However, if the body does not make enough, it can be found from food sources in tiny amounts.  In fact, many countries add it automatically to certain foods like milk. Supplements can be easily found in most developed countries.
What it does change
As a hormone, Vitamin D does many things in the body. It was first discovered as the substance which could prevent and cure rickets. It controls the levels of calcium ions and phosphates in the blood, as well as calcium and magnesium absorption in the intestines. It helps bones grow and form. It is also good for the immune system.
Different kinds change
In total, there are 5 different forms, D1 to D5. The most common ones are D2 and D3 (see images).
D2 (also called ergocalciferol) is produced by fungi. It is similar to D3, but not exactly the same.
Getting enough change
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. Less than 25 micrograms (1000 IU) per day, but up to 100 mcg (4000 IU) per day is considered safe. A recent panel of Vitamin D researchers concluded that at least 20-25 mcg (800-1000 IU) per day would help most adults.
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.
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