Vitamin D

There are many people who are deficient in vitamin D and awareness of this has been increasing. To know you are deficient, you would need to have your  25-hydroxyvitamin D level drawn. Levels less than 20 ng/ml is commonly considered deficient and levels of 30-50 ng/ml are desired. It is thought that deficiency is growing globally and there are a number of theories as to why this is. The most common reasons include not taking in enough vitamin D and not enough sun exposure. It is recommended that people who do not have regular sun exposure intake 600-1000 IU's per day. Some patients will need much larger amounts, but this varies from person to person. Vitamin D3 is often preferred over D2 because the body naturally makes D3. Vitamin D is important for bone health, immune function, and aids in the absorption of other nutrients, like calcium. 

UpToDate: http://www-uptodate-com.proxy.lib.uiowa.edu/contents/vitamin-d-deficiency-in-adults-definition-clinical-manifestations-and-treatment?source=search_result&search=treatment+of+vitamin+d+deficiency&selectedTitle=1~150