Vitamin D deficiency may occur from a lack in the diet, poor absorption, or having a metabolic need for higher amounts. If one is not eating enough vitamin D and does not receive enough ultraviolet sun exposure over an extended period, a deficiency may arise. People who cannot tolerate or do not eat milk, eggs, and fish, such as those with a lactose intolerance or who follow a vegan diet, are at higher risk for a deficiency.