well actually a vitamin is defined as an essential nutrient that your body cannot manufacture on its own, so it must be gained from an outside source.
i live in the upper midwest and its winter here. its been in the teens during the day and below zero at night. its also been cloudy. so there is simply no opportunity to get sunlight on my bare skin. you really cant sun bathe when its cloudy and 15 degrees. thus i need to get my vitamin D from somewhere, and that means either cod liver oil, fish, milk, or vitamin D supplements. i know of no other sources, do you?