ingestion of caffeine appears to stimulate the
release of fatty acids from the fat stores, at least in
well-trained athletes, thus increasing the plasma
concentration of FFA. However, studies have
provided a conflicting picture of the effect on
endurance performance in man (Spriet 1995).
With regard to all these attempts to increase
the plasma concentration of fatty acids, one must
bear in mind that during submaximal exercise
only a small percentage (7–15%) of the arterial
plasma FFA concentration is extracted (Turcotte
et al. 1992). Moreover, from the literature it seems
as if there is a fairly linear relationship between
FFA availability and FFA uptake and oxidation
until a FFA concentration of approximately
700 mmol·l–1. Beyond this concentration, no
further uptake and oxidation of FFA appears in
non-trained subjects despite a further increase in
circulating FFA availability (Turcotte et al. 1992;
Kienset al. 1993). It seems, however, that the con-
centration at which saturation occurs is some-
what higher in trained subjects (Fig. 14.1) (Kiens
et al. 1993). By using stable isotopes, Romijn et al.
(1995) evaluated the relationship between fatty
acid availability and oxidation in six endurance-
trained cyclists. They were studied during
30 min of exercise at 84% of V
.
o2max., on two differ-
ent occasions: once during a control trial when
plasma FFA concentrations were normally low
(0.2–0.3 mmol · l–1) and again when plasma FFA
concentration was maintained between 1 and
2 mmol · l–1 by intravenous infusion of lipid-
heparin. In the control trial, total fat oxidation
amounted to 27± 3 mmol · kg–1· min–1. Even
though the availability of FFA in the lipid-
heparin infusion trial was increased severalfold,
the total fat oxidation only increased to an
average of 34± 4 mmol·kg–1· min–1 (Fig. 14.2).
Thus, the contribution of fat oxidation to energy
expenditure increased from approximately 27%
during control to approximately 35% during
lipid-heparin infusion (P<0.05).
Summarizing these findings, it appears that in
those studies in which the plasma FFA concentra-
tion was succesfully elevated, no clear effects on
endurance performance were demonstrated. A
reason for this could be that the FFA uptake
plateaus around 700–1000mmol · l–1. Another
explanation might be that increasing the fatty
acid oxidation at a given power output is not of
importance for endurance.
In dietary intervention studies lasting 3–5
days, the prevailing concept is that endurance
performance after consuming a carbohydrate-adaptations to a high fat diet 195
20015010050
–19
–27
0 60 120
Exercise (min)FFA uptake (μmol.min–1)(b)***†0 60 120*
*********Delivery of FFA (μmol.min–1)(a)3210Fig. 14.1(a) Delivery of free fatty acids (FFA) (fatty
acid concentration times plasma flow), and (b) net
uptake of FFA during 2 h of dynamic knee-extensor
exercise with either the non-trained () or the
endurance-trained () thigh. *, P<0.05 compared with
resting values; **, P<0.05 compared with previous
measurements; †,P<0.05 between non-trained and
trained. Adapted from Kiens et al. (1993).