912
UNIT 4
Maintenance of the Body
24
Fat-soluble
vitamins (A, D, E, and K) bind to ingested lipids
and are absorbed along with their digestion products. Anything
that interferes with fat absorption also interferes with the uptake
of fat-soluble vitamins. Except for vitamin K, fat-soluble vita-
mins are stored in the body, and pathologies due to fat-soluble
vitamin toxicity, particularly excess vitamin A, are well docu-
mented clinically.
As we describe in the next section, metabolism uses oxy-
gen, and during these reactions some potentially harmful
free radicals are generated. Vitamins C, E, and A (in the form
of its dimer beta-carotene) and the mineral selenium are
antioxidants
that neutralize tissue-damaging free radicals.
Te whole story of how antioxidants interact in the body is
Initially vitamins were given letter designations that indi-
cated the order of their discovery. Although more chemically
descriptive names have been assigned to them, this earlier ter-
minology is still commonly used.
Vitamins are either water soluble or fat soluble.
Water-
soluble
vitamins—the B-complex vitamins and vitamin C—are
absorbed along with water from the gastrointestinal tract. (Te
exception is vitamin B
12
: ±o be absorbed, it must bind to intrin-
sic factor, a stomach secretion.) Te body’s lean tissue stores in-
significant amounts of water-soluble vitamins, and any ingested
amounts not used within an hour or so are excreted in urine.
Consequently, health problems resulting from excessive levels
of these vitamins are rare.
Table 24.2
Vitamin Requirements of Humans
VITAMIN
MAJOR DIETARY SOURCES
MAJOR FUNCTIONS IN THE BODY
SYMPTOMS OF DEFICIENCY
OR EXTREME EXCESS
Water-Soluble Vitamins
Vitamin B
1
(thiamine)
Pork, legumes, peanuts,
whole grains
Coenzyme used in removing
CO
2
from organic compounds
Beriberi (nerve disorders—tingling,
poor coordination, reduced heart
function)
Vitamin B
2
(riboflavin)
Dairy products, meats,
enriched grains, vegetables
Component of coenzymes
FAD and FMN
Skin lesions such as cracks at corners
of mouth
Niacin (B
3
)
Nuts, meats, grains
Component of coenzyme
NAD
1
Skin and gastrointestinal lesions,
nervous disorders
Liver damage
Vitamin B
6
(pyridoxine)
Meats, vegetables, whole
grains
Coenzyme used in amino acid
metabolism
Irritability, convulsions, muscular
twitching, anemia
Unstable gait,
numb feet, poor coordination
Pantothenic acid (B
5
)
Most foods: meats, dairy
products, whole grains, etc.
Component of coenzyme A
Fatigue, numbness, tingling of hands
and feet
Folic acid (folacin) (B
9
)
Green vegetables, oranges,
nuts, legumes, whole grains
Coenzyme in nucleic acid and amino
acid metabolism
Anemia, birth defects
May mask deficiency of vitamin B
12
Vitamin B
12
Meats, eggs, dairy products
Coenzyme in nucleic acid
metabolism; maturation of red blood
cells
Anemia, nervous system disorders
(numbness, loss of balance)
Biotin
Legumes, other vegetables,
meats
Coenzyme in synthesis of fat,
glycogen, and amino acids
Scaly skin inflammation,
neuromuscular disorders
Vitamin C (ascorbic acid)
Fruits and vegetables,
especially citrus fruits,
broccoli, cabbage, tomatoes,
green peppers
Used in collagen synthesis (such as for
bone, cartilage, gums); antioxidant;
aids in detoxification; improves iron
absorption
Scurvy (degeneration of skin, teeth,
blood vessels), weakness, delayed
wound healing, impaired immunity
Gastrointestinal upset
Fat-Soluble Vitamins
Vitamin A (retinol)
Provitamin A (beta-carotene)
in deep green and orange
vegetables and fruits; retinol
in dairy products
Component of visual pigments;
maintenance of epithelial tissues;
antioxidant; helps prevent damage
to cell membranes
Blindness and increased death rate
Headache, irritability, vomiting, hair
loss, blurred vision, liver and bone
damage
Vitamin D
Dairy products, egg yolk;
also made in human skin in
presence of sunlight
Aids in absorption and use of calcium
and phosphorus; promotes bone
growth
Rickets (bone deformities) in children,
bone softening in adults
Brain,
cardiovascular, and kidney damage
Vitamin E (tocopherol)
Vegetable oils, nuts, seeds
Antioxidant; helps prevent damage
to cell membranes
Degeneration of the nervous system
Vitamin K (phylloquinone)
Green vegetables, tea; also
made by colon bacteria
Important in blood clotting
Defective blood clotting
Liver damage and anemia
Source:
From Jane B. Reece, CAMPBELL BIOLOGY, 9th Edition, © 2011. Reprinted by permission of Pearson Education, Inc., Upper Saddle River, N. J.
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