Vitamin A - Advanced Nutrition II

  • Created by: Ann
  • Created on: 19-01-17 20:37

Define Vitamin A

Vitamin A = all compounds that can form retinoids or retinol that can be a precursor to vitamin A; a generic term that possess the biological activity of all-trans retinol

  • Vitamin A is also an all encompassing term for retinoids and carotenoids but are not all interchangable
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Forms of Vitamin A

  • B-carotene is a carotenoid with the most of the vitamin A activity. 
  • retinol can convert irreversibly to retinoic acid or retinyl ester
  • Retinol can also reversibly convert to retinal. (main form of biological activity)
  • You may consume retinyl ester, retinoic acid, and beta-carotene.
  • Beta-carotene is a carotenoid with the most vitamin A activity.
  • All-trans retinol is a retinol with a alcohol group. All bonds are in transformation and it contains an alcohol group. Can find in animal origin.

               β – carotene


retinal retinol retinoic acid


                   retinyl ester

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Structure of Vitamin A

  • All-trans retinol is a retinol with a alcohol group. All bonds are in transformation and it contains an alcohol group. Can find in animal origin.
  • Retinal - has an aldehyde group
  • Retinal and all-trans retinol can convert back and forth reversibly.
  • Retinoic acid - with an acid group
  • All-trans retinol can form retinoic acid in growth
  • Retinyl esters - with a lipid group

Retinol (alcohol group) - animal origin Retinylpalmitate – reduced form Retinal (aldehyde group) – retinol reversibly converts to retinal (oxidized form) Retinoic Acid (carboxylic acid) – retinol irreversibly converts to retinoic acid – ex: in growth Retinyl Esters (ester group)

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Structure of carotenoids

  • Beta carotene is the one carotenoid with vitamin A activity. 
  • Beta-carotene can be split to form two molecules of retinal
  • mg= microgram
  • 12 mg of carotene need to = 1 mg retinol
  • Ex. 36 mg of carotene = 3 mg retinol
  • preformed vitamin A has more activity beta carotene takes more of it to convert.
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Dietary Sources of Vitamin A

  • preformed vitamin A is more toxic and only found in animal sources
  • Preformed -retinyl ester (retinyl palmitate ex. of retinyl ester)
  • Liver
  • Egg Yolk
  • Dairy (fortified in United States)
  • Margarines (fortified)
  • Tuna, Sardines, Heerring
  • Cod Liver Oil
  • Beta-Carotene = exclusively in fruits and vegetables
  • Orange color - Carrots, squash, papayas, grapefruit, pumpkin
  • Green color - chlorofil over shadows the orange color - spinach, peas, broccoli, watermelon
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Digestion and Absorption of vitamin A

  • animal sources and vegetable sources of (protein bound carotenoids and retinyl esters)
  • pepsin cleaves proteins (amino acids) 
  • carotenoid and retinyl esters hydrolases, esterases and lipases
  • cleaves fatty acids
  • free caroteinoids and free reitnol
  • fatty acids combine with phospholipids, monoacylglycerol, and cholesterol and bile (makes it water soluble
  • incorporates into micelle
  • true of all fats and fat soluble vitamins
  • goes across intestinal cell 
  • can either have beta carotene, retinol, or carotenoids
  • most are incorporated into chylomicrons that go into lymphatic system
  • beta carotene - can form two retinals (retinoic acid goes through the blood to the liver) or the retinal gets incorporated into chylomicrons combining with other 
  • Vitamin A is primarily stored in the liver so it will be released into the blood stream as its needed.
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Functions of Vitamin A; What other functions are i

helps your vision = how you respond to bright light when its dark out

your eyes adjustment to light and dark

It's a specific form of Vitamin A : 11-cis retinal. This is a type of double bond cis is a c form. 

its on the 11th carbon there is a double bond in the cis form. whereas for all the other biological function it is all trans double bonds or it is a 9-cis retinal. 

Other functions that are important:

cellular differenciation, growth and reproduction, bone health (osteoblasts and osteoclasts) and immune system

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Describe T-cell antibody response in regards to Vi

Immune system - vitamin A regulates T-cell antibody response - body is exposed to an antigen (adaptive immune response)

You have a vaccination that includes an antigen and your body mounts an antibody response so that the response is already formed. 

Vitamin A regulates t-cell antibody response.

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Describe what night blindness is, and how your eye

Deficency causes Night Blindness - failure to resynthesize cis retinal. 

Light hits the corea and retina in the back of your eye. 

Within the retina there are cone cells and rode cells. 

Within rod cells are made up of rhodopsin.

