The summer sun, with enough skin exposed. I think it's NIH.gov with the info that, on the east coast, from Boston and up, for at least four months, Nov. through Feb., sunlight will produce no vitamin D in humans.
Did that need more commas?
I get skin cancer. My doc advised me to stay out of the sun for the rest of my life. He said that vitamin D was important and I need to force it, and I told him I take multivitamins he said “Wah, you need way, way more than that”.