Vitamin D is a fat-soluble vitamin and 80%-90% is obtained through exposure to sunlight. Ultraviolet rays from sunlight strike the skin and trigger vitamin D synthesis. Very few foods contain vitamin D.
The major role of vitamin D is to maintain normal blood levels of calcium and phosphorus. Vitamin D helps the body absorb calcium, which forms and maintains strong bones. it is used to improve bone health and decrease fractures.
Vitamin D may also protect against osteoporosis, boosting the immune system, preventing autoimmune diseases, and cancer. However since Vitamin D is a fat soluble vitamin it can build up in the system and become toxic so it must be monitored.