Firstly, D is a HOROMONE, NOT a 'vitamin'...look that up and you'll see I'm right. If you go out side, the sunlight ACTIVATES the D HORMONE, that's sitting dormant inside of you, and you body reaps the benefits, healthy-wise. Secondly. If you live somewhere that doesn't give you enough sunlight, when you go outside, then that's nature's ways of telling you, you shouldn't live there and to go live elsewhere were you CAN get enough sunlight, when you go outside, daily.