(averaging approximately 2.5 h) after consuming
a fat or a balanced diet even though initial muscle
glycogen levels amounted to only 76±4 mmol ·
kg–1wet weight on the fat diet vs. 143±10 mmol ·
kg–1wet weight on the balanced diet. In the
study by Lambert et al. (1994), where the
endurance test to exhaustion, at 60% of V
.
o2max.,
was performed as the last of three consecutive
tests, muscle glycogen stores on the carbohy-
drate diet amounted to 77±5 mmol · kg–1 wet
weight prior to the endurance test, and exercise
time to exhaustion lasted only 43±9min,
whereas when on the fat diet, exercise time to
exhaustion was 80±8 min, when muscle glyco-
gen levels averaged 32±6 mmol · kg–1wet weight
prior to the test. In these studies, endurance time
to exhaustion after consumption of a fat diet was
not impaired but in fact even improved despite
an initial glycogen content fourfold and twofold
lower, respectively, than in the carbohydrate
trials. Also, in the study by Helge et al. (1996),
muscle glycogen levels prior to exercise were sig-
nificantly different after 7 weeks’ adaptation to
the fat diet (128±6 mmol · kg–1wet weight) and
the carbohydrate diet (153±7 mmol · kg–1 wet
weight). However, the rate of muscle glycogen
breakdown during exercise was similar in both
trials and muscle glycogen stores were not
depleted in either group at exhaustion. This was
even more conspicuous after 8 weeks, when a
carbohydrate diet had been consumed for 1 week
after 7 weeks’ adaptation to a fat diet. In this case
muscle glycogen concentrations at exhaustion
were as high as resting values before initiating
the dietary intervention period. These observa-
tions indicate that content of muscle glycogen
prior to an endurance test does not seem to be
closely correlated to submaximal performance
time when adaptation to a fat diet for more than
14 days has been induced, whereas after acute or
a few days’ dietary manipulation, exercise time
to exhaustion seems more closely related to
initial muscle glycogen content (Christensen &
Hansen 1939; Bergström et al. 1967; Galbo et al.
1979).
The hypothesis that manipulation of dietary
fat can improve endurance performance by
200 nutrition and exercise
increasing fat oxidation and decreasing carbohy-
drate oxidation can probably be true for the rat.
However, in man there are no scientific data to
support this notion inasmuch as those few labo-
ratory studies purporting to show a benefit suffer
from serious methodological flaws. It has also
been hypothesized that if a combination of
training and the intake of a fat-rich diet was
performed, then a subsequent brief switch to a
carbohydrate-rich diet should create optimal
conditions for increased endurance because a
high-fat oxidative capacity is combined with
large glycogen stores. This hypothesis may arise
from studies in rats which have demonstrated, as
mentioned earlier, that endurance performance
time was increased after prolonged fat adapta-
tion and a subsequent brief exposure to a carbo-
hydrate-rich diet (Conlee et al. 1990; Lapachet et
al.1996). However, these findings are not sup-
ported in man. In the study by Helge et al. (1996),
trained subjects switched to a carbohydrate diet
(65 E% CHO, 20 E% fat) for another week, after 7
weeks’ adaptation to a fat diet, while continuing
their supervised training programme (T-FAT/
CHO group). Another group, also participating
in the same training programme, followed a car-
bohydrate diet through all 8 weeks (T-CHO
group). An endurance test to exhaustion per-
formed after the 8th week revealed that exercise
time, at the same relative exercise intensity (70%
V.
o2max.) as at the 7-week endurance test was
modestly increased by 18%, from 65±7 min at 7
weeks to 77±9 min in the T-FAT/CHO group.
This exercise time was, however, 26% shorter
than endurance time to exhaustion in the T-CHO
group (Fig. 14.4). It is of note that in the T-FAT/
CHO group the muscle glycogen stores were sig-
nificantly higher initially (738±53 mmol · kg–1
dry weight) than in the T-CHO group (561±
22 mmol · kg–1 dry weight). Moreover, blood
glucose concentrations were significantly higher
during exercise and at exhaustion in the
T-FAT/CHO group than in the T-CHO group.
Even so, endurance performance was still
shorter in the T-FAT/CHO group. These data
give no support to the belief that several weeks’
adaptation to a fat diet followed by a few days on