Vitamins
Vitamins
A vitamin is an organic compound, essential in small amounts for the proper functioning of the human body, that must be obtained from dietary sources because the body cannot synthesize it.
Vitamins differ from the major classes of foods (carbohydrates, lipids, and proteins) in the amount required; for vitamins it is microgram or milligram quantities per day compared with 50–200 grams per day for the major food categories.
Vitamins
• Solubility characteristics divide the vitamins into two major classes: the water- soluble vitamins and the fat (lipid)-soluble vitamins. There are nine water-soluble
vitamins and four fat-soluble vitamins.
Vitamins
Water-soluble vitamins must be constantly replenished in the body because they are rapidly eliminated from the body in the urine. They are carried in the bloodstream, are needed in frequent, small doses, and are unlikely to be toxic except when taken in unusually large doses.
The fat-soluble vitamins are found dissolved in lipid materials. They are, in general, carried in the blood by protein carriers, are stored in fat tissues, are needed in periodic doses, and are more likely to be toxic when consumed in excess of need.
WATER-SOLUBLE VITAMINS
• The nine water-soluble vitamins, vitamin C and eight B
vitamins, got their names from the labels B and C on the test tubes in which they were first collected. Later, test tube B
was found to contain more than one vitamin.
Vitamin C
• Vitamin C, which has the simplest structure of the 13 vitamins, exists in two active forms in the human body: an oxidized form and a reduced form.
Vitamin C
The most completely characterized role of vitamin C is its function as a cosubstrate in the formation of the structural protein collagen, which makes up much of the skin, ligaments, and tendons and also serves as the matrix on which bone and teeth are formed.
Specifically, biosynthesis of the amino acids hydroxyproline and hydroxylysine
(important in binding collagen fibers together) from proline and lysine requires the presence of both vitamin C and iron.
Vitamin C also functions as a general antioxidant for water-soluble substances in the blood and other body fluids.
Vitamin C
• Vitamin C is also involved in the metabolism of several
amino acids that end up being converted to the hormones norepinephrine and thyroxine.
• An intake of 100 mg/day of vitamin C saturates all body tissues with the compound. After the tissues are saturated, all additional vitamin C is excreted.
Vitamin B
Thiamin (vitamin B1) Ribofl avin (vitamin B2)
Niacin (nicotinic acid, nicotinamide, vitamin B3) Vitamin B6 (pyridoxine, pyridoxal, pyridoxamine) Folate (folic acid)
Vitamin B12 (cobalamin)
Pantothenic acid (vitamin B5) Biotin
Vitamin B
• The major function of B vitamins within the human body is as components of coenzymes. Unlike vitamin C, all of the B vitamins must be chemically modified before they become functional within the coenzymes.
Vitamin B
• In their function as coenzymes, some of the B vitamins do not remain permanently bonded to the apoenzyme that they are associated with.
• This means that they can be repeatedly used by various enzymes. This reuse (recycling) diminishes the need for large amounts of the B vitamins in biochemical systems.
Vitamin B
• Fruits, in general, are very poor sources of B vitamins and that only certain vegetables are good B vitamin sources.
• Vitamin B12 is unique among the vitamins in being found almost exclusively in food derived from animals.
• Both niacin and folate have been linked positively to improvement in cardiovascular health. Adding prescription-strength, extended-release niacin to cholesterol-lowering statin medications slows the
progression of atherosclerosis among people with coronary heart disease and low HDL levels better than statin therapy alone.
FAT-SOLUBLE VITAMINS
• The four fat-soluble vitamins are designated using the
letters A, D, E, and K. Many of the functions of the fat-soluble vitamins involve processes that occur in cell membranes.
• The structures of the fat-soluble vitamins are more
hydrocarbon-like, with fewer functional groups than the water-soluble vitamins. Their structures as a whole are
nonpolar, which enhances their solubility in cell membranes.
Vitamin A
• Normal dietary intake
provides a person with both preformed and precursor forms (provitamin forms) of vitamin A.
