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CHAPTER EIGHTEEN
Nutrition and Metabolism
The B-complex vitamins include the following:
1.
Thiamine,
or
vitamin B
1
. In its pure form, thiamine is
a crystalline compound called thiamine hydrochloride.
Exposure to heat and oxygen destroys it, especially in
alkaline environments. (See F
g. 18.18 for its molecular
structure.)
Thiamine is part of a coenzyme called
cocarboxylase,
which oxidizes carbohydrates. SpeciF cally, thiamine is
required for pyruvic acid to enter the citric acid cycle (see
chapter 4, p. 122); in the absence of this vitamin, pyruvic
acid accumulates in the blood. Thiamine also functions
as a coenzyme in the synthesis of the sugar ribose, which
is part of the nucleic acid RNA.
Thiamine is primarily absorbed through the wall of
the duodenum and is transported by the blood to body
cells. Only small amounts are stored in the tissues, and
excess is excreted in the urine.
Vitamin B
1
oxidizes carbohydrates, so cellular
requirements vary with caloric intake. It is recommended
that an adult diet contain 0.5 milligram (mg) of thiamine
for every 1,000 calories ingested daily. Good sources of
thiamine are lean meats, liver, eggs, whole-grain cereals,
leafy green vegetables, and legumes.
Excess thiamine is not as common as excesses of
fat-soluble vitamins, due to the excretion of thiamine
in urine. Toxicity effects include vasodilation, cardiac
dysrhythmias, headache, weakness, and convulsions.
A mild deF
ciency of thiamine produces loss of
appetite, fatigue, and nausea. Prolonged deF
ciency
leads to a disease called
beriberi,
which causes
About 1 in every 200 to 400 newborns develops vitamin K def
ciency
because oF an immature liver, poor transFer oF vitamin K through the
placenta, or lack oF intestinal bacteria that can synthesize this vitamin.
The def
ciency causes “hemorrhagic disease oF the newborn,” which
is associated with abnormal bleeding that typically appears during
the second to f
Fth day oF liFe. Injections oF vitamin K shortly aFter birth
prevent this condition. Adults may develop vitamin K deFiciency iF
they are treated with antibiotic drugs that kill the intestinal bacteria
that manuFacture the vitamin. People with cystic Fibrosis develop
vitamin K def
ciency, and/or def
ciency oF other Fat-soluble vitamins,
because they cannot digest Fats well.
PRACTICE
42
Where in the body is vitamin K synthesized?
43
What is the Function oF vitamin K?
44
Which Foods are good sources oF vitamin K?
Water-Soluble Vitamins
The water-soluble vitamins include the B vitamins and vita-
min C. Cooking and food processing destroy some of them.
The
B vitamins
are several compounds essential for normal
cellular metabolism. They help oxidize (remove electrons
from) carbohydrates, lipids, and proteins during cellular
respiration. The B vitamins are usually in the same foods,
so they are called the
vitamin B complex.
Members of this
group differ chemically and functionally.
TABLE
18.8
|
Fat-Soluble Vitamins
Vitamin
Characteristics
Functions
Sources and
RDA
*
for Adults
Conditions Associated with
Excesses
De±
ciencies
Vitamin A
Exists in several Forms;
synthesized From carotenes;
stored in liver; stable in heat,
acids, and bases; unstable in
light
An antioxidant necessary For synthesis
oF visual pigments, mucoproteins,
and mucopolysaccharides; For normal
development oF bones and teeth; and
For maintenance oF epithelial cells
Liver, f
sh, whole milk, butter,
eggs, leaFy green vegetables,
yellow and orange
vegetables and Fruits
4,000–5,000 IU
**
Nausea,
headache,
dizziness, hair
loss, birth deFects
Night blindness,
degeneration
oF epithelial
tissues
Vitamin D
A group oF steroids; resistant to
heat, oxidation, acids, and bases;
stored in liver, skin, brain, spleen,
and bones
Promotes absorption oF calcium and
phosphorus; promotes development oF
teeth and bones
Produced in skin exposed to
ultraviolet light; in milk, egg
yolk, f
sh liver oils, Fortif
ed
Foods
400 IU
Diarrhea,
calcif
cation oF
soFt tissues, renal
damage
Rickets, bone
decalcif
cation
and weakening
Vitamin E
A group oF compounds; resistant
to heat and visible light;
unstable in presence oF oxygen
and ultraviolet light; stored in
muscles and adipose tissue
An antioxidant; prevents oxidation oF
vitamin A and polyunsaturated Fatty
acids; may help maintain stability oF cell
membranes
Oils From cereal seeds, salad
oils, margarine, shortenings,
Fruits, nuts, and vegetables
30 IU
Nausea,
headache,
Fatigue, easy
bruising and
bleeding
Rare, uncertain
ects
Vitamin K
Exists in several Forms; resistant
to heat but destroyed by acids,
bases, and light; stored in liver
Required For synthesis oF prothrombin,
which Functions in blood clotting
LeaFy green vegetables,
egg yolk, pork liver, soy oil,
tomatoes, cauli²
ower
55–70 µg
Jaundice in
Formula-Fed
newborns
Prolonged
clotting time
*
RDA = recommended daily allowance.
**
IU = international unit.
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