- Vitamin D synthesis:
- Begins in the:
- Keratinocytes of the skin
- Subsequently, hydroxylation occurs:
- In the liver to yield 25-hydroxyvitamin D
- The final step in the conversion of vitamin D to its active form occurs in the kidney:
- Where a second hydroxylation reaction takes place:
- To yield 1,25-dihydroxyvitamin D
- Where a second hydroxylation reaction takes place:
- Begins in the:
- Sun- light plays a key role:
- In the initial synthesis step in the skin:
- Persons who are not exposed to sunlight:
- Require supplemental vitamin D through dietary intake
- Persons who are not exposed to sunlight:
- In the initial synthesis step in the skin:
7-dehydrocholesterol (provitamin D3) in the skin absorbs ultraviolet B (UVB) radiation with wavelengths of 290 to 315 nm and is converted to pre-vitamin D3.
Pre-vitamin D3 undergoes thermal isomerization to vitamin D3
Continued exposure to UVB radiation can result in the breakdown of pre-vitamin D3 and vitamin D3 to inactive photoproducts
Dietary vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) are absorbed in the gastrointestinal tract, incorporated into chylomicrons, and transported via the lymphatic system into systemic circulation.
Vitamin D (vitamin D2 and vitamin D3) from the diet and skin enters the circulation bound to the vitamin D-binding protein.
As a fat-soluble molecule, it can be taken up by adipose tissue and stored for later use.
Circulating vitamin D is metabolized in the liver to 25-hydroxyvitamin D [25(OH)D] by the enzyme vitamin D-25-hydroxylase.
Vitamin D-25-hydroxylase activity is inhibited by 25(OH)D (negative feedback).
25(OH)D re-enters the circulation and is metabolized in the kidney and other tissues to the active metabolite 1,25-dihydroxyvitamin D [1,25(OH)2D] by 25(OH)D-1α-hydroxylase.
Renal production of 1,25(OH)2D is inhibited by elevated serum levels of phosphorus, calcium and fibroblast growth factor 23 (FGF-23).
Parathyroid hormone enhances renal production of 1,25(OH)2D.
Catabolism of 25(OH)D and 1,25(OH)2D into biologically-inactive molecules is primarily mediated by the cytochrome P-450 enzymes CYP24 and CYP3A4.
#Arrangoiz #ParathyroidSurgeon #ParathyroidExpert #CancerSurgeon #HeadandNeckSurgeon #EndocrineSurgery #Miami #VitaminDSynthesis #CalciumHemostasis #Hyperparathyroidism #MountSinaiMedicalCenter
Rodrigo Arrangoiz MS, MD, FACS, FSSO 