Adaptations to a High Fat Diet

Chapter 14
Adaptations to a High Fat Diet
BENTE KIENS AND JØRN W. HELGE
Historical perspective
It is known from practical experience, obtained
from the large number of polar expeditions
occurring from the middle of the last century,
that the dietary intake of indigenous people and
their domestic animals (sledge dogs) had a very
high fat content. As early as 1908, August and
Marie Krogh studied the metabolism of the
Greenland Eskimo where food consumption was
calculated, based on observations made by the
Danish explorer Rink in 1855 (Krogh & Krogh
1913). Despite such a fat-rich diet, these indigenous people and their dogs seemed to maintain normal work capacity and normal body
function.
In the laboratory, several groups of scientists
from the turn of the century have tried to elucidate what substrate is oxidized in muscle during
exercise. Short-term dietary changes, mostly
to fat-rich and carbohydrate-rich diets, were
applied. Zuntz et al. (1901) demonstrated that
respiratory quotient (RQ) values during mild
exercise after a fat-rich diet were of a magnitude
that suggested an almost exclusive oxidation
of fat. This was later supported by Krogh and
Lindhard (1920) and Marsh and Murlin (1928).
In the studies by Krogh and Lindhard (1920),
the subjects were asked to describe their food
intake and the perception of daily living chores
and exercise sessions while eating a fat-rich or a
carbohydrate-rich diet for 3–5 days. The subjects
almost uniformly described that exercise was
performed easily after consumption of the carbo-
192
hydrate diet, while exercise was performed with
severe difficulty after consumption of the fat diet.
Krogh and Lindhard (1920) also demonstrated
that the muscular efficiency, measured on a
Krogh bicycle ergometer positioned within a
Jaquet respiration chamber, was some 10–11%
more effective while carbohydrates were oxidized than while fat was oxidized. These findings were later supported by Hill (1924) and
Marsh and Murlin (1928).
The work by Christensen and Hansen (1939)
revealed a lower respiratory exchange ratio
(RER) during exercise and a shorter endurance
performance time at a submaximal exercise
intensity after 3–5 days’ adaptation to a fat diet
than after 3–5 days’ adaptation to a carbohydrate
diet. Thus, interest in the influence of diet on
work capacity is not new, but during the last 50
years, focus has mainly been on the role of
dietary carbohydrates for enhancing physical
performance. However, because athletes today
participate in physically demanding events of
ever-increasing duration, it has been speculated
whether habitually eating a high-fat diet could
provide some of the adaptations that are produced by habitual physical exercise and thus
improve physical performance.
Endurance performance in rats
In animals, the effect of adaptation to a fat-rich
diet on endurance performance has mostly been
investigated in rodents and less often in dogs and
other animals. Studies in rats adapted to a
adaptations to a high fat diet
fat-rich diet have shown a positive effect on
endurance performance. However, in most
studies, fat-rich diets that are practically carbohydrate free have been used. For instance, in the
study by Miller et al. (1984), endurance performance 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 composition to those diets utilized in the study by
Miller et al. (1984). In both of these studies, training 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 interventions 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 carbohydraterich 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 carbohydrate 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 carbohydrate diet (69 E% carbohydrates, 10 E% fat). They
193
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 performance 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 demonstrated that after 4 weeks of training and adaptation to a fat-rich diet containing 15 E%
carbohydrates, endurance performance was similarly enhanced compared with that of rats which
had been exposed to a carbohydrate-rich diet
(Helge et 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, followed by 3 weeks on a 40 E% fat diet. Then the
rats were trained on a treadmill for 16 weeks
while fed ad libitum one 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 significance 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 highfat 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-
194
nutrition and exercise
tigated whether animals adapted to a prolonged
high-fat diet could tolerate a second bout of exercise following 3 days of recovery consuming a
carbohydrate-rich diet compared with animals
adapted to a prolonged high-carbohydrate diet
after consuming a fat-rich diet. Even though
Conlee and co-workers (1990) found that fat-fed
rats ran equally long as carbohydrate-fed rats,
switching the diet for the last 3 days resulted in
better endurance performance by fat-adapted
animals switched to the carbohydrate diet for 3
days than carbohydrate-fed animals continued
on the carbohydrate diet for 3 more days. Also, in
the study by Lapachet et al. (1996), when training
was combined with diet for 8 weeks, rats ran
approximately 40% longer when the rats, after fat
adaptation, switched to the carbohydrate diet for
3 days than when the rats were fed only a carbohydrate diet. Thus, in rats endurance performance time was increased after prolonged fat
adaptation and a subsequent brief exposure to a
carbohydrate-rich diet.