• Preformed vitamin A forms are called retinoids. The retinoids include retinal, retinol, and retinoic acid.
Vitamin A
• Foods derived from animals, including egg yolks and dairy products, provide compounds (retinyl esters) that are easily hydrolyzed to retinoids in the intestine.
• Foods derived from plants provide carotenoids which serve as precursor forms of vitamin A. The major carotenoid with vitamin A activity is beta-carotene (b carotene), which can be cleaved to yield two molecules of vitamin A.
Vitamin A
• Vitamin A has four major functions in the body.
• 1. Vision. In the eye, vitamin A (as retinal) combines with the
protein opsin to form the visual pigment rhodopsin. Rhodopsin participates in the conversion of light energy into nerve impulses that are sent to the brain.
• 2. Regulating Cell Differentiation. Cell differentiation is the process whereby immature cells change in structure and function to become specialized cells.
Vitamin A
• 3. Maintenance of the Health of Epithelial Tissues. Epithelial tissue covers outer body surfaces as well as lining internal cavities and tubes. Lack of vitamin A (as retinoic acid) causes such surfaces to become drier and harder than normal. Vitamin A’s role here is related to cellular differentiation involving mucus- secreting cells.
• 4. Reproduction and Growth. In men, vitamin A participates in sperm development. In women, normal fetal development during pregnancy requires vitamin A.
Vitamin D
• The two most important members of the vitamin D family of molecules are vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol).
• Vitamin D3 is produced in the skin of humans and animals by the action of sunlight (ultraviolet light) on its precursor
molecule, the cholesterol derivative 7-dehydrocholesterol (a normal metabolite of cholesterol found in the skin).
Vitamin D
• Vitamin D2 (ergocalciferol) differs from vitamin D3 only in the side-chain
structure. It is produced from the plant sterol ergosterol through the action of light.
Vitamin D
• The principal function of vitamin D is to maintain normal blood levels of calcium ion and phosphate ion so that bones can
absorb these ions.
• Vitamin D stimulates absorption of these ions from the
gastrointestinal tract and aids in their retention by the kidneys.
• Vitamin D triggers the deposition of calcium salts into the organic matrix of bones by activating the biosynthesis of calcium-binding proteins.
Vitamin E
• There are four forms of vitamin E: alpha-, beta-, delta-, and gamma-tocopherol. These forms differ from each other structurally in what
substituents (-CH3 or -H) are present at two positions on an aromatic ring.
Vitamin E
The tocopherol form with the greatest biochemical activity is alpha-tocopherol.
Gamma-tocopherol is the main form of vitamin E in vitamin-E rich foods.
The primary function of vitamin E in the body is as an antioxidant—a compound that protects other compounds from oxidation by being oxidized itself. Vitamin E is
particularly important in preventing the oxidation of polyunsaturated fatty acids in membrane lipids. It also protects vitamin A from oxidation.
Vitamin E has also been found to be involved in the conversion of arachidonic acid (20:4) to prostaglandins
Vitamin K
• Vitamin K1, also called
phylloquinone, has a side chain that is predominantly saturated; only one carbon–carbon double bond is
present. It is a substance found in plants.
• Vitamin K2 has several forms, called menaquinones, with the various forms differing in the length of the side chain. Menaquinone side chains have several carbon–carbon double bonds, in contrast to the one carbon–
carbon double bond present in phylloquinone.
Vitamin K
• Vitamin K2 is found in animals and humans and can be
synthesized by bacteria, including those found in the human intestinal tract.
• Typically, about half of the human body’s vitamin K is
synthesized by intestinal bacteria and half comes from the diet.
Vitamin K
• Vitamin K is essential to the blood-clotting process. Over a dozen different proteins and the mineral calcium are
involved in the formation of a blood clot.
• Vitamin K is essential for the formation of prothrombin and at least five other proteins involved in the regulation of
blood clotting.
