If UVB is required for vitamin D metabolism, how would one maintain normal vitamin D levels with restricted sun exposure? How much sun exposure is necessary?

In the southern United States, daily short UVB exposure (5 minutes for lightly pigmented people and 10 minutes for darkly pigmented people) of a small area of skin (face, hands, and arms) will supply ample vitamin D for the body’s needs. However, at latitudes greater than 40 degrees, winter sun exposure does not produce active vitamin D. Therefore, residents of the northern United States should take supplemental vitamin D of at least 400 IU daily. Fortunately, proper vitamin D levels can be easily maintained by eating a vitamin D–fortified diet. Eggs, beef liver, and oily fish, such as salmon, catfish, herring, mackerel, and tuna, are excellent sources of vitamin D. Many foods, such as milk, cereals, and bread are fortified with vitamin D, and most multivitamins contain vitamin D.

Cicarma E, Porojnicu AC, Lagunova Z, et al: Sun and sun beds: inducers of vitamin D and skin cancer, Anticancer Res 29 (9):3495– 3500, 2009.

Weinstock MA, Moses AM: Skin cancer meets vitamin D: the way forward for dermatology and public health, J Am Acad Dermatol 61(4):720–724, 2009.