[AlexandrB]

To get enough vitamin D you need ~10 minutes a day[1] in direct sunlight with 25% skin exposed.

> In spring and summer, 25 percent of the body (the hands, face, neck and arms) is exposed to the sun, and in these seasons, about 8 to 10 minutes of sun exposure at noon produces the recommended amount of vitamin D.

[1] https://www.uclahealth.org/news/article/ask-the-doctors-roun...

[layer8]

At noon, in summer, in Spain. And probably 25% of your skin needs to be hit by direct perpendicular sunlight, and not just be exposed. In any case, don’t assume, go measure your Vitamin D levels.

[jimbokun]

So 20 minutes then, just to be sure.

[layer8]

Sunlight intensity varies greatly. Depending on where you live, season and cloudiness, you may only get a small fraction of the amount implied above. Again, there is no substitute for measuring your levels. Speaking from experience.

[snapplebobapple]

Or take your shirt off to increase exposed area...

[jfengel]

Anecdata-point: In the summer I regularly run for hours at a time (marathon training), without a shirt and without sunscreen. (I've got a good tan which seems to keep me from getting sunburned.) And yet I regularly test as low Vitamin D. A daily Vitamin D supplement suffices to keep the numbers in range.

I assume that this is a personal thing, something about my specific metabolism or skin or whatever that doesn't produce vitamin D particularly well. I have no idea if the supplement actually improves my health outcome -- I was generally healthy both before and after adding supplements.

My advice, such as it is: don't take any supplement without a doctor telling you it's needed. But if a doctor says it's necessary, yeah, do that.