Carbohydrate and fat stores
Carbohydrates (CHO) are stored in the body as
the glucose polymer called glycogen. Normally,
about 300–400 g of glycogen is stored in the
muscles of an adult human. Skeletal muscle con-
tains a significant store of glycogen in the sar-
coplasm. The glycogen content of skeletal muscle
at rest is approximately 54–72 g · kg–1dm (300–
400 mmol glucosyl units · kg–1dm). The liver also
contains glycogen; about 90–110 g are stored in
the liver of an adult human in the postabsorptive
state, which can be released into the circulation in
order to maintain the blood glucose concentra-
tion at about 5 mmol · l–1(0.9 g · l–1). Fats are stored
as triacylglycerol mainly in white adipose tissue.
This must first be broken down by a lipase
enzyme to release free fatty acids (FFA) into the
circulation for uptake by working muscle. Skele-
tal muscle also contains some triacylglycerol
(about 50 mmol·kg–1dm) which can be used as
energy source during exercise following lipoly-
sis, and this source of fuel may become relatively
more important after exercise training. Fat stores
in the body are far larger than those of CHO
(Table 2.3) and fat is a more efficient storage form
of energy, releasing 37 kJ · g–1, whereas CHO
releases 16 kJ · g–1. Each gram of CHO stored also
retains about 3 g of water, further decreasing the
efficiency of CHO as an energy source. The ener-
gy cost of running a marathon is about 12 000 kJ;
if this could be achieved by the oxidation of
fat alone, the total amount of fat required would
be about 320 g, whereas 750 g of CHO and an
additional 2.3 kg of associated water would be
required if CHO oxidation were the sole source
of energy. Apart from considerations of the
weight to be carried, this amount of CHO
exceeds the total amount normally stored in the
liver, muscles and blood combined. The total
storage capacity for fat is extremely large, and for
most practical purposes the amount of energy
stored in the form of fat is far in excess of that
required for any exercise task (Table 2.3).
Protein is not stored, other than as functionally
important molecules (e.g. structural proteins,
enzymes, ion channels, receptors, contractile
proteins, etc.), and the concentration of free
amino acids in most extracellular and intracellu-
lar body fluids is quite low (e.g. total free amino
acid concentration in muscle sarcoplasm is about
50 mmol · l–1). It is not surprising, then, that in
most situations, CHO and fats supply most of the
energy required to regenerate ATP to fuel muscu-
lar work. In most situations, protein catabolism
contributes less than 5% of the energy provision
for muscle contraction during physical activity.
Protein catabolism can provide both ketogenic
and glucogenic amino acids which may eventu-
ally be oxidized either by deamination and con-
version into one of the intermediate substrates in
the TCA cycle, or conversion to pyruvate or ace-
toacetate and eventual transformation to acetyl-
CoA. During starvation and when glycogenbiochemistry of exercise 29
Table 2.3Energy stores in the average man.
Mass (kg) Energy (kJ) Exercise time (min)Liver glycogen 0.08 1 280 16
Muscle glycogen 0.40 6 400 80
Blood glucose 0.01 160 2
Fat 10.5 388 500 4856
Protein 12.0 204 000 2550Values assume a body mass of 70 kg and a fat content of 15% of body mass. The value for blood glucose includes the
glucose content of extracellular fluid. Not all of this, and not more than a very small part of the total protein, is
available for use during exercise. Also shown are the approximate times these stores would last for if they were the
only source of energy available during exercise at marathon running pace (equivalent to an energy expenditure of
about 80 kJ·min-^1 ).