Vit A Flashcards
What is vitamin A?
A group of structurally related compounds classified as retinoids
-naturally occuring or synthetic compound that is structurally similar to all-trans retinol (with or without vit A activity)
-including retinol, retinal, retinoic acid, and retinyl esters
Includes provitamin A carotenoids like β-carotene found in plant foods.
What is the main form of Vit A?
All trans retinol
What are dietary forms of preformed vit A? what does this mean?
Retinol (retinyl palmitate and retinyl acetate), retinyl ester,retinal, and retinoic acid
these are biologically active forms of vit A that are found in animal derived foods
What are the characteristics of fat-soluble vitamins?
Stored in liver and adipose tissue, require lipids for absorption, higher risk for toxicity due to accumulation
Includes vitamins A, D, E, K.
What are the types of retinoids? how do they differ?
Preformed vitamin A and provitamin A carotenoids
Preformed sources include animals; provitamin sources include plants.
How is retinol transported in the circulation during fasted vs postprandial states?
Bound to retinol-binding protein (RBP) in fasting state/non-fasting state and packaged as retinyl esters in chylomicrons postprandially
RBP levels are often used as a proxy for retinol levels.
Where are retinoids stored in the body? how are they stored?
Stored as retinyl esters (retinyl palmitate) in hepatic stellate cells (HSCs) in the liver
HSCs store over 90% of total hepatic vitamin A, while only representing ~5% of cells in the liver
What are common ester forms of stored retinoids?
Retinyl palmitate (major), retinyl linoleate, retinyl oleate, retinyl stearate
These serve for long-term storage of vitamin A.
What is the role of 11-cis-retinal?
It is an active metabolite that is crucial for vision, acts as a chromophore in rhodopsin
Converts to all-trans-retinal upon light exposure.
What is all-trans-retinoic acid’s function?
Regulates gene expression by binding to nuclear receptors
It is not involved in vision and its synthesis is tightly regulated.
What is the importance of various metabolic forms of vit A? explain the metabolic pathway for excretion
Metabolic intermediated or breakdown products are essential to regulate excretion
Retinol → Retinal → Retinoic acid → hydroxylation → glucuronidation
Non-active metabolites serve no functional role but are crucial for homeostasis.
What is a Retinol Activity Equivalent (RAE)? what is the the conversion for b-carotene and other provitamin carotenoids?
A standardization of vitamin A activity from multiple sources
1:12 ug for b-carotene
1:24 ug for other carotenoids
1 µg retinol = 1 RAE, 12 µg β-carotene = 1 RAE, 24 µg other carotenoids = 1 RAE.
What are common trends for Vit A in terms of RDA? what is the RDA for men vs women >19yr old?
1) men have»_space; RDA than women
2) increased requirements during pregnancy and lactation to account for fetal growth and breast milk production
900 ug for men and 700 ug for women
What are the dietary sources of vitamin A? what foods should be eaten to meet vit A requirements?
Animal sources provide preformed vitamin A; plant sources provide provitamin A carotenoids
- a balanced diet should provide ample amounts of Vit A
Examples include liver, dairy, carrots, and spinach.
Fill in the blank: Approximately ____ of vitamin A intake comes from preformed sources and ____ from provitamin sources
2/3 and 1/3
what ways can Vit A status be assessed?
symptoms, circulating retinol, and tracer studies
Are circulating levels of retinol an accurate assessment of Vit A status?
Circulating levels do not always correlate with hepatic Vit A levels because the liver secretes retinol at a steady state
-circulating levels are able to be maintained until the liver is almost completely empty of its Vit A stores
Why aren’t liver biopsys done to assess vit A status? what can be used instead?
They are unethical and impractical
A multi-compartment model is used to reflect stores in the liver
-uses a tracer (labelled retinoid) and analyzes isotope dilution
What are the two major forms of dietary Vitamin A? where are they found?
Preformed Vitamin A
-Retinol, retinal, retinoic acid
Provitamin A
-Beta-carotene
Retinol is found in animal sources, while beta-carotene is found in plant sources.
What are the key processes involved in Preformed Vitamin A absorption?
1) hydrolysis
2) uptake into enterocytes
3) esterification and chylomicron packaging
4) secretion of chylomicrons
comparable process to other lipid absorption
What enzymes are involved in the absorption of retinyl esters? how do they work? what type of Vit A is this? how does this differ from provitamin A absoprtion?
1) REH (Retinyl Ester Hydrolase; hydrolyzes retinyl esters into free retinol)
2) LRAT (Lecithin:retinol acyltransferase which transfers an acyl group to retinol to aid in packaging it into chylomicrons)
Retinyl esters are preformed Vit A sources, this differs from provitamin A absorption due to the need to be hydrolysed prior to entorocyte absorption
How is provitamin A absorbed? what enzymes are involved? what is their function?
Uptake into enterocytes, esterification and chylomicron packaging + secretion
1) CMO (carotenoid cleavage enzyme)
2) LRAT (lecithin:retinol acyltransferase; reduces retinol to retinol ester to be packaged into chylomicrons)
What is the role of the liver in Vitamin A metabolism? what percentage of retinyl esters are taken up by the liver?
The liver stores Vitamin A and regulates its supply to the body
About 75% of retinyl esters from chylomicrons are taken up by the liver.
what is the role of hepatocytes and stellate cells? where are they loctaed?
hepatocytes:
1) upatke of chylomicron retinyl ester
2) secretion of retinol into circulation
stellate cells: store Vit A in the liver as retinyl ester in lipid droplets
Both cells are found in the liver