Rhodopsin is made up of opsin protein and 11-cis retinal. 

When light hits 11-cis retinal it causes 11-cis retinal and opsin to cleave (disband). 

11-cis retinal is then converted to trans-retinol (this allows brain to adjust for low light)

The impulse to your brain happens and your eyes adjust to low light. (dark to light)

When trans-retinol converts to cis-retinol this adjust for high light. (light to dark)

This process is recycled some vitamin A is lost but your not going to lose all of you vitamin A. 

You will need a constant supply of Vitamin A to make this process happen. When there is a deficency it causes night blindness. 

When there is a deficency the eyes will not adjust back to low light after a bright high light situation such as driving at night.

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Gene Expression

  • All trans retinoic acid or 9-cis retinoic acid (two forms of vitamin A)
  • binding  either to one or two types of intracellular lipid binding proteins
  • which then being to either an rar or a rxr receptor this forms either a homodimer or a heterodimer
  • If they were both rar or both rxr they would be homodimer. 
  • they are bound through vitamin A and also bound through a retinoic acid response element (RARE) on the DNA. 
  • It changes the mRNA transcriptor so it increases or decreases the mRNA from that gene. 
  • In the case of Vitamin A, if Vitamin A is not bound to the receptor you may have inhibited the trascription. If it is bound it may release the inhibition and allow the growth factors to have normal transcription and the proteins to develop as normal. Vitamin A promotes transcription of specific genes and growth.
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Describe cellular differentiation in regards to ep

Cellular Differentiation - when one cell becomes a specific type of cell

epithelial cells which protect all of your membranes (including your skin) and retinoic acid can 

direct the differentitation of immature skin cells into mature epidermal cells

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What is one of the main symptoms of a more severe

when you have a vitamin A deficency (more severe than night blindness) you may develop keratin producing cells that replace the mucus secreting cells in (eyes, skin, gi tract, trachea)

instead of secreting mucus you have a hardened keratin substance (causes blindness) with severe vitamin A deficency and the cornea forms a tough exterior and people will go blind

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What is involved with presenting the antigen to pr

In immune cells, you have cells in the bone marrow that are progenitor cells that can become dendritic cells that are a type of cell in the immune response. Those are involved with presenting the antigen that show disease to cells to produce antibodies.

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what do retinoids due in regards to cells (cancer

Cancer response retinoids may inhibit the cell cycle in cancer. Cancer is metastasizing cells cells that are out of control and retinoids may stop the differenciation apoptosis and cause cell death. 

Impaired growth witihin cells can happen but, the mechanisms are unclear. It may however, be linked to gene expression.

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Carotenoids. describe.

Carotenoids are not necessary the same as vitamin A. Beta-carotene can form Vitamin A but, Beta-caratene can stay Beta-carotene in the body. 

Carotenoids function as antioxidants in your cell membrane and in lipoproteins 

Lipoproteins like LDL for example oxidized LDL may cause heart attack or stroke. 

Lipid peroxidation occurs in membranes caused by unpaired electrons destroying membrane integrity and allows the cellular contents to leak out. 

The damaged oxidative stress may react with protein, lipids, or DNA. 

Beta carotene and lycopene (another carotenoid) will quench the free radicals and stop the reaction. 

Donate an electron and stop the propogation of the reaction, terminate it, and inhibit furhter oxidative stress.

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How does vitamin A protect against macular degener

there are three cartoenoids: Beta carotene, lutein, and zeaxanthin that inbit oxidation of cellular membranes inthe macula. (the center of the retina in the eye)

By inhibiting the oxidation of the celullar membranes that inhibits oxidative stress

Vitamin A may also play a role in reducing cateracts. Oxidative stress may play a role in cateracts.

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Why should people consume antioxidants?

High antioxidants reduce the risk of heart disease and cancer (in food)

Supplements do not reduce the risk of mortality.

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Specifics of retinol; and what are the symptoms of

You need:

Unit:retinol activity equivalents (RAE)

1 mg of retinol = 12 mg beta-carotene RDA

-900 mg RAE for men

-700 mg RAE for women

DV = 5000 IU = 1500 ug

1 IU = 3.33 ug

Upper limit 3000 ug RAE or 10,000 IU 

Toxcity causes Hypervitaminosis A

-Acute - nausea, vomitting, blurred vision, headache, dizziness

-Chronic - dry itchy skin, alopecia (lose your hair), ataxia (problems walking), bone/muscle pain, liver damage

-Teratogenic - birth defects

Carotenoids - non-toxic, yellow discoloration of skin

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