02 Vitamin A Flashcards
What is vitamin A?
Vitamin A is group of fat-soluble vitamins AKA retinoids, that are obtained from two resources:
(1) pre-formed retinoids.
(2) pro-vitamin carotenoids.
Where are pre formed retinoids found in our food ?
Retinoids are found in animal sources such as liver, kidney, eggs, and dairy products
Pre-formed retinoids include?
(a) retinol and its esterified form, retinyl ester (retinyl-palmitate) that can be further converted into (b) retinal and (c) retinoic acid.
Retinyl ester is an esterified form of………….. .
retinyl ester (retinyl-palmitate) that can be further converted into
Retinol
retinal and retinoic acid.
The most important pro-vitamin carotenoid is ……………
beta- carotene which has the highest vitamin A activity.
beta- carotene is found in ……….
plants such as dark or yellow vegetables and carrots.
Beta-carotene is a provitamin and it is an ………………….. . It is converted into vitamin A as needed by the body, so there is no risk of overdose or toxicity.
Antioxidant
The active forms of vitamin A are ………………
retinal and retinoic acid.
Both pro-vitamin A and preformed vitamin A must be metabolized…………………. to the active forms of vitamin A.
intracellularly
……………. is hydrolyzed in the intestine to yield two retinal molecules
Beta-carotene
Beta-carotene is hydrolyzed in the intestine to yield ………….. molecules by the enzyme………….
two retinal
oxygenase
What is the storage form of vitamin A in the liver?
Retinol ester
Retinol ester is the storage form of vitamin A in the liver. When appropriate, it is hydrolyzed to generate ………………. , which binds to………………….. before being released in the bloodstream.
all- trans-retinol
retinol binding protein (RBP)
We get Beta-carotene from …………….
It is hydrolised in …………… by rhe enzyme ………….. to form ……….
Plant foods
Intastine
Oxygenase
2 retinal molecules
Retinol can be converted into …………, or stored in the liver as …………
Retinal Retinyl palmitate ( retinyl ester )
We can get retinyl esters in our food from ……..
Animal products
After hydrolization of retinol ester, we get all-trans-retinol which binds to RBP( retinol binding protien). This bined complex is released in ……………… then circulates and binds to a protien called …………… which delivers all-trans- retinol to the peripheral tissue.
Blood stream
Transthyretin
Inside cells, all-trans-retinol can be either …………….. or ……………
re- esterified and stored as retinyl esters
or oxidized to all- trans-retinal then to retinoic acid (RA).
……….. acts as a hormone to affect gene expression and thereby influence numerous physiological processes
Retinoic Acid
Within the nucleus, …………. binds to specific nuclear receptor known as ……………. which are transcription factors.
Retinoic acid (RA)
retinoic acid receptors (RAR)
retinoic acid receptors (RAR) are…………. factors in the nuclues.
transcription
By regulating the expression of over 500 retinoid- responsive genes (including several genes involved in vitamin A metabolism itself), …………. plays major roles in cellular proliferation and differentiation.
retinoic acid
The retina is located at the back of the eye and it contains two types of light-sensitive photoreceptor cells − known as ………………
rods and cones.
Rods are extremely sensitive to…………….. . However, they are unable to distinguish color, and the images they relay are not well defined.
motion, shape and dim light
cones perform in ………….. giving detailed colored views, but fairly insensitive at…………. .
bright light
low light levels
light strikes the ………….. which is located at the back of the eye and transfers the information to the nearby rods and cones. From there, signals pass through ……………………. on their way to the retinal highway leading to the ………. nerve.
retinal pigment epithelium (RPE)
bipolar cells and ganglion cells
Optic
Just at the center of the retina lies a small depression called the ……….. (yellow spot), at its center lies the ………… which contains exclusively/only …………. .
Macula
Fovea
Cones
To form “high resolution images”, the light must fall on the……….. , and that limits the acute vision angle to about ….. degrees. Since the fovea provides the sharpest and most detailed information, the eyeball is continuously moving when seeing objects, so that light from the object of primary interest falls on this region.
fovea
15
In ……. light, fovea constitutes a second blind spot since it is exclusively composed of cones which have …… light sensitivity
Low
Low
All-trans-retinol is transported to the retina via the…………. and accumulates in ………… .
Then, these all-trans-retinol are ……………. To form …………….. which can be oxidized to form ………….. .
Then this ……………. is shuttled to the rods where it binds to a protein called……… to form the visual pigment rhodopsin (AKA visual purple).
circulation RPE retinol pigment epithelium Isomerised 11-cis-retinol 11- cis-retinal 11-cis-retinal opsin
What catalyzes back the isomerization of 11-cis-retinal to all-trans- retinal?
