Vitamin D

Question 1

What is vitamin D?

Answer 1

Classified as a fat soluble vitamin, however can be synthesised by the skin from cholesterol in the presence of UVB

Question 2

Explain the mechanism and pathway involved in synthesising vitamin D

Answer 2

In the presence of UVB, 7-dehydrocholesterol is converted to cholecalciferol (D3)

This then travels to the liver where it is converted to 25-hydroxyvitamin D3 (also known as 25(OH)D3) by the 25-hydroxylase enzyme

25(OH)D3 then travels to the kidneys where it is further hydroxylated to 1,25(OH)D3 by the 1alpha-hydroxylase enzyme. This is now calcitriol

Question 3

What is calcitriol?

Answer 3

The active form of vitamin D which has a hormone like activity

Question 4

What enzyme is responsible for the first hydroxylation reaction of cholecalciferol?

Answer 4

25-hydroxylase

Question 5

What does 1-alpha-hydroxylase catalyse?

Answer 5

The conversion of 25(OH)D3 to 1,25(OH)D3 in the kidneys

Question 6

How is vitamin D transported through the body?

Answer 6

From the liver it is bound to lipoproteins or vitamin D binding protein (DBP)

Question 7

What are the functions of vitamin D?

Answer 7

• Calcium and phosphorus homeostasis
• Aids in the absorption of calcium from the GIT: calcitriol acts on gene transcriptors to upregulate calcium transporters which allow entry into the enterocyte
• regulation of renin, insulin and parathyroid hormone
• regulation of immune function

Question 8

Explain the mechanism in which calcitriol assists in regulating blood calcium levels

Answer 8

Low blood calcium levels are recognised by the parathyroid gland

This stimulates the release of PTH and 1,25(OH)D3

Together these mobilise calcium from bone

PTH also reduces calcium excretion from the kidneys and stimulates more 1,25(OH)D3 to be synthesised

1,25(OH)D3 also increases the absorption of calcium from the GIT

Question 9

What are vitamin D equivalents?

Answer 9

1μg cholecalciferol = 0.2μg 25(OH)D3

1μg = 40IU

1 IU = 0.025μg cholecalciferol

1 IU = 0.005μg 25(OH)D3

Question 10

What is caused by a vitamin D deficiency?

Answer 10

In children: rickets
- bowed legs and short stature

In adults: osteomalacia or earl stages of osteoprosis

Question 11

What is caused by a vitamin D toxicity?

Answer 11

Does not occur through sunlight alone, only supps

Creates hypercalcaemia

Question 12

What are dietary sources of vitamin D?

Answer 12

• oily fish
• milk
• eggs