Not Vitamin Dallas. (Forgive me.)
When I went in for my annual physical this year, I got my bloodwork done (typically I do it every couple of years right now - guess I'm getting to that age that it'll get more frequent). My doctor added in the vitatmin D test because he's been seeing more and we live in Ohio. Turns out, I do have a deficiency.
(My other bloodwork is normal. My cholesterol is 202, which is borderline high according to the internet. My LDL is 125. He wasn't concerned at all about my cholesterol, so I won't be either.)
Apparently a normal vitamin D level in the blood is 30 to 100. Mine is 6, so definitely low. My doctor prescribed me a supplement that I'll take once a week for five months. Then I'll go back for retesting. I'm guessing between spring/summer running and the supplement, my level should be within the normal range. He said once it's within normal, OTC supplements should be able to keep it there.
I'm making other changes - skipping the sunscreen on short runs (well, except for my face and my tattoos) and trying to take my vitamins/supplements (like fish oil) regularly. I also got Carnation Instant Breakfast to have in replacement of my chocolate milk that I have for breakfast once or twice a week (along with a banana and whatever nut butter I have around). (Most milk is fortified with vitamin D because it helps the absorption of calcium.) Other nutritional sources for vitamin D include fish and eggs, two things I don't consume much of.
We apparently get most of our vitamin D from the sun. In Ohio, that can be hard enough, but considering that I try to use sunscreen religiously, it's even more difficult.
Vitamin D is pretty important because you need it to maintain normal levels calcium and phosphorus (the Mayo Clinic says it can lead to rickets and bone problems). There's evidence that it might protect you from osteoporosis and high blood pressure, as well as provide some protection against some cancers and autoimmune diseases.