224
U N I T 2
Integrative Body Functions
used for muscle contraction is derived largely from
energy sources that are stored in muscle cells and then
released as the muscle contracts. Because most of our
energy sources come from the nutrients in food that is
eaten, the ability to store energy and control its release is
important. Normally, energy utilization is balanced with
energy expenditure. When the intake of food consistently
exceeds energy expenditure, the excess energy is stored
as fat, and the person becomes overweight. Conversely,
when food intake does not meet energy expenditure, fat
stores and other body tissues are broken down and the
person loses weight.
More than 90% of body energy is stored as triglycer-
ides in the fat cells of the body. The body has a limited
ability to store dietary carbohydrates and proteins as
energy sources. Dietary carbohydrates are largely con-
verted to glucose, which is stored as glycogen in liver
and skeletal muscle cells. Liver glycogen stores reach
a maximum of approximately 200 to 300 g after a
high-carbohydrate meal, after which the liver begins to
convert some of the excess glucose to triglycerides for
storage in fat cells. The amino acids from protein in the
diet are stored mainly in the form of structural proteins,
enzymes, nucleoproteins, and other types of cellular
proteins. After all the cells have reached their limits, the
excess amino acids are converted to glucose and used
for energy or stored in the liver as glycogen or in adipose
tissue as triglycerides.
Triglycerides, which contain no water, have the high-
est caloric content of all nutrients and are an efficient
form of energy storage. When calorie intake is restricted
for any reason, the triglycerides in fat cells are broken
down, and their fatty acids and glycerol released as
energy sources. Fat cells synthesize triglycerides from glu-
cose and fatty acids. Insulin is required for fat storage. It
promotes glucose transport through the cell membrane
of fat cells. Some of the glucose is used to synthesize fatty
acids, but more importantly it forms large amounts of
α
-glycerol phosphate. This compound supplies the glyc-
erol that combines with fatty acids to form triglycerides.
Therefore, in the absence of insulin, the storage of tri-
glycerides in adipose tissue is almost blocked.
Fat cells, or
adipocytes
, are modified fibroblasts that
store almost pure triglycerides in quantities as great as
80% to 95% of their total cell volume.
1
Adipocytes
occur singly or in small groups in adipose connective
tissue, entire regions of which are committed to triglyc-
eride storage. Collectively, adipocytes constitute a large
body organ that is metabolically active in the uptake,
synthesis, storage, and mobilization of lipids. In addi-
tion, adipose tissue provides insulation for the body, fills
body crevices, and protects body organs.
2
Early studies suggested that fully differentiated adi-
pocytes do not undergo further cell division, and thus
that the number of fat cells is fixed in early childhood.
This theory proposed that subsequent gains in adipose
tissue represented increases in fat cell size. This is no
longer considered to be true, since adipose tissue in
adults is now known to contain
preadipocytes
, capable
of forming new fat cells, and fat deposition can result
from proliferation of these immature adipocytes.
3
Some medications can also increase fat cell numbers.
For example, the thiazolidinedione (TZD) class of anti-
diabetic drugs can stimulate the formation of new fat
cells from the preadipocytes, allowing increased uptake
of glucose into these cells (and storage as fat), resulting
in the desired reduction in serum glucose levels but with
unwanted weight gain.
4
In contrast, some drugs can
cause loss of fat cells. This occurs in persons who are
HIV-positive and are treated with highly active antiret-
roviral therapy (see Chapter 16). The mechanism of fat
loss is not known; however, it may be due to increased
apoptosis (programmed cell death) of the adipocytes.
There are two types of adipose tissue: white (unilocu-
lar) and brown (multilocular).
2
White fat
, which despite
its name is cream colored or yellow, is the predominant
form of adipose tissue in adults. The adipocytes of white
fat are large spherical cells that become polyhedral or oval
when crowded in adipose tissue. The functions of white
fat include energy storage, endocrine and adipocytokine
secretion, insulation, and cushioning of vital organs. In
the connective tissue under the skin, the white fat layer
has significant insulating functions. Concentrations are
found under the skin of the abdomen, buttocks, axilla,
and thigh. Sex differences in the thickness of this fatty
layer in different parts of the body account, in part, for
the differences in body composition between males and
females. Internally, white adipose tissue is preferentially
located in the greater omentum, mesentery, and retro-
peritoneal space and is usually abundant around the kid-
ney. It is also found in the orbits around the eyeballs, in
the bone marrow, and between other tissues, where it
fills spaces. It retains this structural function even during
reduced caloric intake, when the lipid content of adipose
tissue elsewhere has been depleted.
2
Brown fat
differs from white fat in terms of its ther-
mogenic capacity (its ability to produce heat). The color
of brown fat reflects the presence of iron in its abundant
mitochondria. Brown fat mitochondria produce a spe-
cific protein called uncoupling protein-1 (UCP-1) that
releases the energy generated from metabolism as heat.
It is found abundantly in newborns, in whom thermo-
genesis is critical because of their proportionally greater
heat loss as compared to adults and their reduced ability
to shiver. Historically, adults were thought to have only a
small amount of brown fat; however, recent studies have
confirmed that moderate deposits of brown fat are com-
monly present in adults and can be stimulated by sev-
eral factors including cold and the sympathetic nervous
system.
Nutritional Status
The body obtains the energy needed to perform its vari-
ous functions and maintain the integrity and health of its
cells from the various foods that are consumed in the diet.
Because different foods contain different proportions of
proteins, carbohydrates, fats, minerals, and vitamins,
appropriate balances must be maintained among these
constituents so that all segments of the body’s metabolic
systems can be supplied with the prerequisite materials.
