Vitamin D3 is good for achieving an optimal level of vitamin D in the blood. The National Institutes of Health Office of Dietary Supplements says vitamin D is necessary for calcium and phosphorus absorption, immune function, nervous system activity, cell formation and muscle function.
Although vitamin D is found in some foods, it is not found in great enough quantities to satisfy the body's minimum requirements. The only ways to get the amount of vitamin D necessary are to take supplements or spend time in the sun, according to the Vitamin D Council. When the skin is exposed to sunlight, it produces vitamin D. How much vitamin D is produced depends on the amount of skin exposed, the time of day, the color of the person's skin and the location. The lighter someone's skin, the easier it is for that person to produce vitamin D.
For those who are concerned about spending too much time in the sun, supplements are a good alternative. The Vitamin D Council recommends taking vitamin D3 supplements rather than D2 supplements when possible. D3 is the type of vitamin D produced when the skin is exposed to the sun. These supplements are generally safe, but there are risks associated with taking too much of the vitamin. Side effects of too much vitamin D include headache, dry mouth, vomiting, nausea and fatigue, according to WebMD.