D3 is made in the skin from cholesterol, and changed into a more active form by the liver. However, the skin will not make it unless enough ultraviolet light shines on it. As sunlight contains ultraviolet light, getting enough sun is one way of getting enough D3.
Many things can keep the skin from making enough D3. Winter sunlight may be too weak. Melanin, which protects skin from damage, also keeps it from making D3, which is why people with darker skin are more prone to deficiency. Older people are also prone, because aging skin makes less D3, even with enough sunlight. Clothing, glass, sunscreens and sunblocks also shield the skin from getting enough ultraviolet light to make D3.
It is hard to know how much supplemental Vitamin D, if any, is needed. Some current recommendations from health organizations (such as the Institute of Medicine) have recently increased. They are still typically less than 25 micrograms (1000 IU) per day, but up to 100 mcg (4000 IU) per day is considered safe[1]. A recent panel of Vitamin D researchers concluded that at least 20-25 mcg (800-1000 IU) per day would be beneficial for most adults[2].
Very few foods naturally contain much D3. Fish do, especially oily ones, such as salmon, sardine and mackerel. Many kinds of edible mushrooms contain some D2, like shiitake. Mushrooms grown in full sunlight tend to have more.
The U.S. Institute of Medicine is involved in a study that will likely result in an increase in the recommended daily intake of vitamin D. Research shows vitamin D affects nearly every area of the body and low levels of vitamin D can have serious consequences.
It's the vitamin associated with the sun that we absorb through our skin, and increasingly block out when we use sunblock, to avoid getting skin cancer.
Vitamin D is also available in some foods, but as the world gets fatter, Vitamin D gets trapped in body fat.
What doctors are now discovering is that many people don't have enough vitamin D.