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Vitamins
Chapter 5
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Vitamins
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In 1913 thiamin was discovered as the first vitamin, the “vital amine” necessary to prevent the
deficiency disease beriberi
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Today, 13 vitamins have been identified as important for human nutrition
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Understanding Vitamins
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Organic compounds made of carbon, hydrogen, oxygen, and sometimes nitrogen or other
elements
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Facilitate biochemical reactions within cells
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Essential to life
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Individual molecules
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Do not provide energy (calories), but they are needed for metabolism of energy
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Needed in microgram or milligram quantities
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Understanding Vitamins (cont’d)
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Vitamins are chemically defined
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Extremely complex chemical substances
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Absorption rates of natural and synthetic vitamins sometimes differ because of different
chemical forms of the same vitamin
Vitamins are susceptible to destruction
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Vitamins in food are susceptible to destruction and subsequent loss of function
Vitamins may exist in more than one form
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Different forms perform different functions in the body
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Provitamins
Vitamins are essential
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With few exceptions, the body cannot make vitamins
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Some vitamins are coenzymes
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Many enzymes cannot function without a coenzyme, and many coenzymes are vitamins
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Understanding Vitamins (cont’d)
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Some vitamins are antioxidants
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Free radicals are produced continuously in cells as they burn oxygen during normal
metabolism
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Oxidize body cells and DNA in their quest to gain an electron and become stable
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Antioxidants protect body cells from being oxidized (destroyed) by free radicals
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Major antioxidants are vitamin C, vitamin E, and beta-carotene
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Understanding Vitamins (cont’d)
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Some vitamins are used as food additives
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Some foods have vitamins added to them simply to boost their nutritional content
Vitamins as drugs
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In megadoses, vitamins function like drugs, not nutrients
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Vitamin Classifications Based on Solubility
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Fat-soluble vitamins
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Characteristics
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Sources are the fat and oil portion of foods
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Absorption: fat encased in chylomicrons that enter the lymphatic system before
circulating in the bloodstream
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Transportation through the blood occurs by attaching to protein carriers
because fat is not soluble in watery blood
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When consumed in excess stored primarily in liver and adipose tissue
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Vitamin Classifications Based on Solubility (cont’d)
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Fat-soluble vitamins
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Characteristics (cont’d)
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Can be toxic if consuming high intakes through supplements
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Generally do not have to be consumed daily because the body can retrieve
them from storage as needed
Vitamin Classifications Based on Solubility (cont’d)
Vitamin A
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Preformed vitamin A is found only in animal sources
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Also includes provitamin A carotenoids
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Natural plant pigments found in deep yellow and orange fruits and vegetables
and most dark-green leafy vegetables
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Beta-carotene, lutein, and lycopene are among the most commonly known
carotenoids
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Vitamin Classifications Based on Solubility (cont’d)
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Vitamin A (cont’d)
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Best known for its roles in normal vision, reproduction, growth, and immune system
functioning
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Body can store up to a year’s supply of vitamin A
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Only preformed vitamin A is toxic in high doses
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Beta carotene is nontoxic
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Hypercarotenemia
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Vitamin Classifications Based On Solubility (cont’d)
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Vitamin D
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Unique in that the body has the potential to make all of the vitamin D it needs if
exposure to sunlight is optimal and liver and kidney function are normal
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Cholecalciferol
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Another distinctive feature of vitamin D is that it acts like a hormone because it is
synthesized in one part of the body (skin) and stimulates functional activity elsewhere
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Vitamin Classifications Based on Solubility (cont’d)
Vitamin D (cont’d)
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Primary function of vitamin D is to maintain normal blood concentrations of calcium and
phosphorus
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Vitamin D is important for immune function
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It is possible to fulfill the vitamin D requirement by taking a daily 15-minute walk in the
sun under optimal conditions
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Winter, living in northern latitudes, and being of black race and older age are associated
with low vitamin D synthesis
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Vitamin Classifications Based on Solubility (cont’d)
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Vitamin D (cont’d)
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Dietary source is considered essential because few people meet optimal conditions
