- D3 (cholecalciferol) from exposure of skin (stratum basale) to sun, ingestion of fish, milk, plants.
- D2 (ergocalciferol) from ingestion of plants, fungi, yeasts.
- Both converted to 25-OH D3 (storage form) in liver and to the active form 1,25-(OH)2 D3 (calcitriol) in kidney.
- Function
- ↑ intestinal absorption of Ca2+ and PO43.
- ↑ bone mineralization at low levels (mediated by osteoblast release of osteocalcin (calcium binder) and phosphatases that promote bone mineralization.
- ↑ bone resorption at higher levels.
- Regulation
- ↑ PTH, ↓ Ca2+, ↓ PO43- → 1,25-(OH)2D3 production.
- 1,25-(OH)2D3 feedback inhibits its own production.
- ↑ PTH→ ↑Ca2+ reabsorption and ↓ PO43- reabsorption in the kidney.
- Deficiency
- Rickets in children (deformity, such as genu varum “bow legs”[A]), osteomalacia in adults (bone pain and muscle weakness), hypocalcemic tetany.
- Caused by malabsorption, ↓ sun exposure, poor diet, chronic kidney disease.
- Give oral vitamin D to breastfed infants.
- Deficiency is exacerbated by pigmented skin, premature birth.
- Excess
- Hypercalcemia, hypercalciuria, loss of appetite, stupor. Seen in granulomatous disease (↑ activation of vitamin D by epithelioid macrophages).