I have a vitamin D deficiency, and went on vacation to the beach, and after that I immediately went to France (great meat there, and great eggs :D). The weather wasn't that bad, but as soon as there was a lot of sun I immediately went outside and read on a chair. Sometimes I did that for 3 hours at 30°C (86°F). I usually ended up quite red, but I'm not a person that gets burned easily for some reason (I would have to stay in the sun all day to get burned a bit, and even then I'd like it). Is it bad to stay in the sun that long? After all, I had a vitamin D deficiency, and need it ...
Once your skin begins to turn pink, you are not absorbing vitamin D any more, only damaging your skin. You can gradually build up the amount of time it takes to get to that point...the darker your skin, the less efficient your body is at making the conversion of vitamin D from sunlight, and vice versa.
Burning = bad. Sun in moderate amounts = good. Sun time built up gradually to create base tan & lack of burning when exposed for a longer time = great. 15 to 30 minutes will give light skinned people in temperate latitudes midsummer 10-20K IU of D.
As someone who works outdoors, needs vitamin D, and has had several melanoma scares, too much sun can be bad. Personally, I'll use sunscreen if I'm going to be working in an open field for more than a few hours—though this isn't ideal. Alternate with some shade before your skin burns.
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