Metabolic bone disease - TABLE

Question 1

Metabolism of vitamin D

Answer 1

Vitamin D metabolism is dependent on sunlight and enzymatic conversion in the liver and kidneys. Ultraviolet light converts 7-dehydrocholesterol to colecalciferol (vitamin D3). Colecalciferol may also be obtained from the diet.
Colecalciferol is converted to calcidiol (25-hydroxyvitamin D) by vitamin D 25-hydroxylase in the LIVER.
Calcidiol is converted to calcitriol (1,25-dihydroxyvitamin D) by 1-alpha-hydroxylase in the kidney.

Ergocalciferol (vitamin D2) is obtained from the diet and is metabolised in a similar manner as colecalciferol.

Vitamin D maintains calcium and phosphate homeostasis through its actions on the GI tract, the kidneys, bone, and the parathyroid glands. Vitamin D is obtained from the diet, or it can be produced from a sterol precursor (7-dehydrocholesterol) in the skin following exposure to UV-B light.
Sequential hydroxylation of vitamin D is required to produce the metabolically active form of vitamin D. Hydroxylation occurs first in the liver and then in the kidneys and produces 1,25-dihydroxyvitamin D. Dysfunction in any of these metabolic steps results in osteomalacia and secondary hyperparathyroidism in adults