In rats the literature reveals a fairly uniform
positive effect of fat-rich, virtually carbohydratefree diet on endurance performance in rats,
whereas there is an apparent discrepancy regarding the effect of dietary fats on endurance performance in man.
Endurance performance in man
It is well known from the classic literature that
increasing the dietary fat relative to carbohydrates results in increased fat and decreased
carbohydrate utilization during submaximal
exercise (Christensen & Hansen 1939). Thus, it
has been hypothesized that increasing the availability of fatty acids for oxidation might increase
the oxidation of fat and spare carbohydrate and
furthermore increase performance. Due to this
hypothesis, acute dietary and pharmacological
methods have been used to enhance the availability of fatty acids for oxidation. In the study by
Griffiths et al. (1996), eight subjects consumed
either a fat-rich meal (65 E% fat, 28 E% carbohydrate, 7 E% protein) or a carbohydrate meal (2
E% fat, 80 E% carbohydrate, 18 E% protein) and
were followed over the next 6 h, while resting.
Prior to the fat meal the plasma concentration of
free fatty acids (FFAs) amounted to 400 mmol · l–1.
One hour after ingestion of the fat meal, the
plasma concentration of FFA had decreased to
200 mmol · l–1, whereafter plasma FFA increased
continuously to 500 mmol · l–1 at 4 h and approximately 550 mmol · l–1 at 6 h. Thus, the intake of
80 g fat, as in this study, was not associated with
any particular increase in circulating fatty acids
during the following 6 h. Studies have established that glucose feeding prior to exercise produces hyperglycaemia, inducing stimulation of
insulin secretion, which in turn depresses the
exercise-induced lipolysis and increases RQ,
indicating an increased participation of carbohydrates in the total energy expenditure. The question is whether fat feeding prior to exercise
would enhance the oxidation of fat at the
expense of carbohydrate during exercise. This
question was addressed in the study by Satabin
et al. (1987). Nine trained male subjects either
were fasting or ingested a pre-exercise meal
(1.7 MJ, 400 kcal) 1 h prior to a submaximal exer.
cise test (60% of Vo2max.) to exhaustion. The meals
contained either medium-chain triacylglycerols,
long-chain triacylglycerols or glucose. During
exercise, plasma insulin concentrations were
decreased in all conditions. The FFA concentrations were increased similarly after the two lipid
meals and in the fasting situation and markedly
higher than that in the glucose trial, and RQ
was significantly lower in the lipid trials and in
the fasting condition than in the glucose trial.
Despite the enhanced fat oxidation during exercise, after the consumption of a fat meal, no
differences in endurance time (approximately
110 min) between any of the four dietary trials
were seen. Also, in studies in which intralipidheparin was infused during exercise, the availability of fatty acids was markedly increased. In
the study by Hargreaves et al. (1991), a sparing of
muscle glycogen during exercise was not seen,
whereas a decreased rate of glycogen degradation was found in another study (Dyck et al.
1993). Endurance performance was, however,
not measured in any of these studies. Also, the
adaptations to a high fat diet
3
Delivery of FFA (µmol.min–1)
**
(a)
FFA uptake (µmol.min–1)
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;
Kiens et al. 1993). It seems, however, that the concentration at which saturation occurs is somewhat 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 endurancetrained cyclists. They were studied during
.
30 min of exercise at 84% of Vo2max., on two different 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 lipidheparin. In the control trial, total fat oxidation
amounted to 27 ± 3 mmol · kg–1 · min–1. Even
though the availability of FFA in the lipidheparin 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 concentration was succesfully elevated, no clear effects on
endurance performance were demonstrated. A
reason for this could be that the FFA uptake
195
2
**
**
**
*
1
*
0
0
60
200
120
**†
150
*
100
50
–19
–27
0
(b)
60
Exercise (min)
120
Fig. 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 (䊊) or the
endurance-trained (䊉) 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).
plateaus around 700–1000 mmol · 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-
196
nutrition and exercise
2.0
1.5
1.0
0.5
0.0
(a)
40
*
Fat oxidation (µmol.kg–1.min–1)
Plasma FFA concentration (mmol.l–1)
2.5
Control
30
20
10
0
Lipid
heparin
*
Control
(b)
rich diet is superior to that when a fat-rich diet
is consumed. Thus, in the classic study by
Christensen and Hansen from 1939, three trained
subjects consumed either a fat-rich diet (containing only 5 E% carbohydrates) or a carbohydraterich diet (90 E% carbohydrates) for 3–5 days.
