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Sep 24
Vitamin A metabolism involves several steps in the body.
1. Absorption: Vitamin A is absorbed in the small intestine as retinyl esters or beta-carotene. Retinyl esters are broken down into retinol, which is the active form of vitamin A.
2. Transport: Retinol is then transported to the liver where it is converted into retinyl esters and stored in the liver cells.
3. Storage: The liver stores vitamin A in the form of retinyl esters until it is needed by the body.
4. Release: When the body requires vitamin A, the stored retinyl esters are released into the bloodstream and transported to target tissues.
5. Activation: In the target tissues, retinyl esters are converted back into retinol, which can then be further metabolized into its active forms, such as retinoic acid.
6. Function: Retinoic acid plays a crucial role in various physiological processes, including vision, immune function, cell growth, and differentiation.
7. Excretion: Excess vitamin A is excreted from the body through urine.
1. Absorption: Vitamin A is absorbed in the small intestine as retinyl esters or beta-carotene. Retinyl esters are broken down into retinol, which is the active form of vitamin A.
2. Transport: Retinol is then transported to the liver where it is converted into retinyl esters and stored in the liver cells.
3. Storage: The liver stores vitamin A in the form of retinyl esters until it is needed by the body.
4. Release: When the body requires vitamin A, the stored retinyl esters are released into the bloodstream and transported to target tissues.
5. Activation: In the target tissues, retinyl esters are converted back into retinol, which can then be further metabolized into its active forms, such as retinoic acid.
6. Function: Retinoic acid plays a crucial role in various physiological processes, including vision, immune function, cell growth, and differentiation.
7. Excretion: Excess vitamin A is excreted from the body through urine.
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Google Gemini AI
Sep 24
Metabolism of Vitamin A
Absorption:
- Fat-soluble vitamin, absorbed in the small intestine with dietary fat.
- Converted to retinol in the liver (carotenoids from plants are converted to retinol in the small intestine).
Transport:
- Retinol is bound to retinol-binding protein (RBP) and transported in the blood.
- Transthyretin (TTR) carries a complex of retinol, RBP, and prealbumin.
Storage:
- Stored in the liver as retinyl esters in hepatic stellate cells.
Mobilization:
- Retinyl esters hydrolyzed to retinol when needed.
- Retinol bound to RBP for transport to tissues.
Activation:
- Retinol converted to retinoic acid (RA) in target tissues by retinol dehydrogenases (RDHs).
- RA is the active form of vitamin A.
Mechanism of Action:
- RA binds to retinoic acid receptors (RARs) and retinoid X receptors (RXRs) in the nucleus.
- RARs and RXRs regulate gene expression involved in cell growth, differentiation, and metabolism.
Metabolism and Excretion:
- RA is metabolized to polar metabolites, such as 4-hydroxyretinoic acid (4-OHRA) and 4-ketoretinoic acid (4-KRA).
- Metabolites excreted in urine and bile.
Regulation:
- Vitamin A metabolism regulated by various factors, including:
- Dietary intake
- Hepatic storage capacity
- Activity of enzymes involved in metabolism (e.g., RDHs)
Absorption:
- Fat-soluble vitamin, absorbed in the small intestine with dietary fat.
- Converted to retinol in the liver (carotenoids from plants are converted to retinol in the small intestine).
Transport:
- Retinol is bound to retinol-binding protein (RBP) and transported in the blood.
- Transthyretin (TTR) carries a complex of retinol, RBP, and prealbumin.
Storage:
- Stored in the liver as retinyl esters in hepatic stellate cells.
Mobilization:
- Retinyl esters hydrolyzed to retinol when needed.
- Retinol bound to RBP for transport to tissues.
Activation:
- Retinol converted to retinoic acid (RA) in target tissues by retinol dehydrogenases (RDHs).
- RA is the active form of vitamin A.
Mechanism of Action:
- RA binds to retinoic acid receptors (RARs) and retinoid X receptors (RXRs) in the nucleus.
- RARs and RXRs regulate gene expression involved in cell growth, differentiation, and metabolism.
Metabolism and Excretion:
- RA is metabolized to polar metabolites, such as 4-hydroxyretinoic acid (4-OHRA) and 4-ketoretinoic acid (4-KRA).
- Metabolites excreted in urine and bile.
Regulation:
- Vitamin A metabolism regulated by various factors, including:
- Dietary intake
- Hepatic storage capacity
- Activity of enzymes involved in metabolism (e.g., RDHs)
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