Light,

UVB, heat

Cholecalciferol vitamin D3

DBP

DBP

Vitamin D

Pool

Liver

7-Dehydro-cholesterol

DBP

→ 25-(OH)D

1 Hydroxyl-apatite

Bone

Fat, bile acids

Chylomicrons

Vitamin D from food

Skin

#Parathyroid

#PTH

secretion

Circulating

25-(OH)D

DBP

Kidney

• 25-(OH)D

1,25-(OH,) D

1,25-(OH)w D

† Ca?+-retention

Metabolite

#Bile excretion

DBP

Low

Plasma

Ca?+

concentration

absorbtion

Mucosa

Intestine