Very few foods in nature contain vitamin D. The best sources are fatty fish such as salmon, tuna, mackerel, and fish liver oil. Small amounts are found in beef liver, cheese, and egg yolks. Vitamin D in these items is primarily in the form of vitamin D3 and its metabolites. Some types of mushrooms can provide vitamin D2 in small amounts.
In the 1920's vitamin D was recognized for its role in preventing rickets. After this time, some foods began to be fortified with vitamins (milk, cereal, etc.) to help prevent this disease. To this day, fortified foods provide most of the vitamin D in the American diet. For example, almost all of the U.S milk supply is voluntarily fortified with 100 IU/cup of vitamin D. In Canada, milk is fortified by law with 35 - 40 IU / 100 mL, as is margarine at greater than or equal to 530 IU/100 g. Vitamin D3, or cholecalciferol, is the form of vitamin D found in milk.
