Vitamin D (D2) comes from plant sources, such as wild mushrooms, as well as fortified foods, such as milk or cereal products. Its strength is typically measured in international units, which is abbreviated as “IU” on labeling. The 50,000 IU capsules are prescription only, while lower strengths are available over-the-counter. Vitamin D is less expensive to produce and therefore is the form most commonly found in fortified food products.
Vitamin D3 mainly comes from animal sources such as fish oil, fatty fish, liver, and egg yolks. When your skin is exposed to sunlight, it produces vitamin D3. For this reason, it is sometimes referred to as the sunshine vitamin. Its strength is also measured in international units. All forms of vitamin D3 are available over-the-counter.