Vitamin D is a fat soluble vitamin that plays a vital role in the human body. It has many functions, and a lack of vitamin D can lead to many health problems. Bone Growth. Vitamin D is essential in bone growth. Its main role in the body is to increase the flow of calcium into the bloodstream. It does this by promoting absorption of calcium from food. Without vitamin D, calcium would not be absorbed into the body. Therefore, vitamin D is equally important to the maintenance of bone health as is calcium.