Cholecalciferol from skin and diet
Stimulus · 7-dehydrocholesterol UVB 290-315 nm
7-dehydrocholesterol in the epidermis is converted by UVB light to previtamin D3, which isomerises to cholecalciferol (vitamin D3). The skin supplies most of the body's vitamin D; diet adds D3 from animal sources and D2 (ergocalciferol) from fungi and fortified foods. Both are inert until hydroxylated twice.
Detail
- Skin
- 7-dehydrocholesterol + UVB at 290-315 nm gives previtamin D3, which isomerises thermally to cholecalciferol; this is 80-90% of supply in most people
- Diet
- Cholecalciferol (D3) from oily fish, egg yolk and liver; ergocalciferol (D2) from fungi and fortified foods
- Absorption
- Fat-soluble: needs bile salts, pancreatic lipase and an intact small bowel
- Transport
- Carried on vitamin D-binding protein; stored in adipose tissue, which is why obesity lowers measured 25(OH)D
When it goes wrong
Deeply pigmented skin, veiling, institutional care, high latitude or consistent sun avoidance
Low 25(OH)D leading to secondary hyperparathyroidism, osteomalacia in adults and rickets in children
Fat malabsorption: coeliac disease, cholestasis, cystic fibrosis, bariatric surgery
Deficiency of the fat-soluble vitamins A, D, E and K, with osteomalacia and a raised alkaline phosphatase
Exclusively breastfed infant of a vitamin D-deficient mother
Nutritional rickets: bowed legs, widened wrists, rachitic rosary, delayed fontanelle closure and hypocalcaemic seizures
Practise this structure
1 question in the bank tagged Vitamin D.