Of all the foods that provide vitamin D, cod liver oil has the most abundant amount. It provides 1,360 international units of vitamin D, which is 340 percent of the recommended daily value. The vitamin D content of mushrooms can be increased by exposing them to ultraviolet light while they grow.
Vitamin D is unusual in that it's one of the few vitamins manufactured by the human body. Because of this, some scientists believe it is actually a hormone. The body makes vitamin D when the skin is exposed to the sun's UV rays. The zoosterol 7-dehydrocholesterol, a molecule that's found in the skin, is converted into a form of vitamin D. Ideally, this type of vitamin D is then transported to the liver, where it's made into another form of vitamin D. Finally, this second form of vitamin D is sent to the kidneys, which produce the most potent form of the vitamin for the body's use.