Vitamin D is a fat soluble vitamin that plays a role in many important bodily functions. Vitamin D is best known for working with calcium in the body to build and maintain strong bones. It is also involved in regulating the immune system and cells, which could help to prevent cancer. Vitamin D also protects against Osteomalacia, which is a softening of the bones in adults.
Rickets, a disease that causes soft and/or weak bones can result if a child has a Vitamin D deficiency. It is important to get the recommended amount of Vitamin D as a child because most bone is formed when you are young. The average recommendation for children is: 2500 units between the ages of 5-10 years old. Milk is a great source of Vitamin D. If you are lactose intolerant or just choose to not drink milk, then access to direct sunlight exposure, at least 45 minutes per week, will help your body produce Vitamin D. Taking a vitamin D and calcium supplement also can reduce the rate of bone loss and could help prevent Osteoporosis.
There is no way to know if the above recommendations are correct for you however. The ONLY way to know for sure is to test your blood. You may need four to five times the recommended Vitamin D. Ideally, your blood level should be 25(OH)D (the marker that is most strongly associated with overall health) and 60ng/ml (nanograms per milliliter of blood).