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Vitamin D occurs naturally in only a few foods
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Many experts believe the AI for vitamin D is set too low
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Overt deficiency of vitamin D causes poor calcium absorption
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Rickets and osteomalacia
Current upper limit for vitamin D is set at 50 micrograms (2,000 IU)
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Vitamin Classifications Based on Solubility (cont’d)
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Vitamin E
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Generic term that describes a group of at least 8 naturally occurring compounds
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Alpha-tocopherol is considered the most biologically active form of vitamin E
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Functions as the primary fat-soluble antioxidant in the body
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Protects polyunsaturated fatty acids (PUFAs) and other lipid molecules, such as LDL
cholesterol, from oxidative damage
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Vitamin Classifications Based on Solubility (cont’d)
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Vitamin E (cont’d)
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Need increases as the intake of PUFA increases
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Megadoses may help protect against atherosclerosis, some types of cancer, cataracts,
age-related macular degeneration, Parkinson’s disease, and Alzheimer’s disease
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Deficiency can occur in very specific instances
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Premature infants
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May occur secondary to a genetic abnormality or malabsorption syndromes
such as cystic fibrosis and short bowel syndrome
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Vitamin Classifications Based on Solubility (cont’d)
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Vitamin E (cont’d)
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Signs and symptoms of deficiency include peripheral neuropathy, ataxia, and ultimately
death
Vitamin K
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Occurs naturally in 2 forms
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Phylloquinones, found in plants
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Menaquinones, which are synthesized in the intestinal tract by bacteria
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Vitamin Classifications Based on Solubility (cont’d)
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Vitamin K (cont’d)
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Coenzyme essential for the synthesis of prothrombin
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Clinically significant vitamin K deficiency is defined as vitamin-K–responsive
hypoprothrombinemia
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Characterized by an increase in prothrombin time
Deficiency does not occur from inadequate intake but may occur secondary to
malabsorption syndromes or the use of certain medications that interfere with vitamin
K metabolism or synthesis, such as anticoagulants and antibiotics
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Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins
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Thiamine
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Coenzyme in the metabolism of carbohydrates and branched-chain amino acids
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Beriberi has been virtually eliminated
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Deficiency is usually seen only in alcoholics
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No adverse effects have been noted from high intakes of thiamin
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Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins (cont’d)
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Riboflavin
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Vitamin B2 is an integral component of the coenzymes flavin adenine
dinucleotide (FAD) and flavin mononucleotide (FMN) that function to release
energy from nutrients in all body cells
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Milk and dairy products contribute the most riboflavin to the diet
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Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins (cont’d)
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Biochemical signs of inadequate riboflavin status can appear after only a few
days of poor intake
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Elderly and adolescents are at greatest risk for riboflavin deficiency
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Deficiency symptoms include sore throat, cheilosis, stomatitis, glossitis, and
dermatitis
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Certain diseases, such as cancer, heart disease, and diabetes, precipitate or
exacerbate riboflavin deficiency
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Vitamin Classifications Based on Solubility (cont’d)
Water-soluble vitamins (cont’d)
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Riboflavin (cont’d)
Niacin
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Vitamin B3 exists as nicotinic acid and nicotinamide
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Body can make it from the amino acid tryptophan
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Niacin requirements are stated in niacin equivalents (NEs)
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Part of the coenzymes nicotinamide adenine dinucleotide (NAD) and
nicotinamide adenine dinucleotide phosphate (NADP), which are involved in
energy transfer reactions in the metabolism of glucose, fat, and alcohol in all
body cells
Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins (cont’d)
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Niacin (cont’d)
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Pellagra, the disorder caused by severe niacin deficiency, is rare in the United
States and usually is seen only in alcoholics
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Niacin deficiency may be treated with niacin, or tryptophan, or both
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Large doses of niacin in the form of nicotinic acid (1 g to 6 g/d) are used
therapeutically to lower total cholesterol and LDL-cholesterol and raise highdensity lipoprotein (HDL)–cholesterol
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Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins (cont’d)
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Vitamin B6
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Vitamin B6 and pyridoxine are group names for 6 related compounds that
include pyridoxine, pyridoxal, and pyridoxamine
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Helps plays a role in the synthesis, catabolism, and transport of amino acids and
in the conversion of tryptophan to niacin
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Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins (cont’d)
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Vitamin B6 (cont’d)
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Deficiencies of vitamin B6, folic acid, and vitamin B12 lead to an increase in
blood homocysteine levels
