Introduction
In the search for strategies to improve athletic
performance, recent interest has focused on
several nutritional procedures which may,
theoretically, promote fatty acid (FA) oxidation,
attenuate the rate of muscle glycogen utilization
and improve exercise capacity (for reviews, see
Chapter 14 and Lambert et al. 1997; Hawley et al.
1998). The aim of this chapter is to provide the
reader with a general overview of the role of
endogenous fat as an energy source for muscular
contraction, to discuss the effects of exercise
intensity and duration on the regulation of fat
metabolism, and to give a synopsis of some of the
factors which may potentially limit FA mobiliza-
tion, uptake and oxidation by human skeletal
muscle during exercise.
Fat as an energy source for
physical activity
The human body utilizes carbohydrate (CHO),
fat and, to a lesser, extent protein as fuel for mus-
cular work. Fat as an energy source has several
advantages over CHO: the energy density of
fat is higher (37.5 kJ · g–1(9 kcal · g–1) for stearic
acid vs. 16.9 kJ · g–1(4 kcal · g–1) for glucose), there-
fore the relative weight as stored energy is lower.
FAs provide more adenosine triphosphate (ATP)
per molecule than glucose (147 vs. 38 ATP).
However, in order to produce the equivalent
amount of ATP, the complete oxidation of FA
requires more oxygen than the oxidation of CHO
(6 vs. 26 molecules of oxygen per molecule of
substrate for the complete oxidation of glucose
and stearic acid, respectively). Using CHO as a
fuel, 21 kJ (5 kcal) of energy are available for
each litre of oxygen used, whereas only 19.7 kJ
(4.7 kcal) per litre of oxgyen are available when
fat is the sole fuel oxidized: this may be impor-
tant when the oxygen supply is limited.
On the other hand, for every gram of CHO
stored as glycogen, approximately 2 g of water
are stored (Holloszy 1990). Consequently, the
amount of glycogen stored in muscle and liver is
limited to about 450 g in an average-sized adult.
Of interest is that although skeletal muscle com-
prises up to 40% of body mass in well-trained
individuals, CHO utilization by muscle in the
resting or postabsorptive state is minimal,
accounting for less than 10% of total glucose
turnover (Felig & Wahren 1975).
Fat can be stored in much greater amounts. In a
healthy, untrained male, up to 20 kg of fat can be
stored, mainly in adipose tissue: in the obese
individual, the fat store may exceed 100 kg. Even
in highly trained athletes with much lower levels
of adipose tissue, endogenous fat stores still far
exceed the requirements of all athletic pursuits.
Both FA stored in adipose tissue and fat enter-
ing the circulation after a meal can serve as
potential energy sources for the muscle cell (Fig.
13.1). For humans ingesting a typical Western
diet (approximately 35% of energy from fat), FAs
are comprised of approximately 40% oleate, 25%
palmitate, 15% stearate and 10% linoleate. The
remainder is thought to be a mixture of both