According to the Mayo Clinic, a vitamin D deficiency takes place when the level of vitamin D in a person's body is lower than normal. Vitamin D is produced by the body when it is exposed to sunlight.
Humans can also ingest vitamin D by eating certain foods that include milk, yogurt and fatty fish, such as halibut, trout and tuna. Some groups of people, especially those who are older or those who have dark skin, may become deficient in vitamin D and require a supplement. People who believe that they may have a vitamin D deficiency should speak with their doctor as vitamin D deficiency can impact bone health.