Vitamins
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Organic (carbon-based) compounds that are required for many reactions in the
body
Fat-Soluble Vitamins-A,D,E & K
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Require bile for absorption (incorporated into micelles)
Travel from gut to liver in chylomicrons
Body can store them to some degree
A,D, and (possibly) K are toxic at high doses
Water-Soluble Vitamins
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B vitamins and vitamin C
Absorbed from lumen of gut on their own
Do not travel in lipoproteins
With exception of B12, aren’t stored to a high degree in body
Toxicity potential is low for most
Vitamin A
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The first vitamin to be discovered
Body can use it as retinol (alcohol), retinal (aldehyde), or retinoic acid
Important in vision, epithelial tissues, bones
Vitamin A precursors
Some (not all) carotenoids
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Beta-carotene is most potent
Only a small fraction of it is converted to vitamin A in the body
Current thinking-12 µg beta-carotene is converted to 1 µg vitamin A
For other pro-vitamin A carotenoids, the ratio is more like 24 µg to 1 µg
vitamin A
Vitamin A Absorption
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Retinyl esters in diet are digested to retinol & fatty acid
Retinol and carotenoids are incorporated into micelles to get absorbed
Conversion of beta-carotene to retinal occurs in small intestine
Retinol and carotenoids enter chylomicrons in intestinal cell
In the liver
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Vitamin A is stored (90%) in liver--special cells called stellate cells
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Retinol picked up by tissues
When needed, liver takes vitamin A out of stores and binds it to retinolbinding protein and transthyretin and sends it into bloodstream
Functions of Vitamin A
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Vision
– Retina
– Cornea
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Maintenance of epithelium
Immune function
Reproduction
Cell differentiation
Vitamin A Deficiency
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The number one cause of preventable blindness in children worldwide
Also a major cause of death in young children
Early sign--night blindness
Xerophthalmia-later stages in which cornea dries out and may rupture
Other signs of vitamin A deficiency
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“Goose flesh”
Increased susceptibility to infections
Changes in epithelia of the lungs, gut and reproductive tract
RDA for Vitamin A
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700 µg females, 900 µg males
1 µg retinol activity equivalent
– 1 µg retinol
– 12 µg beta-carotene
– 12 µg alpha-carotene or beta-cryptoxanthin
Sources of vitamin A
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Preformed--milk and dairy products, egg yolk, liver
From carotenoids--carrots, sweet potato, mango, cntaloupe, spinach
– Vitamin A from fruit seems to be more bioavailable than from veggies
Vitamin A Toxicity
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Acute--eating polar bear liver
Chronic--overuse of supplements for many years
Can cause liver failure
UL = 3000 µg/day from preformed vitamin A (NOT carotenoids)
The Carotenoids
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May be important to health independent of vitamin A
Lutein in eye health
Lycopene in prostate gland and heart disease
More to come during antioxidants
Vitamin D
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Will be covered with calcium
Vitamin E
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Alpha-, beta-, gamma-, and delta- tocopherols and tocotrienols
Alpha-tocopherol is most active
Is not involved in specific enzyme’s activity
Exerts effect as an antioxidant
Originally discovered as required for reproduction in rodents (NOT humans)
Absorption, Circulation, Storage
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Absorption similar to vitamin A
Also travels to liver in chylomicrons
Not stored to great extent in liver
Incorporated into VLDL
Stored 90% in fat tissue
Function
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Prevents lipids in membranes and lipoproteins from oxidation
Due to its function, it is important in preventing heart disease, cancer, and
protecting the integrity of tissues
Vitamin E Deficiency
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Almost never seen in humans
Hemolysis--rupture of red blood cell membrane
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In rats, sterility, death of fetuses in utero
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Fats, particularly polyunsaturated
Other animals--white muscle disease
Sources of vitamin E
Wheat germ oil
Nuts
Vitamin E toxicity
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Only risk is with persons who are taking blood thinners
Vitamin E can interfere with absorption of vitamin K
RDA for Vitamin E
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15 mg per day
UL = 1000 mg per day
Vitamin K
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“Koagulation” and a whole lot more
Varieties of vitamin K
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Phylloquinones (K1)-found in plants
Menaquinones (K2)--produced by intestinal bacteria
Menadione (K3)--synthetic
Blood Clotting
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Vitamin K is responsible for making changes in 7 clotting proteins
These proteins bind calcium and help form the blood clot
Vitamin K in bone
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Responsible for activating the protein osteocalcin, which binds calcium in the
bone
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Two other bone proteins also require vitamin K