Absorption of a photon of light catalyzes the isomerization of 11-cis-retinal back to all-trans- retinal that is released from the opsin molecule.
So 11-cis-retinal goes back to all-trans- retinal by putting visible light on it
The photoisomerization of 11-cis-retinal back to all-trans- retinal triggers a cascade of events, leading to the generation of nerve impulses conveyed/carried by the………….. to the brain’s visual cortex for interpretation.
optic nerve
All-trans-retinal which came out of the photoisomerization is converted to ……………….., then transported to the ……….. cells thereby completing the visual cycle.
all-trans-retinol
RPE
the visual cycle that apears in fovea is similar for cones that contain ………………………. Opsin protiens.
red, green, or blue
Vitamin A is also essential for mammalian eye development. Thus, vitamin A is required for the normal functioning of the retina, dim-light vision, motion and color vision. True or false?
True
Vitamin A is important in Resistance to infections specially…………… . This “anti-infective” effect, in part, is due to the necessity for vitamin A in normal immune responses.
measles
Epithelial cell integrity: Many epithelial cells (such as……………..) appear to require vitamin A for proper differentiation and maintenance
Goblet cells
Lack of vitamin A leads to dysfunction of many epithelia - the skin becomes ………………. , and mucus secretion is ……………. .
keratinized and scaly
suppressed/stopped
Other functions of vitamin A?
Resistance to infections
Epithelial cell integrity
Bone remodeling
Reproduction & growth
Bone remodeling: Normal functioning of ……………… is dependent upon vitamin A.
osteoblasts and osteoclasts
Reproduction & growth: Normal levels of vitamin A are required for ……………….. . Similarly, normal …………………. in females require adequate availability of vitamin A.
sperm production
menstrual cycles
Vitamin A and most retinoids are highly toxic when taken in large amounts, and the most common cause of this disorder is …………… .
excessive supplementation
Vitamin A and most retinoids are highly toxic when taken in large amounts. In contrast, excessive intake of carotenoids are not reported to cause disease. True or false ?
True
Toxicity occurs when …………… are saturated with the vitamin, free unbound vitamin A begins to destroy cells. In contrast, excessive intake of carotenoids are not reported to cause disease.
retinol binding protein (RBP)
What is RDA ?
Recommended Dietary Allowance (RDA): Average daily level of intake sufficient to meet the nutrient requirements of nearly all healthy individuals; often used to plan nutritionally adequate diets for individuals.
Recommended Dietary Allowance (RDA): Average …….. level of intake sufficient to meet the nutrient requirements of nearly all healthy individuals; often used to plan nutritionally adequate diets for individuals.
daily
Recommended Dietary Allowance (RDA) Varies according to …… and …….. and ……. .
Age
Sex
Health condition
The content of vitamin A in foods is often measured as ………………….. .
retinol activity equivalents (RAEs)
1 RAE = 1 microgram (mcg) of retinol, 12 mcg of beta- carotene, or 3.33 International Unit (IU) of vitamin A.
1 IU = 0.3 mg of retinol.
1 RAE = ………………
1 RAE = 1 microgram (mcg) of retinol, 12 mcg of beta- carotene, or 3.33 International Unit (IU) of vitamin A.
1 IU = 0.3 mg of retinol.
Vitamin A supplements: Vitamin A is available in multivitamins and as a stand-alone supplement, often in the form of ……………. Or …………… .
retinyl acetate or retinyl palmitate
Multivitamin supplements typically contain ………….. of vitamin A.
750 - 3,000 mcg(microgram) RAE
which = 2,500 –10,000 IU
The weight-loss drug …………. reduces the body’s ability to absorb fat-soluble vitamins such as vitamin A, increasing the risk of deficiency.
Orlistat (Xenical)
Because the liver stores rather large amounts of retinol, deficiency states typically take several months to develop. True or false?
True
Vitamin A deficiency causes Ocular manifestations like?
- XN= night blindness
- XIA= Conjunctival xerosis
- XIB= bitot spots
- X2= Corneal Xerosis
- X3A = Corneal Ulceration Less than 1/3 of corneal Surface
X3B = Corneal Ulceration More than 1/3 of Corneal Surface
• XS= corneal scar
• XF= xeropthalmic fundus
How is Ocular manifestations treated?
Early stages of xeropthalmia can be reversed by administration of massive doses of vitamin A (200,000 IU) orally on 2 successive days.
- Short-term Prophylaxis: mega doses on regular basis.
- Long-term prophylaxis: dietary intervention.
All children with corneal ulcers are given vitamin A whether a deficiency is suspected or not. True or false?
True
Vitamin A supplements should be given to all infants and young children (0–59 months), pregnant women and postpartum women within 6 weeks after delivery. True or false?
True
