Vitamin D is made in the skin under the influence of sunlight. The amount of sunlight needed to synthesize adequate amounts of vitamin D varies, depending upon the person’s age, skin color, sun exposure, and underlying medical problems. The production of vitamin D from the skin decreases with age. In addition, people who have darker skin need more sun exposure to produce adequate amounts of vitamin D, especially during the winter months.

Another important source of vitamin D is foods, where it may occur naturally (in fatty fish, cod-liver oil, and [to a lesser extent] eggs). In the United States, commercially fortified cow’s milk is the largest source of dietary vitamin D, containing approximately 100 international units of vitamin D per 8 ounces. Vitamin D intake can be estimated by multiplying the number of cups of milk consumed per day by 100 (two cups milk = 200 international units vitamin D). In other parts of the world, cereals and bread products are often fortified with vitamin D.

Although vitamin D is found in cod liver oil, some fish oils also contain high doses of vitamin A. Excessive vitamin A intake can be associated with side effects, including liver damage and fractures.