fat-rich diet have shown a positive effect on
endurance performance. However, in most
studies, fat-rich diets that are practically carbo-
hydrate free have been used. For instance, in the
study by Miller et al. (1984), endurance perfor-
mance was evaluated after rats were exposed to a
diet consisting of 78% of total energy intake (E%)
as fat, 1 E% carbohydrate and 15 E% protein, or
a diet containing 69 E% carbohydrate, 11 E%
fat and 20 E% protein for 1 and 5 weeks. They
demonstrated that rats ran for a longer time after
adaptation to the fat diet than on the normal diet
already after only 1 week’s adaptation to the diet
(45±5 min vs. 42±4 min) and this difference was
even larger after 5 weeks’ adaptation (47±4 min
vs. 35±3 min). These findings are in contrast
to those of Conlee et al. (1990), who report
unchanged endurance performance time when
rats had been exposed for 4–5 weeks to either a
fat- or carbohydrate-rich diet, similar in compo-
sition to those diets utilized in the study by
Milleret al. (1984). In both of these studies, train-
ing status of the rats was not altered during the
dietary intervention period. However, if both
training and a fat diet induce adaptations that
increase the fat oxidative capacity, then it might
be reasoned that combining the two interven-
tions could result in an additive effect and in turn
could optimize endurance capacity. In the study
by Simi et al. (1991), where 12 weeks of training
in combination with the intake of either a
fat-rich diet (no carbohydrates included) or a
carbohydrate-rich diet (no fat included), rats ran
for a longer time after adaptation to training and
the fat-rich diet than those on a carbohydrate-
rich diet. Rats fed the carbohydrate diet were all
exhausted before 7 h of exercise, whereas half of
the fat-fed rats had to be stopped after 7.5 h of
running before becoming exhausted. However,
in that study untrained rats fed the fat-rich diet
also ran longer (68±5 min) than those fed the car-
bohydrate diet (42±4 min).
In the study by Lapachet et al. (1996), rats were
trained 5 days per week, for 2 h at a time on a
treadmill for 8 weeks while fed either a fat diet
(79 E% fat, 0 E% carbohydrates) or a carbohy-
drate diet (69 E% carbohydrates, 10 E% fat). They
found a 31% longer endurance performance time
in the fat-adapted rats than in the rats adapted to
the carbohydrate diet.
In summary, it appears that endurance perfor-
mance time in rats is not shorter but mostly
longer in fat-fed than in carbohydrate-fed rats,
both in rats adapted to training and in sedentary
rats. In these studies, the fat diets contained no
carbohydrates and a very high proportion of fat.
In a recent study, however, findings demon-
strated that after 4 weeks of training and ad-
aptation to a fat-rich diet containing 15 E%
carbohydrates, endurance performance was sim-
ilarly enhanced compared with that of rats which
had been exposed to a carbohydrate-rich diet
(Helgeet al. 1998). This study demonstrated no
effect of dietary composition on exercise time to
exhaustion in either sedentary (mean running
time to exhaustion, 50±3 min) or trained rats
(153±8 min). In the study by Tollenar (1976),
similar findings were obtained. In that study, rats
were initially fed a stock diet for 4 months, fol-
lowed by 3 weeks on a 40 E% fat diet. Then the
rats were trained on a treadmill for 16 weeks
while fed ad libitumone of three different diets
consisting of 20 E%, 40 E% or 70 E% of fat. Data
revealed that dietary fat content had no effect on
running time to exhaustion. These findings lead
to the conclusion that the relative proportion of
carbohydrate–fat content in the diet is of signifi-
cance in the adaptation to dietary fat and thus
on running time to exhaustion in rats. Enhanced
performance is apparently only observed
when the fat-rich diet is virtually free from
carbohydrates.
An interesting idea to investigate is whether
prolonged exposure to a fat-rich diet followed by
brief exposure to a carbohydrate-rich diet per se
could improve endurance performance further.
The reasoning behind such a speculation is that a
prolonged fat-diet regimen might induce a high-
fat oxidative capacity. Then after switching to a
carbohydrate-rich diet, muscle glycogen stores
are maximized and thus the muscle is provided
with both a high-fat oxidative capacity and with
large muscle glycogen stores. This approach was
first addressed by Conlee et al. (1990), who inves-