for body made one.

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