Common effects of vitamin D include helping the body absorb and use calcium for strong bones and correcting low levels of vitamin D in people who don't get enough. Problems associated with low levels of vitamin D include the bone-weakening conditions osteoporosis and osteomalacia, both of which can lead to fractures. Vitamin D could also have a positive effect on other conditions such as certain cancers, type I diabetes, and certain autoimmune diseases. As most commonly eaten foods don't contain much of vitamin D, it can be difficult to get enough of it without taking a supplement.
Vitamin D helps to prevent or correct problems caused by low levels of this vitamin, most of which are related to bone growth. In children, inadequate vitamin D causes rickets, a bone-softening disease that occurs because the body can't use calcium efficiently without vitamin D. A number of digestive problems can cause adult rickets, also known as osteomalacia, in which an inability to absorb vitamin D leads to calcium loss from the bones. Somewhere between 10 and 40 percent of adults may be deficient in vitamin D. Correcting deficiency problems is one of the effects of vitamin D and can help prevent related problems.
One of the proposed effects of vitamin D is a reduced risk of some cancers. In particular, this includes colon, breast, and prostate cancer. The reason for this isn't clear, but it may be due to the positive effects of vitamin D on the immune system.
Low levels of vitamin D have been linked to depression and seasonal affective disorder (SAD), a condition that has some features of depression and occurs in winter. The lack of natural vitamin D synthesis in winter, which naturally takes place when the skin is exposed to sunlight, may be related to SAD. One of the effects of vitamin D could be its ability to prevent some autoimmune diseases like rheumatoid arthritis and multiple sclerosis.
Side effects of vitamin D are fairly rare. Some doctors recommend taking up to 1,000 international units (IU) of vitamin D daily. Greater amounts than this are probably unnecessary and could possibly be harmful if taken over a long period of time. It should be remembered that, in addition to supplements, total daily intake also includes vitamin D from the diet, fortified foods like milk, and the vitamin D that is naturally produced in the skin.