Q. How can I be sure I'm getting enough vitamin D during the winter months?

A. Vitamin D promotes the body's absorption of calcium, important to maintaining healthy bones and teeth. Your skin makes vitamin D when exposed to the sun's ultraviolet rays. Fortunately, this process is very efficient; you need to expose only a small amount of skin (without sunscreen)—the face and arms, for example—to direct sunlight for 10 to 15 minutes three times a week to meet your D needs. It is also possible to get natural vitamin D from foods such as oily fish, organ meats, and egg yolks. However, the limited sunshine in winter can make it difficult to get adequate vitamin D without supplementation.

During the shorter, darker days of winter, supplements can be just as effective (though not as fun) as a sun bath. The National Academy of Sciences recommends 200 international units (IU) a day for adults 50 and younger, 400 IU for people 51 to 70, and 600 IU for anyone older than 70. However, many researchers believe that current recommendations are outdated—and much too low. If you get little sun, some researchers suggest taking as much as 1,000 to 3,000 IU daily. For those of us who get some sun and need only a moderate boost, a safe bet is to take a multivitamin with 400 IU of vitamin D, and include some vitamin-D-fortified foods and beverages in the diet. A cup of fortified milk or orange juice, for example, contains 100 IU, and a serving of most fortified breakfast cereals provides around 40 IU.

This Q&A was written by Heather Jones, a registered dietitian, freelance writer, and nutrition professional based in Washington, D.C.