I thought you could get Vitamin D from sunlight?
I asked that same question, and the doc laughed. He said literally that some of his patients go to Florida for extended periods, come back "black" and still have Vitamin D levels that "suck."
He said Vitamin D3 supplements are the way to go. Since mine "sucked" so bad, he put me on 15,000 IU's per day for two months, then cut back to 10,000 for a month. Then a blood test to confirm the results.
The thing that prompted all this for me was a preponderance of fever blisters that I'd been getting over the past 6-12 months or so. He said my immunity levels probably "sucked." He looked at the results of a January blood test that I'd had done and said, "Yep, Vitamin D levels 'suck.'"
The doc is not a young guy -- been around for awhile. I liked him (first time I'd seen him) because he pulled no punches.
I didn't ask him any questions about his favorite word -- "suck." I figured, leave well enough alone.
ETA: I'm no chemist, but I believe that sunlight helps convert Vitamin D into Calcium in the body. Something like that, anyway.