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Supplements of vitamin B6 have been used for a variety of other conditions,
although supportive evidence is lacking
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High intake of vitamin B6 from food does not pose any danger
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Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins (cont’d)
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Vitamin B6 (cont’d)
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Deficiencies of vitamin B6 are uncommon but are usually accompanied by
deficiencies of other B vitamins
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Secondary deficiencies are related to alcohol abuse and to other drug therapies
such as isoniazid, the antituberculosis drug that acts as a vitamin B6 antagonist
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Vitamin Classifications Based on Solubility (cont’d)
Water-soluble vitamins (cont’d)
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Folate
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Generic term for this B vitamin that includes both synthetic folic acid found in
vitamin supplements and fortified foods and naturally occurring folate in food
such as green leafy vegetables, dried peas and beans, seeds, liver, and orange
juice
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Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins (cont’d)
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Folate (cont’d)
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Major function is in the synthesis of DNA
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Folate is recycled through the intestinal tract
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Folate deficiency impairs DNA synthesis and cell division and results in
macrocytic anemia and other clinical symptoms
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Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins (cont’d)
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Folate (cont’d)
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Adequate intake of folate before conception and during the first trimester of
pregnancy reduces the risk of neural tube defects
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Upper limit for folic acid is 1,000 micrograms/ day
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Consistently high intakes of folate can mask vitamin B12 deficiency
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Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins (cont’d)
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Vitamin B12
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Cobalamin
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Vitamin B12 has important role in maintaining the myelin sheath around nerves
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Large doses of folic acid can alleviate the anemia caused by vitamin B12
deficiency (a function of both vitamins), but folic acid cannot halt the
progressive neurologic impairments that only vitamin B12 can treat
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Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins (cont’d)
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Vitamin B12 (cont’d)
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Only water-soluble vitamin that does not occur naturally in plants
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Requires an intrinsic factor
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Deficiency symptoms may take 5 to 10 years or longer to develop
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People with pernicious anemia require parenteral injections of vitamin B12
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Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins (cont’d)
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Vitamin B12 (cont’d)
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Recommended that people over 50 obtain most of their requirement from
fortified foods or supplements
Other B vitamins
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Pantothenic acid
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Biotin
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Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins (cont’d)
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Non-B vitamins
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Inositol
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Choline
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Carnitine
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Vitamin Classifications Based on Solubility (cont’d)
Water-soluble vitamins (cont’d)
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Vitamin C
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Ascorbic acid
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Found in citrus fruits and juices
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Prevents scurvy
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Water-soluble antioxidant that protects vitamin A, vitamin E, PUFA, and iron
from destruction
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Vitamin Classifications Based on Solubility (cont’d)
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Water-soluble vitamins (cont’d)
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Vitamin C (cont’d)
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Newest RDA for vitamin C represents an increase from the previous
recommendation
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No clear and convincing evidence that large doses of vitamin C prevent colds
Phytochemicals
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Literally plant chemicals
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When eaten in the “package” of fruit, vegetables, whole grains, or nuts, these
chemicals work together with nutrients and fiber to promote health
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Vitamins in Health Promotion
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Vitamins A (as carotenoids), C, and E are of concern for American adults based on dietary intake
data or evidence of public health problems
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A food group approach rather than actual intake calculations is “good enough” to assess vitamin
adequacy
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“Vitamins of concern” are found almost exclusively in plants
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Vitamins in Health Promotion (cont’d)
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What about supplements?
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People who choose to take an all-purpose multivitamin should select one that provides
100% of the daily value (DV) for vitamins with an established DV
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The USP stamp ensures the quality of supplements, but not the safety or benefits
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High-cost supplements are not necessarily superior to lower-cost ones
Vitamins in Health Promotion (cont’d)
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Can supplements be used as insurance against poor food choices?
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Supplements are not substitutes for healthy food: “supplement” means “add to,” not
“replace”
A word of caution
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Supplements taken to augment an intake that is already adequate are unnecessary