Exercise to exhaustion at approximately 65–70%
of maximal oxygen uptake revealed an average
endurance time on the carbohydrate diet of
210 min, which was markedly longer than when
on the fat diet (90 min). Also, when intermittent
exercise (30 min running followed by 10 min rest)
at 70% of maximal oxygen uptake was performed in trained men, endurance performance
time to exhaustion was significantly impaired
after consuming a fat diet, consisting of 76 E%
fat, 13.5 E% protein, for 4 days (62 ± 6 min) compared with when a carbohydrate-rich diet (77 E%
carbohydrate, 13.5 E% protein) was consumed
for 4 days (106 ± 5 min) (Galbo et al. 1979). Also,
the short-term studies by Bergström et al. (1967)
and Karlsson and Saltin (1971) suggested that
3–7 days of fat diet were detrimental to exercise
performance. Thus, it is evident from these brief
dietary manipulations that ‘fat-loading’ impairs
endurance performance. However, in these
short-term dietary studies, the primary goal was
to determine the extent to which muscle glycogen content could be altered by varying the
dietary regimen after depletion of the glycogen
Lipid
heparin
Fig. 14.2 (a) Plasma free fatty acid
(FFA) concentrations, and (b) total
fat oxidation during a 20–30-min
exercise period for six subjects
during a control trial and during
intralipid infusion. Subjects
exercised for 30 min at 85% of
maximal oxygen uptake. *,
P < 0.05 compared with control
trial. Adapted from Romijn et al.
(1995).
stores and subsequently to ascertain the relation
between the individual muscle glycogen content
and the capacity for prolonged exercise. Thus,
these short-term carbohydrate-restricted diets
probably reflect rather acute responses to
changes in diet.
Longer-term adaptation to fat-rich diets may,
on the other hand, induce skeletal muscle adaptations, metabolic as well as morphological,
which in turn could influence exercise performance. It has been known for a long time that
endurance training induces several adaptations
in skeletal muscle such as increased capillarization, increased mitochondrial density, increased
activity of several oxidative enzymes (Saltin &
Gollnick 1983) and, furthermore, as recently
shown, an increased content of fatty acid binding
protein in the sarcolemma (FABPpm) (Kiens et al.
1997), parameters that all are suggested to play a
significant role in enhancing lipid oxidation.
It might be speculated that a way to influence
the fat oxidative system further, is to increase the
substrate flux of fatty acids through the system
by increasing the fat content of the diet. This
might result in further adaptations in the fat
oxidative capacity, providing possibilities for an
increased fat oxidation, a sparing of carbohydrates and an increasing endurance performance. Thus, in the study by Muoio et al. (1994),
five well-trained runners followed a dietary
adaptations to a high fat diet
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 Vo2max. for 30 min and then at 75–80% of
.
Vo2max. until exhaustion, was longer following
the fat diet (91 ± 10 min) than after both the
normal, mixed (69 ± 7 min) and the carbohydraterich 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 (approximately 430 g · day–1). Furthermore, a maximal
exercise test was performed before the submaximal endurance test only separated by a short
break, and this inevitably confounds the interpretation of dietary effects on endurance performance. 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.
Lambert et 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 libitum or 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 Vo2max. (8.3 ± 2 vs. 12.5 ±
4 min for HF and HC, respectively) were not
impaired after the fat diet. Moreover, during a
197
subsequent prolonged submaximal exercise test
.
at approximately 60% Vo2max., endurance performance 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 glycogen content twofold lower (32 ± 6 mmol · kg–1 wet
weight) than in the carbohydrate-adapted trial
(78 ± 5 mmol · kg–1 wet weight). However, the
subjects performed three consecutive tests on the
same day only separated by short rest intervals
and the submaximal endurance test to exhaustion 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 (45min 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% carbohydrate, 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 consumed all three diets. The exercise experiments
were performed with 2-week intervals at power
.
outputs equal to 50% and 70% of Vo2max.. The
subjects maintained their training throughout
the 2 months required to complete a series of
experiments. It was reported that exercising at
.
50% Vo2max. time to exhaustion was not different
between the three diets. However, maximal possible work time was 270 min and due to that,
several of the subjects were stopped before they
.
were exhausted. At 70% Vo2max., exercise time to
exhaustion was not different between the standard (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). Submaximal endurance performance was studied in
five well-trained bicyclists fed a eucaloric balanced diet (EBD) for 1 week, providing 147–
198
nutrition and exercise
210 kJ · kg–1 · day–1 (35–50 kcal · kg–1 · day–1) , 1.75 g
protein · kg–1 · day–1 and the remainder of calories
as two-thirds carbohydrates and one-third fat.
