712
UNIT FIVE
Vitamin E
includes a group of compounds, the most
active of which is
alpha-tocopherol.
This vitamin is resistant
to the effects of heat, acids, and visible light but is unstable
in bases and in the presence of ultraviolet light or oxygen.
Vitamin E is a strong antioxidant.
Vitamin E is found in all tissues but is primarily stored in
the muscles and adipose tissue. It is also highly concentrated
in the pituitary and adrenal glands.
The precise functions of vitamin E are unknown, but it
is thought to act as an antioxidant by preventing oxidation of
vitamin A and polyunsaturated fatty acids in the tissues. It
may also help maintain the stability of cell membranes.
Vitamin E is widely distributed among foods. Its richest
sources are oils from cereal seeds such as wheat germ. Other
good sources are salad oils, margarine, shortenings, fruits,
nuts, and vegetables. Excess vitamin E may cause nausea,
headache, fatigue, easy bruising and bleeding, and muscle
weakness. This vitamin is so easily obtained that deF ciency
conditions are rare.
PRACTICE
39
Where is vitamin E stored?
40
What are the functions of vitamin E?
41
Which foods are good sources of vitamin E?
Vitamin K,
like the other fat-soluble vitamins, is in sev-
eral chemical forms. One of these, vitamin K
1
(phylloqui-
none), is found in foods, whereas another, vitamin K
2
, is
produced by bacteria (
Escherichia coli
) that normally inhabit
the human intestinal tract. These vitamins resist the effects of
heat but are destroyed by oxidation or by exposure to acids,
bases, or light. The liver stores them to a limited degree.
Vitamin K primarily functions in the liver, where it is
necessary for the formation of several proteins needed for
blood clotting, including
prothrombin
(see chapter 14,
p. 539). Consequently, deficiency of vitamin K prolongs
blood clotting time and may increase risk of hemorrhage.
Excess vitamin K may occur in formula-fed infants, causing
jaundice, hemolytic anemia, and hyperbilirubinemia.
The richest sources of vitamin K are leafy green vege-
tables. Other good sources are egg yolk, pork liver, soy oil,
tomatoes, and caulifl ower.
Table 18.8
summarizes the fat-
soluble vitamins and their properties.
Natural foods are often poor sources of vitamin D, so
it is often added to food during processing. ±or example,
homogenized, nonfat, and evaporated milk are typically forti-
F ed with vitamin D.
Fortif
ed
means essential nutrients have
been added to a food where they originally were absent or
scarce.
Enriched
means essential nutrients have been partially
replaced in a food that has lost nutrients during processing.
Excess vitamin D, or
hypervitaminosis D,
produces diar-
rhea, nausea, and weight loss. Over time it may also calcify
certain soft tissues and irreversibly damage the kidneys.
In children, vitamin D deficiency results in
rickets,
in
which the bones and teeth fail to develop normally
(f g. 18.11)
.
In adults or in the elderly who have little exposure to sun-
light, such a deF ciency may lead to
osteomalacia,
in which
the bones decalcify and weaken due to disturbances in cal-
cium and phosphorus metabolism. Risk of developing vita-
min deF ciency increases in people who stay out of the sun or
liberally use sun block to prevent skin cancer. However, just
F ve minutes of sun exposure two to three times a week can
maintain skeletal health without elevating skin cancer risk.
Older people are usually outdoors less than younger individu-
als, so the Institute of Medicine suggests that daily vitamin D
intake increase with age (
table 18.7
).
PRACTICE
35
Where is vitamin D stored?
36
What are the functions of vitamin D?
37
Which foods are good sources of vitamin D?
38
What are symptoms of vitamin D excess and deF
ciency?
FIGURE 18.11
Vitamin D deF
ciency causes rickets, in which the
bones and teeth do not develop normally. Note this boy’s bowed legs.
TABLE
18.7
|
Vitamin D Requirements
Increase with Age
Age Range (Years)
International Units of Vitamin D
<50
200
50–70
400
70+
600
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