Nutrition, Metabolism, and Body Temperature Regulation
Carbo loading is standard practice among marathon runners
and distance cyclists, because studies have shown that it im-
proves performance and endurance.
When too little glucose is available to stoke
the “metabolic furnace,” glycerol and amino acids are converted
, the process of forming new (
molecules, occurs in the liver.
Gluconeogenesis takes place when dietary sources and glucose
reserves have been used up and blood glucose levels are begin-
ning to drop. Gluconeogenesis protects the body, especially the
nervous system, from the damaging eﬀects of low blood sugar
) by ensuring that ATP synthesis can continue.
Check Your Understanding
What name is given to the chemical reaction in which
glycogen is broken down to its glucose subunits?
What does carbo loading accomplish?
For answers, see Appendix H.
When more glucose is available than can immediately be oxi-
dized, rising intracellular ATP concentrations eventually inhibit
glucose catabolism and cause glucose to be stored as glycogen
or fat. Because the body can store much more fat than glycogen,
fats account for 80–85% of stored energy.
When high ATP levels begin to “turn oﬀ” gly-
colysis, glucose molecules are combined in long chains to form
glycogen, the animal carbohydrate storage product. ±is pro-
cess is called
, right side).
Glycogenesis begins as glucose entering cells is phosphor-
ylated to glucose-6-phosphate and then converted to its isomer,
. ±e terminal phosphate group is cleaved
oﬀ as the enzyme
catalyzes the attachment of
glucose to the growing glycogen chain. Liver and skeletal mus-
cle cells are most active in glycogen synthesis and storage.
On the other hand, when blood glucose levels
drop, glycogen lysis (splitting) occurs. ±is process is known as
ĭ-sis) (Figure 24.13, le² side). ±e
oversees phosphorylation and
cleavage of glycogen to release glucose-1-phosphate, which is
then converted to glucose-6-phosphate, a form that can enter
the glycolytic pathway to be oxidized for energy.
In muscle cells and most other cells, the glucose-6-phosphate
resulting from glycogenolysis is trapped because it cannot cross
the cell membrane. However, hepatocytes (and some kidney
and intestinal cells) contain
, an enzyme
that removes the terminal phosphate, producing free glucose.
Because glucose can then readily diﬀuse from the cell into the
blood, the liver can use its glycogen stores to provide blood
sugar for other organs when blood glucose levels drop. Liver
glycogen is also an important energy source for skeletal muscles
that have depleted their own glycogen reserves.
Athletes and Carbohydrates
A common misconception is
that athletes need to eat large amounts of protein to improve
their performance and maintain muscle mass. Actually, a diet
rich in complex carbohydrates results in more glycogen storage
in muscle, and is more eﬀective than high-protein meals for
sustaining intense muscle activity.
Notice that the emphasis is on
a candy bar before an athletic event to provide “quick” energy
does more harm than good because it stimulates insulin secre-
tion, which favors glucose use and retards fat use at a time when
fat use should be maximal. Building muscle protein or avoiding
its loss requires not only extra protein, but also extra (protein-
sparing) complex carbohydrate calories to meet the greater en-
ergy needs of increasingly massive muscles.
Endurance athletes—long-distance runners in particular—
are well aware of the practice of glycogen loading, popularly
called “carbo loading,” for endurance events. Carbo loading
“tricks” the muscles into storing more glycogen than they nor-
mally would. It generally involves eating a carbohydrate-rich
diet (75% of energy intake) for three to four days before an en-
durance event while decreasing activity. ±is practice increases
muscle glycogen stores to as much as twice the normal amount.
(all tissue cells)
(present in liver,
Glycogenesis and glycogenolysis.
supplies exceed demands, glycogenesis (conversion of glucose to
glycogen) occurs. Falling blood glucose levels stimulate glycogenoly-
sis (breakdown of glycogen to release glucose).