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; it is required to absorb calcium and phosphate from the gut into the bloodstream. Vitamin D is mostly produced in the skin in response to sunlight and is also absorbed from food eaten (about 10% of vitamin D is absorbed this way) as part of a healthy balanced diet. The liver and kidneys convert vitamin D (produced in the skin and taken up in the diet), into the active hormone, which is called calcitriol(1,25-dihydroxyvitamin D). Active vitamin D helps to increase the amount of calcium the gut can absorb from food into the bloodstream and also prevents calcium loss from the kidneys. Vitamin D modifies the activity of bone cells and is important for the formation of new bone in children and adults.
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 (catalyst) that 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 lack of sun exposure. It may also occur in case of diseases affecting the intestine, liver and kidney which affect the ability to absorb or process Vitamin D. If you have severely low vitamin D levels 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, muscle weakness, fatigue and 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.
Oily fish such as sardines, mackerel and salmon are good dietary sources of vitamin D. Calcium can be found in cow’s milk and dairy products. In the UK, foods such as breakfast cereals and margarine are fortified with vitamin D. Adequate exposure to sunlight is important, especially between April and October, for around 15 – 30 minutes daily. Public Health England recommends vitamin D supplements are taken by people in at risk groups (for example, women who are pregnant or breast-feeding, young children and those with osteoporosis).
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. Hypercalcaemia 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: Nov 2021