Vitamin D is a fat-soluble chemical found in egg yolks and butter, and it may be formed by a reaction of sunlight on skin. It is essential for the balance of calcium and phosphorus in the bones and bloodstream, but it is not used routinely in the treatment of disease. Vitamin D is actually composed of a number of chemicals including calcitriol, cholecalciferol and ergocalciferol.
The level of vitamin D in a blood sample can be measured. The normal value is 11 to 46 µmol/L in adults and 7 to 35 µmol/L in children. A false normal result can occur in patients with high cholesterol or blood fat levels, and some gall bladder diseases.
In children a lack of vitamin D causes rickets, while in adults it can result in osteoporosis, muscle weakness and osteomalacia. Excess vitamin D may cause hypercalcaemia (high blood calcium levels), constipation and nausea.
Vitamin D supplements may be used in those who have minimal exposure to sunlight due to climate (higher latitudes), location (live indoors eg. nursing home) and clothing (eg. some Muslim women). In combination with calcium supplements, vitamin D is also useful in the management of osteoporosis.