palmitate and oleate are known). Palmitate is a
saturated 16-C FA (CH 3 (CH 2 ) 14 COOH) whose
kinetics closely resemble those of most other
long-chain FAs (Havel et al. 1964). As such, the
rate of appearance (Ra) of palmitate gives an
index of the release of FAs into the plasma. The
Ra of glycerol, on the other hand, gives an index
of whole-body lipolysis. Rates of total fat and
CHO oxidation are determined by indirect
calorimetry. The use of these methods has
allowed estimates to be made of the rates of lipid
kinetics, including the contribution to energy
expenditure from peripheral lipolysis occurring
in the adipocytes and from intramuscular
lipolysis.
During low-intensity exercise (25% of maxi-
mum oxygen uptake (V
.
o2max.)), peripheral lipo-
lysis is strongly stimulated, with little lipolysis of
intramuscular TG (Fig. 13.2). Similarly, CHO oxi-
dation appears to be met exclusively by blood
glucose with little or no muscle glycogen utiliza-
tion. Ra of FA into the plasma and their oxidation
are highest during exercise at 25% of V
.
o2max., and
decline progressively as the exercise intensity
186 nutrition and exercise
increases. Conversely, although intramuscular
TG (and glycogen) do not contribute signifi-
cantly to energy production during low-intensity
work, fat oxidation is highest during exercise at
about 65% of V.
o2max.(Fig. 13.2). At this intensity,
lipolysis in both peripheral adipocytes and intra-
muscular TG stores attains its highest rates, and
these two sources contribute about equally to the
rate of total fat oxidation. With an increase
in exercise intensity to 85% of V.
o2max., total fat
oxidation falls. This is mainly due to a suppres-
sion in the Ra of FA into the plasma, presumably
caused by the increases in circulating plasma
catecholamines, which stimulate muscle
glycogenolysis and glucose uptake. Lipolysis of
intramuscular TG does not increase substantially
with an increase in exercise intensity from 65%
to 85% of V.
o2max., indicating that lipolysis of
peripheral adipose tissue and lipolysis of intra-
muscular TG are regulated differently. Further
evidence for this hypothesis comes from studies
which have increased FA delivery (by intra-
venous infusion of lipid and heparin) during
intense (85% of V.
o2max.) exercise in well-trained
subjects (Romijn et al. 1995). These data reveal
that even when plasma FA concentration is artifi-
cially maintained above 1 mm, this only partly
restores fat oxidation to those (higher) levels seen
at more moderate intensity (65% of V.
o2max.) exer-
cise. Taken collectively, these observations indi-
cate that factors other than FA availability play
an important role in the regulation of FA oxida-
tion during high-intensity exercise (see following
sections).
With regard to the effects of exercise duration
on fat metabolism, there is little change in either
the rates of total fat or total CHO oxidation after
2 h compared with the first 30 min of exercise at
25% of V.
o2max.. However, at an intensity of 65% of
V.
o2max., there is a progressive increase in the Ra
of FA into the plasma (and presumably their oxi-
dation) and glucose availability over time. After
2 h of cycling at this intensity, there is no change
in either the rates of total fat and total CHO oxi-
dation compared with the situation after 30 min
of exercise. Thus, it is likely that the contribution
of intramuscular substrates (TG and glycogen)Energy expenditure (J.kg–1.min–1
)25
Intensity (% VO2max)65 851400120010008006004002000Muscle glycogen
Muscle triglyceride
Plasma FFA
Plasma
glucose.Fig. 13.2The maximal contribution to energy
expenditure from endogenous fat and carbohydrate,
expressed as a function of increasing exercise intensity.
FFA, free fatty acids; V
.
o2max., maximal oxygen uptake.
From Romijn et al. (1993), with permission from the
American Physiological Society.