My doctor just recommended that I take vitamin D supplements. She did not draw my blood, ask me any questions about my diet or my exposure to sun. Odds are if you have been to the doctor in the past year, you have also been told to take vitamin D supplements.
Very few foods with the exception of fatty fish are good sources of vitamin D. While milk and some cereals are fortified with vitamin D, most people meet their vitamin D requirements through sun exposure. Here is where things take a turn for the absurd. The medical community officially recommends avoiding all sun exposure and if you have to be outside, covering up with protective clothing and wearing sunblock, effectively making it impossible to get adequate vitamin D.
I am not recommending prolonged sun exposure. I have had skin cancer. I wear sunblock 365 days a year. But I miss some spots and don't always re-apply as often as recommended. I am confident I am getting 15 minutes a week of sun exposure, enough to meet my vitamin D requirements. When I pointed this out to my doctor, she responded as if I told her I was getting my vitamins through cigarette smoke. I think in order for public health policy to be effective we must find the balance between what is optimally recommended and what is practical.
Unless you are housebound or living in a nursing home, you are more than likely getting enough incidental sun exposure to meet your vitamin D requirements. I have been a nutritionist long enough to know that vitamin D supplements will be the "next big thing" until they're not.