710
UNIT FIVE
RECONNECT
To Chapter 2, Bonding of Atoms, pages 55–57.
Only foods of animal origin such as liver, F sh, whole
milk, butter, and eggs are sources of vitamin A. However, the
vitamin’s precursor, carotene, is widespread in leafy green
vegetables and in yellow or orange vegetables and fruits.
Excess vitamin A produces peeling skin, hair loss, nausea,
headache, and dizziness, a condition called
hypervitaminosis A.
Chronic overdoses of the vitamin may inhibit growth and
break down bones and joints. “Megadosing” on fat-soluble
vitamins is particularly dangerous during pregnancy. Some
forms of vitamin A, in excess, can cause birth defects.
A deF ciency of vitamin A causes
night blindness,
in which
a person cannot see normally in dim light. Xeropthalmia, a
dryness of the conjunctiva and cornea, is due to vitamin A
Vitamin A is relatively stable to the effects of heat, acids,
and bases. However, it is readily destroyed by oxidation and
is unstable in light.
Vitamin A is important in vision. Retinal is used to syn-
thesize
rhodopsin
(visual purple) in the rods of the retina and
may be required for production of light-sensitive pigments in
the cones as well. The vitamin also functions in the synthesis
of mucoproteins and mucopolysaccharides, in development
of normal bones and teeth, and in maintenance of epithelial
cells in skin and mucous membranes. Vitamin A and beta
carotenes also act as
antioxidants
(an
tı˘-ok
sı˘-dant) by read-
ily combining with oxygen and certain oxygen-containing
molecules that have unshared electrons, which makes them
highly reactive and damaging to cellular structures. These
unstable molecules are called oxygen free radicals, and they
accumulate in certain diseases and with age.
I
n the United States, obesity is epidemic.
Nearly a third of all adults are obese, deF
ned
as 20% above “ideal” weight based on popu-
lation statistics considering age, sex, and build, or
a body mass index above 30. Obesity raises risks
for type 2 diabetes, digestive disorders, heart
disease, kidney failure, hypertension, stroke, and
cancers of the female reproductive organs and
the gallbladder. The body has to support the extra
weight—miles of extra blood vessels are needed
to nourish the additional pounds. Another third
of the adult population of the United States is
overweight. Obesity is the second leading cause
of preventable death, following cigarette smok-
ing. People in the United States are overweight
because of overeating and underexercising. The
average person today consumes 3,700 calories
daily, compared to 3,100 in the 1960s.
Obesity refers to extra pounds of fat. The pro-
portion of fat in a human body ranges from 5%
to more than 50%, with “normal” for males falling
between 12% and 23% and for females between
16% and 28%. An elite athlete may have a body
fat level as low as 4%. ±at distribution also a²
ects
health. Excess poundage above the waist is linked
to increased risk of heart disease, type 2 diabetes,
hypertension, and lipid disorders. The body mass
index (BMI) accounts somewhat for a person’s
build (F
g. 18A). A person who weighs 170 pounds
and is 6 feet tall is slim, whereas a person of the
same weight who is 5 feet tall is obese. The tall
person’s BMI is 23; the short person’s is 33.5.
18.1
CLINICAL APPLICATION
Obesity
29
27
25 27
23
22
21
19
18
17
16
15
15
14
13
Healthy weight
Overweight
Obese
4'6"
4'8"
4'10"
5'0"
5'2"
5'4"
5'6"
5'8"
5'10"
6'0"
6'2"
6'4"
6'6"
6'8"
Developed by the National Center for Health Statistics in collaboration with the
National Center for Chronic Disease Prevention and Health Promotion
31
29
25
24
22
21
20
19
18
17
16
15
14
29
34
31
27
26
24
23
21
20
19
18
17
16
15
31
36
34
29
27
26
24
23
22
20
19
18
17
17
34
39
36
31
29
28
26
24
23
22
21
20
19
18
36
41
Weight in pounds
Height in feet and inches
38
33
31
29
27
26
24
23
22
21
20
19
38
43
40
35
33
31
29
27
26
24
23
22
21
20
40
46
43
37
35
33
31
29
27
26
24
23
22
21
42
48
45
39
37
34
32
30
29
27
26
24
23
22
44
51
47
41
38
36
34
32
30
28
27
26
24
23
46
53
49
43
40
38
36
34
32
30
28
27
25
24
48
56
52
45
42
40
37
35
33
31
30
28
27
26
50
58 60
51
47
44
41
39
37
35
33
31
29
28
26
52
120 130 140 150 160 170 180 190 200 210 220 230 240 250
56
49
46
43
40
38
36
34
32
30
29
28
FIGURE 18A
Body mass index (BMI). BMI equals weight/height
2
, with weight measured
in kilograms and height measured in meters. This chart provides a shortcut—the calculations
have been done and converted to the English system of measurement. The uncolored squares
indicate lower than healthy weight according to this index.
Both heredity and the environment contrib-
ute to obesity. Dozens of genes interact to control
energy balance and therefore body weight. The
observation that identical twins reared in di²
erent
households can grow into adults of vastly differ-
ent weights indicates that environment in³
uences
weight too. Even the environment before birth can
affect body weight later. Individuals born at full
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