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Vitamins
Chapter 5
Copyright © 2010 Wolters Kluwer Health | Lippincott Williams & Wilkins
Vitamins
• In 1913 thiamin was discovered as the first
vitamin, the “vital amine” necessary to prevent
the deficiency disease beriberi
• Today, 13 vitamins have been identified as
important for human nutrition
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Understanding Vitamins
• Organic compounds made of carbon, hydrogen,
oxygen, and sometimes nitrogen or other elements
• Facilitate biochemical reactions within cells
• Essential to life
• Individual molecules
• Do not provide energy (calories), but they are
needed for metabolism of energy
• Needed in microgram or milligram quantities
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Understanding Vitamins (cont’d)
• Vitamins are chemically defined
– Extremely complex chemical substances
– Absorption rates of natural and synthetic
vitamins sometimes differ because of
different chemical forms of the same vitamin
• Vitamins are susceptible to destruction
– Vitamins in food are susceptible to
destruction and subsequent loss of function
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Understanding Vitamins (cont’d)
• Vitamins may exist in more than one form
– Different forms perform different functions in
the body
– Provitamins
• Vitamins are essential
– With few exceptions, the body cannot make
vitamins
• Some vitamins are coenzymes
– Many enzymes cannot function without a
coenzyme, and many coenzymes are vitamins
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Understanding Vitamins (cont’d)
• Some vitamins are antioxidants
– Free radicals are produced continuously in cells
as they burn oxygen during normal metabolism
o Oxidize body cells and DNA in their quest to
gain an electron and become stable
– Antioxidants protect body cells from being
oxidized (destroyed) by free radicals
– Major antioxidants are vitamin C, vitamin E, and
beta-carotene
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Understanding Vitamins (cont’d)
• Some vitamins are used as food additives
– Some foods have vitamins added to them
simply to boost their nutritional content
• Vitamins as drugs
– In megadoses, vitamins function like drugs,
not nutrients
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Question
• Vitamins do not provide calories. What do they
do?
a. Help metabolize energy
b. Function like drugs at normal doses
c. Provide essential nutritional needs at gram
or higher doses
d. Prevent biochemical reactions in the body
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Answer
a. Help metabolize energy
Rationale: Vitamins do not provide energy but
are needed for metabolism of energy.
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Vitamin Classifications Based on Solubility
• Fat-soluble vitamins
– Characteristics
o Sources are the fat and oil portion of foods
o Absorption: fat encased in chylomicrons that enter
the lymphatic system before circulating in the
bloodstream
o Transportation through the blood occurs by
attaching to protein carriers because fat is not
soluble in watery blood
o When consumed in excess stored primarily in liver
and adipose tissue
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Vitamin Classifications Based on Solubility
(cont’d)
• Fat-soluble vitamins
– Characteristics (cont’d)
o Can be toxic if consuming high intakes
through supplements
o Generally do not have to be consumed
daily because the body can retrieve them
from storage as needed
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Vitamin Classifications Based on Solubility
(cont’d)
• Vitamin A
– Preformed vitamin A is found only in animal
sources
– Also includes provitamin A carotenoids
o Natural plant pigments found in deep yellow
and orange fruits and vegetables and most
dark-green leafy vegetables
o Beta-carotene, lutein, and lycopene are
among the most commonly known carotenoids
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Vitamin Classifications Based on Solubility
(cont’d)
• Vitamin A (cont’d)
– Best known for its roles in normal vision,
reproduction, growth, and immune system
functioning
– Body can store up to a year’s supply of vitamin A
– Only preformed vitamin A is toxic in high doses
– Beta carotene is nontoxic
– Hypercarotenemia
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Vitamin Classifications Based On
Solubility (cont’d)
• Vitamin D
– 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
o Cholecalciferol
– 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)
– Primary function of vitamin D is to maintain
normal blood concentrations of calcium and
phosphorus
– Vitamin D is important for immune function
– It is possible to fulfill the vitamin D requirement
by taking a daily 15-minute walk in the sun
under optimal conditions
– 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)
• Vitamin D (cont’d)
– Dietary source is considered essential because few
people meet optimal conditions
o Vitamin D occurs naturally in only a few foods
– Many experts believe the AI for vitamin D is set too
low
– Overt deficiency of vitamin D causes poor calcium
absorption
o 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)
• Vitamin E
– Generic term that describes a group of at least 8
naturally occurring compounds
– Alpha-tocopherol is considered the most
biologically active form of vitamin E
– Functions as the primary fat-soluble antioxidant
in the body
– 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)
• Vitamin E (cont’d)
– Need increases as the intake of PUFA increases
– Megadoses may help protect against atherosclerosis,
some types of cancer, cataracts, age-related macular
degeneration, Parkinson’s disease, and Alzheimer’s
disease
– Deficiency can occur in very specific instances
o Premature infants
o 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)
• Vitamin E (cont’d)
– Signs and symptoms of deficiency include
peripheral neuropathy, ataxia, and ultimately
death
• Vitamin K
– Occurs naturally in 2 forms
o Phylloquinones, found in plants
o Menaquinones, which are synthesized in the
intestinal tract by bacteria
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Vitamin Classifications Based on Solubility
(cont’d)
• Vitamin K (cont’d)
– Coenzyme essential for the synthesis of prothrombin
– Clinically significant vitamin K deficiency is defined as
vitamin-K–responsive hypoprothrombinemia
o 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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Question
• What is one of the functions of vitamin D?
a. Synthesis of prothrombin
b. Protects polyunsaturated fatty acids (PUFAs)
and other lipid molecules
c. Aids in reproduction
d. Helps immune function
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Answer
d. Helps immune function
Rationale: Vitamin D is important for immune
function.
