Dvitamiinien

Dvitamiinien, commonly called vitamin D, refers to a group of fat-soluble secosteroids that includes ergocalciferol (D2) and cholecalciferol (D3). They are essential for calcium and phosphorus homeostasis and bone mineralization, and they also influence immune function and cell growth. The body can synthesize vitamin D in the skin upon exposure to ultraviolet B (UVB) radiation from sunlight and it can be obtained from certain foods such as fatty fish, cod liver oil, egg yolk, and fortified products, or from supplements.

Metabolism involves sequential hydroxylations: in the liver to 25-hydroxyvitamin D [25(OH)D], the main circulating form used to assess status, and in the kidney to the active hormone 1,25-dihydroxyvitamin D [1,25(OH)2D]. The active form binds the vitamin D receptor in many tissues, regulating genes related to intestinal calcium absorption and bone remodeling.

Clinical deficiency impairs bone mineralization, causing rickets in children and osteomalacia in adults; insufficiency has also been linked to increased risk of falls, fractures, and some immune-related conditions. Risk factors include limited sun exposure, older age, darker skin, malabsorption, and certain medications.

Recommended intakes vary by country and age group, commonly expressed as micrograms per day or international units. Many adults are advised to obtain about 10–20 micrograms (400–800 IU) daily, with higher needs for older adults or those with limited sunlight. Safe upper intake levels are set to prevent toxicity, typically around 100 micrograms (4000 IU) per day for adults, though recommendations differ.

Vitamin D toxicity is rare and usually arises from excessive supplementation, leading to hypercalcemia and related complications. Deficiency and sufficiency are assessed by measuring serum 25(OH)D concentrations.