This was followed by 4 weeks of a eucaloric ketogenic diet (EKD), isocaloric and isonitrogenous
with the EBD diet, but providing fewer than
20 g carbohydrates daily. The subjects continued
their normal training throughout the study.
.
Endurance time to exhaustion, at 60–65% Vo2max.,
was longer in three subjects (57%, 30%, 2%) and
shorter in two (36%, 28%) after 4 weeks’ adaptation to EKD, resulting in no statistical difference
in the mean exercise time after the two dietary
trials (147 ± 13 min for EBD vs. 151 ± 25 min for
EKD). However, the big variability in performance time of the subjects makes the results difficult to interpret. A highly significant decrease in
RQ values during the endurance test was found
and in agreement with this a threefold drop in
glucose oxidation and a fourfold reduction in
muscle glycogen use were demonstrated.
To summarize, so far the literature has provided a conflicting picture when the effect of
dietary fat on endurance performance is investigated in man. These disparate results could be
explained by the varied research designs used,
making firm conclusions impossible. Moreover,
dietary manipulations for only 4 weeks may not
be long enough to induce adaptations in skeletal
muscle of importance for endurance exercise
capacity. Also, one might speculate whether
training status, as indicated by maximal oxygen
uptake of the subjects, could be of any significance. In the study by Helge et al. (1998), the
interaction between training and diet was investigated. Fifteen initially non-trained male subjects were randomly assigned to consume a fat
diet (62 E% fat, 21 E% carbohydrate, 17 E%
protein) or a carbohydrate diet (20 E% fat, 65 E%
carbohydrate, 15 E% protein) while following a
supervised training programme for 4 weeks.
Training was performed four times weekly and
each training session alternated between short
.
and long-lasting intervals at 60–85% of Vo2max.,
lasting 60 min. After the 4-week intervention
.
period, Vo2max. was similarly increased by 9% in
both dietary groups (P < 0.05). Endurance perfor-
mance time to exhaustion, measured on a Krogh
.
bicycle ergometer, at 72% of Vo2max. (same
absolute power output as in the initial nontrained trial), was similarly and significantly
increased in both dietary groups both after 2 and
4 weeks of training and dieting (Table 14.1).
Thus, comparing the trained subjects in the fat
group with those in the carbohydrate group after
4 weeks, exercising at the same relative workload
.
(72% of Vo2max.), no differences in exercise time to
exhaustion were found between the two dietary
groups (79 ± 8 min in the fat group vs. 79 ± 15 min
in the carbohydrate group). Thus, it appears that
adaptation to a fat diet in combination with training up to 4 weeks, exercising at a submaximal
.
intensity (60–70% of Vo2max.), does not impair
endurance performance (Phinney et al. 1983;
Helge et al. 1998). However, in the study by Helge
et al. (1996), two groups of non-trained male subjects underwent a 7-week supervised training
programme while consuming either a fat diet (62
E% fat, 21 E% carbohydrate, 17 E% protein) or a
carbohydrate diet (20 E% fat, 65 E% carbohydrate, 15 E% protein). Maximal oxygen uptake
increased similarly in the two groups by 11% (P <
0.05). Time to exhaustion, exercising on a Krogh
.
bicycle ergometer at 82% of pretraining Vo2max.,
was significantly increased, from initial mean
values for the two groups of 35 ± 4 min to 65 ±
7 min in the fat group, but significantly more in
the carbohydrate group (102 ± 5 min). Thus, combining these findings it is apparent that the
Table 14.1 Endurance performance (mean ± SE,
measured in minutes) until exhaustion before and after
2 weeks’ and after 4 weeks’ adaptation to training and
a fat-rich or a carbohydrate-rich diet.
Before
After
2 weeks
After
4 weeks
Fat-rich diet
29.5 ± 4.3
47.8 ± 8.1*
78.5 ± 8.2*
Carbohydraterich diet
31.7 ± 4.3
59.5 ± 10.6*
79.3 ± 15.1*
From Helge et al. (1998).
* P < 0.05 compared to before values.
adaptations to a high fat diet
training-induced increase in endurance performance is less when a major part of daily energy
intake is covered by fat for a period longer than 4
weeks than when carbohydrates made up the
major part of daily energy intake (Fig. 14.3). Furthermore, comparing the trained subjects, exercising at the same relative exercise intensity, time
to exhaustion is significantly shorter when a fat
diet has been consumed for a longer period than
when a carbohydrate diet has been consumed.
Summarizing these studies, it appears that a
further increase in endurance performance will
be impaired when a fat diet is continued beyond
4 weeks.
