Vitamin D Flashcards
How are fat-soluble vitamins (such as vitamin D, A & K) are absorbed?
How does this differ from the absorption of water-soluble vitamins?
Fat-soluble vitamins are incorporated into mixed micelles with other lipids and bile salts in the lumen of the small intestine
the presence of bile salts helps to emulsify and make things more soluble
They enter the enterocyte largely by diffusion.
Water-soluble vitamins are absorbed through the portal vein but requires a transporter to do so.
Fat-soluble vitamins are absorbed into the lymphatic system first. (requires protein carriers to travel into the blood)
- enters the lymph
- enter systemic circulation
- taken up by the liver
What is vitamin D structurally similar to?
Cholesterol (derivative)
What is the RDA for Vitamin D?
RDA Vit D = 600 IU/day
What are the steps of the absorption of Vitamin D from the sun?
Pre-vitamin D3 is synthesized in the upper layers of the skin from 7-dehydrocholesterol (by UVB)
A nonenzymatic conversion of pre-vitamin D3 into vitamin D3 (cholecalciferol) (in the skin)
Vitamin D3 is quickly transported to adipose tissue for storage or liver for activation
In liver cells, several cytochrome P450 enzymes can catalyze the 25-hydroxylation of vitamin D3 (plant-based form would be vitamin D2 or ergocalciferol)
25-hydroxyvitamin D3 is converted to the active form of vitamin D3 (1,25-dihydroxyvitamin D) in a reaction catalyzed by CYP27B1 (primarily in the kidney)
If you wanted to get your vitamin D status assessed, what would you get measured?
Serum 25-hydroxyvitamin D
(25(OH)D)
this is the main indicator of vitamin D status
What is the recommended Vitamin D intake for adults?
Deficiency in adults is defined as <20 ng/mL of 25(OH)D in the blood
Other measurements define vitamin D sufficiency as the blood level of 25(OH)D of at least 30 ng/mL
The principal action of vitamin D is the maintenance of calcium homeostasis, mainly by regulating intestinal calcium absorption.
How does vitamin D increase calcium absorption?
Active transport of calcium through the cells is dependent on vitamin D
What are the two main mechanisms of functioning for vitamin D?
Functions like a hormone
-binds to membrane receptor on target tissues and activates a signal transduction pathway (binding outside/inside the cell that invokes a response of the cell)
Affects gene expression
-binds to the nuclear receptor and affects transcription
What are the two sources of vitamin D?
sunlight (UVB)
food sources
Name some of the food sources of vitamin D
Cholecalciferol (D3)
- only found in animals
- fish liver and fish oils
Ergocalciferol (D2)
- plant form
- present in only small amounts
