Calcitriol (or 1,25-dihydroxyvitamin D); ergocalciferol (vitamin D2); cholecalciferol (vitamin D3); calcidiol (25-hydroxyvitamin D)
Vitamin D is actually a hormone rather than a vitamin. The body makes most of the vitamin D it needs; only about 10% comes from our food. The action of sunlight on our skin produces a substance called cholecalciferol, which is converted by the liver to calcidiol. This is further converted in the kidneys by the enzyme 1α-hydroxylase to calcitriol, the active form of vitamin D. Calcidiol is considered a good indicator of vitamin D levels and is the form that is usually measured by doctors.
The active form of vitamin D is produced primarily by the kidneys, but there are also a number of other tissues in the body that activate vitamin D. Excess cholecalciferol and calcidiol made during the summer are stored in our fat for use during the winter. Vitamin D modifies the activity of bone cells and is important for the formation of new bone in children and adults. It also regulates calcium levels in the blood by helping the body to absorb calcium from food and by preventing calcium loss from the kidneys. Recently, the role of vitamin D as a potent regulator of other functions throughout the body has emerged, although we are only just beginning to fully understand what these are and the significance for our health.
A fall in the concentration of calcium in the bloodstream is detected by the parathyroid glands, which then produce parathyroid hormone. Parathyroid hormone increases the activity of the enzyme 1α-hydroxylase, which produces active vitamin D. This increase in the concentration of calcium together with vitamin D feeds back to the parathyroid glands to stop further parathyroid hormone release. The production of vitamin D is also directly regulated by calcium, phosphate and calcitriol.
Vitamin D deficiency is common in the UK, probably due to lifestyle changes and increased concern about sun exposure. If you have too little vitamin D you are unable to maintain an adequate concentration of calcium in your blood for bone growth. This causes rickets in children and osteomalacia in adults. As the role of vitamin D as a regulator of other functions throughout the body has emerged, it has been suggested that a lack of vitamin D is linked to an inability to fight infections effectively, the development of diabetes, certain cancers, multiple sclerosis, depression, heart disease, high blood pressure, and stroke, although the direct relevance and mechanisms underlying these responses remain unknown.
It is very rare to have too much vitamin D. If you have too much vitamin D the level of calcium in your blood may increase and this causes a condition known as hypercalcaemia, which can cause a number of symptoms such as nausea, vomiting, constipation, tiredness, confusion, depression, headaches, muscle weakness, the need to pass urine more frequently and feeling thirsty. However, this condition is very rare.
Last reviewed: Jan 2015