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Vitamin Classifications Based on Solubility
(cont’d)
• Water-soluble vitamins
– Thiamine
o Coenzyme in the metabolism of carbohydrates
and branched-chain amino acids
o Beriberi has been virtually eliminated
o Deficiency is usually seen only in alcoholics
o No adverse effects have been noted from high
intakes of thiamin
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Vitamin Classifications Based on Solubility
(cont’d)
• Water-soluble vitamins (cont’d)
– Riboflavin
o 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
o Milk and dairy products contribute the most
riboflavin to the diet
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Vitamin Classifications Based on Solubility
(cont’d)
• Water-soluble vitamins (cont’d)
– Riboflavin (cont’d)
o Biochemical signs of inadequate riboflavin status
can appear after only a few days of poor intake
o Elderly and adolescents are at greatest risk for
riboflavin deficiency
o Deficiency symptoms include sore throat, cheilosis,
stomatitis, glossitis, and dermatitis
o 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)
– Niacin
o Vitamin B3 exists as nicotinic acid and nicotinamide
o Body can make it from the amino acid tryptophan
o Niacin requirements are stated in niacin
equivalents (NEs)
o 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
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Vitamin Classifications Based on Solubility
(cont’d)
• Water-soluble vitamins (cont’d)
– Niacin (cont’d)
o Pellagra, the disorder caused by severe niacin
deficiency, is rare in the United States and usually
is seen only in alcoholics
o Niacin deficiency may be treated with niacin, or
tryptophan, or both
o 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)
• Water-soluble vitamins (cont’d)
– Vitamin B6
o Vitamin B6 and pyridoxine are group names
for 6 related compounds that include
pyridoxine, pyridoxal, and pyridoxamine
o 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)
• Water-soluble vitamins (cont’d)
– Vitamin B6 (cont’d)
o Deficiencies of vitamin B6, folic acid, and
vitamin B12 lead to an increase in blood
homocysteine levels
o Supplements of vitamin B6 have been used for
a variety of other conditions, although
supportive evidence is lacking
o High intake of vitamin B6 from food does not
pose any danger
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Vitamin Classifications Based on Solubility
(cont’d)
• Water-soluble vitamins (cont’d)
– Vitamin B6 (cont’d)
o Deficiencies of vitamin B6 are uncommon but
are usually accompanied by deficiencies of
other B vitamins
o 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)
– Folate
o 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)
• Water-soluble vitamins (cont’d)
– Folate (cont’d)
o Major function is in the synthesis of DNA
o Folate is recycled through the intestinal tract
o 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)
• Water-soluble vitamins (cont’d)
– Folate (cont’d)
o Adequate intake of folate before conception
and during the first trimester of pregnancy
reduces the risk of neural tube defects
o Upper limit for folic acid is 1,000 micrograms/
day
o Consistently high intakes of folate can mask
vitamin B12 deficiency
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Vitamin Classifications Based on Solubility
(cont’d)
• Water-soluble vitamins (cont’d)
– Vitamin B12
o Cobalamin
o Vitamin B12 has important role in maintaining
the myelin sheath around nerves
o 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)
• Water-soluble vitamins (cont’d)
– Vitamin B12 (cont’d)
o Only water-soluble vitamin that does not
occur naturally in plants
o Requires an intrinsic factor
o Deficiency symptoms may take 5 to 10 years
or longer to develop
o People with pernicious anemia require
parenteral injections of vitamin B12
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Vitamin Classifications Based on Solubility
(cont’d)
• Water-soluble vitamins (cont’d)
– Vitamin B12 (cont’d)
o Recommended that people over 50 obtain
most of their requirement from fortified foods
or supplements
– Other B vitamins
o Pantothenic acid
o Biotin
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Vitamin Classifications Based on Solubility
(cont’d)
• Water-soluble vitamins (cont’d)
– Non-B vitamins
o Inositol
o Choline
o Carnitine
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Vitamin Classifications Based on Solubility
(cont’d)
• Water-soluble vitamins (cont’d)
– Vitamin C
o Ascorbic acid
o Found in citrus fruits and juices
o Prevents scurvy
o 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)
• Water-soluble vitamins (cont’d)
– Vitamin C (cont’d)
o Newest RDA for vitamin C represents an increase
from the previous recommendation
o No clear and convincing evidence that large doses
of vitamin C prevent colds
– Phytochemicals
o Literally plant chemicals
o 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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Question
• Which B vitamin can cause permanent
neurologic damage and may not demonstrate
signs or symptoms for 10 years?
a. B6
b. B12
c. Folate
d. B2
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Answer
b. B12
Rationale: 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 neurological impairments that
only vitamin B12 can treat. Vitamin B12 deficiency
symptoms may take 5 to 10 years or longer to
develop, because the liver can store relatively
large amounts of B12 and the body recycles B12
by reabsorbing it.
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Vitamins in Health Promotion
• Vitamins A (as carotenoids), C, and E are of
concern for American adults based on dietary
intake data or evidence of public health
problems
• A food group approach rather than actual intake
calculations is “good enough” to assess vitamin
adequacy
• “Vitamins of concern” are found almost
exclusively in plants
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Vitamins in Health Promotion (cont’d)
• What about supplements?
– 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
– The USP stamp ensures the quality of
supplements, but not the safety or benefits
– High-cost supplements are not necessarily
superior to lower-cost ones
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Vitamins in Health Promotion (cont’d)
• Can supplements be used as insurance against
poor food choices?
– Supplements are not substitutes for healthy
food: “supplement” means “add to,” not
“replace”
• A word of caution
– Supplements taken to augment an intake that is
already adequate are unnecessary
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