NUTRITION IN SPORT - Index of

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 contains
a significant store of glycogen in the sarcoplasm.
The glycogen content of skeletal muscle
at rest is approximately 54–72 g · kg –1 dm (300–
400mmol glucosyl units ·kg –1 dm). 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 concentration
at about 5mmol ·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. Skeletal
muscle also contains some triacylglycerol
(about 50 mmol·kg –1 dm) which can be used as
energy source during exercise following lipolysis,
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 37kJ·g –1, whereas CHO
releases 16 kJ·g –1. Each gram of CHO stored also
retains about 3g of water, further decreasing the
efficiency of CHO as an energy source. The energy
cost of running a marathon is about 12000kJ;
if this could be achieved by the oxidation of
Table 2.3 Energy stores in the average man.
biochemistry of exercise 29
fat alone, the total amount of fat required would
be about 320g, whereas 750 g of CHO and an
additional 2.3kg 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 intracellular
body fluids is quite low (e.g. total free amino
acid concentration in muscle sarcoplasm is about
50mmol ·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 muscular
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 eventually
be oxidized either by deamination and conversion
into one of the intermediate substrates in
the TCA cycle, or conversion to pyruvate or acetoacetate
and eventual transformation to acetyl-
CoA. During starvation and when glycogen
Mass (kg) Energy (kJ) Exercise time (min)
Liver glycogen 0.08 1 280 16
Muscle glycogen 0.40 6400 80
Blood glucose 0.01 160 2
Fat 10.5 388500 4856
Protein 12.0 204000 2550
Values assume a body mass of 70kg 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 80kJ·min -1).