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Vitamins n Vitamins are organic compounds required by the body in trace amounts to perform specific functions.Functions of vitamin A a. Visual cycle (retinal) O Vitamin A is a component of the visual pigments (rhodopsin) of rod in the retina O Rod cells are responsible for vision in dim light (night vision) O Rhodopsin consists of 11-cis retinal bound to the protein opsin.ProvitaminA(?-carotene) n It is the plant precursor of vitamin A n It consists of two molecules of retinal linked at their aldehydes n In humans ?-carotene has only about one sixth the activity of retinol.Sources of vitamin A n VitaminA(retinal,retinol) n Are present only in animal tissue e.g. Liver, milk fat and egg n ?-carotene(precursorofvit.A) n Present in dark green vegetables and orange vegetables & fruits.n Whenneeded,itisreleasedandtransportedto extra-hepatic tissues by the plasma retinol-binding protein (RBP).n They regulate synthesis of many body compounds (bone, skin, nerves, brain, blood, etc..) n They prevent nutritional deficiency diseases and allow optimal health at all ages.n Storage: adipose tissues n Overdosage symptoms: yellow coloration of skin

Absorption & storage n VitaminAisabsorbedwithfatintheintestineand secreted into chylomicrons which reach blood.Vitamin A function n Important functions: n Maintenance of healthy epithelium n Retinol and retinoic acid are required for the growth, differentiation, and maintenance of epithelial cells.Recommended dietary allowance (RDA) n 1ugofretinol=6ugof?-carotene=5IU n Thedailyrequirementisabout5000IUofretinol.Retinol<-----> retinal 2.n TheyincludevitamincandvitaminsB.3.4.

Vitamins
n Vitamins are organic compounds required by the body in trace amounts to perform specific functions.
n Most vitamins cannot be synthesized in human body and must be supplied in diet.

What is the role of vitamins in nutrition?
n Vitamins don’t provide energy, but they help in metabolism of macronutrients.
n They regulate synthesis of many body compounds (bone, skin, nerves, brain, blood, etc..)
n They prevent nutritional deficiency diseases and allow optimal health at all ages.

Classification of vitamins
The vitamins are classified into two groups:
n Water soluble vitamins: vit. B and vit. C
n Lipid soluble vitamins:
vitamin A, vitamin D, vitamin E and vitamin K.

Water Soluble Vitamins General Characters:
n Theyarewatersoluble.
n TheyincludevitamincandvitaminsB.
n Formcoenzymesfortheenzymesofmetabolism
n Wheningestedinexcesstheyarereadilyexcretedin urine, so:
n theymustbecontinuouslysuppliedindiet. n Difficulttoinduceatoxicity.
n Verylittlestorage(exceptforB12)

Fat Soluble Vitamins General Characters:
n These are hydrophobic molecules.
n They are absorbed and transported with fat
n Deficiency occurs in case of fat malabsorption n Only vitamin K has coenzyme function.
n They are not readily excreted in urine.
n Excess amounts are stored in liver and can cause toxicity symptoms

Forms of vitamin A

1. BiologicallyactiveformsofvitaminA:
   (Retinol, Retinal and Retinoic acid) 1. Retinol:
   • An alcohol found in animal tissues as retinyl ester with long-chain fatty acids


2. Retinal:
   • An aldehyde derived from oxidation of retinol.
   • Retinal and retinol can be interconverted. Retinol retinal


3. Retinoic acid:
   • An acid derived from oxidation of retinal. It cannot be reduced again. Retinal----------> retinoic acid

Forms of vitamin A
2. ProvitaminA(β-carotene)
n It is the plant precursor of vitamin A
n It consists of two molecules of retinal linked at
their aldehydes
n In humans β-carotene has only about one sixth the activity of retinol.

Sources of vitamin A
n VitaminA(retinal,retinol)
n Are present only in animal tissue e.g. Liver, milk
fat and egg
n β-carotene(precursorofvit.A)
n Present in dark green vegetables and orange
vegetables & fruits.
n Storage: adipose tissues
n Overdosage symptoms: yellow coloration of skin

Absorption & storage
n VitaminAisabsorbedwithfatintheintestineand secreted into chylomicrons which reach blood.
n Then,VitaminAistakenbytheliverwhereitis stored.
n Whenneeded,itisreleasedandtransportedto extra-hepatic tissues by the plasma retinol-binding protein (RBP).

Vitamin A function
n Important functions:
n Maintenance of healthy epithelium
n Retinol and retinoic acid are required for the growth, differentiation, and maintenance of epithelial cells.
n They bind intracellular receptors, and they regulate transcription through specific response elements.
n Itisalsoinvolvedinreproduction&growth n Vision:
n Retinal

Mechanism of action of Vitamin A
n Insidethecell,Retinolisoxidizedtoretinoicacid.
n Retinoicacidbindstointra-nuclearreceptor forming complex which bind to DNA activating the transcription of specific genes e.g. keratin gene.
n Therefore,retinoicacidproducesitseffectlike hormones by affecting gene transcription.

Functions of vitamin A
a. Visual cycle (retinal)
Ø Vitamin A is a component of the visual pigments (rhodopsin) of rod in
the retina
Ø Rod cells are responsible for vision in dim light (night vision)
Ø Rhodopsin consists of 11-cis retinal bound to the protein opsin.
trigger nerve impulse transmitted by the optic nerve to the brain

n Antioxidantaction(β-carotene):
n β-carotene act as antioxidants and protect tissues from toxic effect of some oxidants.

Recommended dietary allowance (RDA)
n 1μgofretinol=6μgofβ-carotene=5IU
n Thedailyrequirementisabout5000IUofretinol. n Dailyintakeof15xRDAcausetoxicity

1. Eye:
   Vitamin A deficiency
   n Night blindness
   n Xerophthalmia (dryness & roughness of cornea)


2. Skinandmucusmembranes:
   n Roughness of skin and mucus membranes of different body systems e.g. urinary system.
   n This leads to increased susceptibility to infection. 3. Growthretardation.


3. Hydrocephalusandincreasedintracranialpressure 5. Degenerationoftestesandabortion(animals)

Therapeutic uses of vitamin A
n VitaminAisusedfortreatingvitaminAdeficiency n Alltransretinoicacidisusedfortreatmentof
psoriasis & leukemia.
n 13-cisretinoicacidisusedfortreatmentofsevere acne.

Vitamin A toxicity
n Headache,nausea,bonepain,andlossofhair, dermatitis and skin itching.
n VitaminAisteratogenicinhigherdoses(>15000 IU/day) if taken during the first two months of pregnancy
n UnlikevitaminA,β-caroteneisnottoxicevenat high doses