Agriculture Reference
In-Depth Information
(fat cells), producing substantial deposits of body fat that can insulate against body
heat loss and serve as an energy reserve. Various procedures have been used to mea-
sure the size of these adipose deposits, and about 59 million adult Americans have
fat stores so large as to be classified as obese. In these individuals, consumption of
energy far exceeds their energy expenditure. Fat is an efficient energy storage form
because every gram of fat can provide more than twice as much energy in support of
body metabolism as can carbohydrate or protein. However, excess body fat is often
accompanied by health problems—a serious consequence of
too much
food and too
little exercise. Unfortunately,
too little
food characterizes the lives of much of the
world's population, and the benefits of a substantial body fat store to buffer short
periods of food scarcity are not available to them.
t
h e r m o g e in e s i s
Specialized body fat deposits, with an unusually rich blood vessel supply, have been
designated brown adipose tissue. They are characterized by the ability to generate heat
in response to food intake or to prolonged cold exposure by a process called
nonshivering
thermogenesis
. Although present in newborns, particularly between the shoulder blades
(interscapular area) and useful in elevating body temperature after birth, brown adipose
tissue appears to play little role in maintaining the body temperature of human adults.
C
e L L
m
e m b r a n e
s
t r u C t u r e
a n D
f
u n C t i o n
Particular fatty acids are found in phospholipids that are critical for cell membrane
structure and function. They provide physical support to the membranes, serve as a
source of physiologically active compounds, and modulate cross-membrane move-
ment of metabolically active substances.
n
u t r i e n t
t
r a n s P o r t
In addition to serving as a source of essential fatty acids, lipids are involved in trans-
port of the lipid-soluble vitamins A, D, E, and K and provitamin A carotenoids.
h
o r m o n a L
a
C t i v i t y
No longer is body fat considered just a source of stored energy. In recent years, white
adipose tissue has been shown to secrete over ten peptide hormones—compounds
that contain two or more amino acids. By means of these hormones (such as leptin,
which affects appetite and the immune system), fat serves also as an endocrine organ,
sustaining energy homeostasis through regulation of glucose and lipid metabolism,
modulating the immune response, and influencing reproduction.
D
i g e is t i o in
a n D
a
b s o r P t i o n
Fats are hydrophobic (have a limited affinity for water). However, the enzymes that
digest them normally function in an aqueous environment. Thus, fat digestion is
