Vitamin D : forms, half-lives, sources

Vitamin D is complicated.

SUMMARY : 

• calcitriol << kidney << calcidiol << liver << calciferol(s) << exposure of fungi, and animal, tissue to  UV

FORMS : 

• Furthest upstream, calciferol(s)
• [ D3 = cholecalciferol ] ... output FROM human skin, and other animal sources subjected to UV 
• Supplementation guidelines ... this is of interest because of the vast range :
• 400-800 iu/day for maintenance
• 2000-6000 iu/day for athletes
• [ D2 = ergocalciferol ] ... output FROM fungi subjected to UV
• There don't seem to be plant sources - some documentation misattributes fungi sources to plants.
• Plasma half-lives are about 1 day, very short, because these are quickly metabolised in the liver.
• But effective somatic half-lives are 15 to 21 days due to storage in and slow release from fat.
• => 3% at 2.5 to 3.3 months
• The most plasma-stable form, [ calcidiol(s) = calcifediol(s) = "25(OH)D" ]
• FROM liver activity upon calciferol(s)
• [ from D3 : 25-hydroxy-cholecalciferol ]
• [ from D2 : ercalcidiol = 25-hydroxy-ergocalciferol ]
• Plasma half-lives are 15-30 days, bound to [ gc-globulin = D-binding protein ]
• => 3% at 2.5-5 months
• THIS IS THE COMMON BLOODWORK
• SUFFICIENT : > [ 30 mg/mL = 75 nmol/L ]
• INSUFFICIENT : [ 20-29 mg/mL ] = [ 52-72.5 nmol/L ]
• DEFICIENT : < [ 20 mg/mL = 50 nmol/L ]
• The active form, [ 1,25-dihydroxycholecalciferol = calcitriol ]
• FROM kidney activity upon calcidiol(s)
• Plasma half-life 15 hours
• => 3% at 3 days