Vitamin D is essential for a healthy body, and is available in two main forms: vitamin D-2, also called ergocalciferol, and vitamin D-3, also known as cholecalciferol. Vitamin D-3 is ultimately metabolized in the body to its active form: 1,25-dihydroxyvitamin D, also called calcitrol. Humans synthesize vitamin D-3 from the sunlight, but this vitamin has various dietary and supplementary sources as well.
Sunlight
Vitamin D is often called the "sunshine vitamin" because it can be synthesized by skin cells after these cells absorb ultraviolet rays from the sun. People with darker skin or those that live far from the equator also make less vitamin D-3. The ability for skin cells to synthesize vitamin D-3 decreases with age, and thus the elderly are more susceptible to a deficiency in vitamin D.
Fatty Fish
The flesh and liver oil of fatty fish are especially good sources of vitamin D3. Cod liver oil is the best dietary source of this vitamin, with 1 tbsp. of this oil providing 340 percent of the daily recommended value. Salmon is the next best source, with 3 oz. providing 112 percent of the recommended daily value.
Fortified Foods
Because of the importance of vitamin D in the diet, many foods are fortified with vitamin D-3. Most milk sold in the United States is fortified with vitamin D, and each cup provides 2.5 micrograms of this vitamin. Additionally, U.S. law requires fortification of infant formula with 1 to 2.5 micrograms of vitamin D. Other types of foods that are sometimes fortified include yogurt, margarine, orange juice and breakfast cereals.
Supplements
Most vitamin D supplements contain the D-3 form of the vitamin. Multivitamins commonly provide 10 micrograms of vitamin D-3, which is two-thirds the daily requirement for people between 1 and 70 years-of-age. Infants younger than 1 year should get 10 micrograms per day and, due to the decreased ability of skin in the elderly to synthesize vitamin D-3, adults over 70 years-of-age should get 20 micrograms of vitamin D per day.
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