regimen lasting 7 days. The runners performed
two different treadmill tests after consuming
either a normal diet, a mixed diet, a moderate
fat diet (38 E% fat, 50 E% carbohydrates) or a
carbohydrate-rich diet (73 E% carbohydrates, 15
E% fat) assigned in this order. Running time, at
85% of V
.
o2max.for 30 min and then at 75–80% of
V
.
o2max.until exhaustion, was longer following
the fat diet (91±10 min) than after both the
normal, mixed (69±7 min) and the carbohydrate-
rich diet (76±8 min). Although these findings
suggest that a 7-day fat diet improves endurance
performance in trained males, several flaws in
the design of the study are obvious. For example,
the diets were not administered randomly and
there was no separation between the different
dietary periods. The fat diet only contained 38
E% fat and can therefore hardly be characterized
as a fat-rich diet. Besides, a dietary carbohydrate
intake of 50 E% resulted in a daily intake of a
fairly high amount of carbohydrates (approxi-
mately 430 g · day–1). Furthermore, a maximal
exercise test was performed before the submaxi-
mal endurance test only separated by a short
break, and this inevitably confounds the inter-
pretation of dietary effects on endurance perfor-
mance. Moreover, during exercise the R-values
were similar in all three diets and although the
concentration of plasma fatty acids was highest
in the fat diet, plasma glycerol concentrations
were lower than in the two other diets. Thus, the
metabolic responses during exercise do not give
support to the concept that the longer running
time was induced by the diet.
Lambertet al. (1994) extended the dietary
intervention period to 14 days. They studied five
endurance-trained cyclists consuming, in a
random order, either a 74 E% carbohydrate diet
(HC) or a 76 E% fat diet (HF), separated by 2
weeks on ad libitumor normal diet, during which
they continued their normal training. The study
revealed that maximal power output (862±94 W
vs. 804±65 W for HF and HC, respectively) and
high-intensity bicycle exercise to exhaustion at
approximately 90% of V
.
o2max.(8.3±2 vs. 12.5±
4 min for HF and HC, respectively) were not
impaired after the fat diet. Moreover, during a
subsequent prolonged submaximal exercise test
at approximately 60% V.
o2max., endurance perfor-
mance was significantly enhanced on the fat diet
compared with when on the carbohydrate diet.
This improvement in submaximal endurance
capacity occurred despite an initial muscle glyco-
gen content twofold lower (32±6 mmol · kg–1wet
weight) than in the carbohydrate-adapted trial
(78±5 mmol · kg–1wet weight). However, the
subjects performed three consecutive tests on the
same day only separated by short rest intervals
and the submaximal endurance test to exhaus-
tion was always performed as the last test. This
design confounds the interpretation of dietary
effects on endurance performance. In contrast, in
the study by Pruett (1970), relatively well-trained
subjects performed intemittent exercise tests (45-
min bouts followed each time by a 15-min rest
period) until exhaustion after consuming either a
standard diet (31 E% fat, 59 E% carbohydrate, 10
E% protein), a fat diet (64 E% fat, 26 E% carbohy-
drate, 10 E% protein) or a carbohydrate diet (8
E% fat, 82 E% carbohydrate, 9 E% protein) for at
least 14 days. Nine subjects participated in the
study and each subject was placed on one of the
three different diets; four of the subjects con-
sumed all three diets. The exercise experiments
were performed with 2-week intervals at power
outputs equal to 50% and 70% of V.
o2max.. The
subjects maintained their training throughout
the 2 months required to complete a series of
experiments. It was reported that exercising at
50%V.
o2max.time to exhaustion was not different
between the three diets. However, maximal pos-
sible work time was 270 min and due to that,
several of the subjects were stopped before they
were exhausted. At 70% V.
o2max., exercise time to
exhaustion was not different between the stan-
dard (175±15 min) and the fat diet (164±19 min),
whereas a longer work time was observed when
on the carbohydrate diet (193±12 min) than
when on the fat diet (164±19 min).
An even longer period of adaptation to a fat
diet was studied by Phinney et al. (1983). Sub-
maximal endurance performance was studied in
five well-trained bicyclists fed a eucaloric bal-
anced diet (EBD) for 1 week, providing 147–