Vitamin D is found naturally in cod liver oil, swordfish, salmon, tuna, eggs and Swiss cheese. Many foods have been fortified with vitamin D like cereal, yogurt, milk and margarine. Sun exposure helps many patients meet their vitamin D requirements as well. The recommended daily value of vitamin D is 400 IU's for children 4 and over and adults. Many people are supplementing to increase their levels of this vitamin due to a vitamin D lacking diet and less sun exposure than necessary.

Rickets and osteomalacia are the most common diseases associated with vitamin D deficiency. Vitamin D deficiencies are usually the result of dietary inadequacy, impaired absorption and use, increased requirement, or increased excretion. A vitamin D deficiency can occur when usual intake is lower than recommended levels over time, exposure to sunlight is limited, the kidneys cannot convert 25(OH)D to its active form, or absorption of vitamin D from the digestive tract is inadequate. Vitamin D-deficient diets are associated with milk allergy, lactose intolerance, ovo-vegetarianism, and veganism.