It is not clear why prolonged elevated dietary
**
100
*
Time to exhaustion (min)
80
*
60
40
20
0
0
2
4
Time (weeks)
7
Fig. 14.3 Endurance performance to exhaustion
measured on a Krogh bicycle ergometer before and
after 2, 4 and 7 weeks of endurance training when
consuming a fat-rich diet ( ) or a carbohydrate-rich
diet (䊐). *, P < 0.05 compared with 0 week in both diets;
**, P < 0.05 compared with the fat-rich diet after 7
weeks. Adapted from Helge et al. (1996, 1998).
199
fat intake attenuates the improvement in
endurance performance in man. One aspect of
significance in the adaptation to dietary fat could
be the capacity of enzymes involved in the fat
oxidation as a strong correlation between bhydroxy-acyl-CoA-dehydrogenase (HAD) activity and fatty acid uptake and oxidation has been
demonstrated in man (Kiens 1997). In the study
by Helge and Kiens (1997), the activity of HAD
was increased by 25% after 7 weeks’ adaptation
to a fat-rich diet, irrespective of whether subjects
were trained or not. Furthermore, after 4 weeks’
adaptation to a fat-rich diet, carnitine palmitoyl
transferase (CPT I) activity was increased by 35%
and hexokinase activity was decreased by 46%
(Fisher et al. 1983). Putman et al. (1993) demonstrated that the PDHa activity, the active form of
pyruvate dehydrogenase (Reed & Yeaman 1987),
was higher after 3 days’ adaptation to a high-fat
diet than after adaptation to a high-carbohydrate
diet. Preliminary data from our laboratory
(unpublished data) also reveal that a fat-rich diet
per se, consumed for 4 weeks, induces a significant increase in the FABPpm. Thus, allowing for
the complexity of this issue, it seems fair to conclude that a fat-rich diet consumed for a longer
period increases the capacity for fatty acid transport and oxidation. Despite this adaptation,
training-induced increases in endurance performance are nevertheless impaired compared with
when a carbohydrate diet is consumed during
training. Thus, the fat oxidative capacity does not
by itself seem to be decisive for endurance. Other
explanations have to be found. Possible mechanisms could be increasing sympathetic activity
with time when a fat-rich diet is consumed or
changes in phospholipid fatty acid membrane
composition induced by dietary fat intake over a
longer time (Helge et al. 1996).
The relation between muscle glycogen content
and the capacity for prolonged submaximal exercise is evident in the brief dietary studies. The
question is whether content of muscle glycogen
is of the same significance for endurance performance during prolonged dietary adaptations.
In the study by Phinney et al. (1983), endurance
.
performance, at 60–65% of Vo2max., was similar
200
nutrition and exercise
(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–1 wet weight on the fat diet vs. 143 ± 10 mmol ·
kg–1 wet weight on the balanced diet. In the
study by Lambert et al. (1994), where the
.
endurance test to exhaustion, at 60% of Vo2max.,
was performed as the last of three consecutive
tests, muscle glycogen stores on the carbohydrate diet amounted to 77 ± 5 mmol · kg–1 wet
weight prior to the endurance test, and exercise
time to exhaustion lasted only 43 ± 9 min,
whereas when on the fat diet, exercise time to
exhaustion was 80 ± 8 min, when muscle glycogen levels averaged 32 ± 6 mmol · kg–1 wet 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 significantly different after 7 weeks’ adaptation to
the fat diet (128 ± 6 mmol · kg–1 wet 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 observations 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
increasing fat oxidation and decreasing carbohydrate oxidation can probably be true for the rat.
However, in man there are no scientific data to
support this notion inasmuch as those few laboratory 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 adaptation and a subsequent brief exposure to a carbohydrate-rich diet (Conlee et al. 1990; Lapachet et
al. 1996). However, these findings are not supported 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 carbohydrate diet through all 8 weeks (T-CHO
group). An endurance test to exhaustion performed after the 8th week revealed that exercise
time, at the same relative exercise intensity (70%
.
Vo2max.) 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 significantly 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
adaptations to a high fat diet
120
*
*
100
**
Time to exhaustion (min)
80
60
201
days) leads to a deterioration of endurance performance when compared with ingestion of a
carbohydrate-rich diet.
3 Adaptation to a fat-rich diet, in combination
with training, for a period of 1–4 weeks does not
attenuate endurance performance compared
with adaptation to a diet rich in carbohydrates,
but when dieting and training are continued for
7 weeks, endurance performance is markedly
better when a carbohydrate-rich diet is
consumed.
4 No benefit is obtained when switching to a carbohydrate-rich diet after long-term adaptation to
a fat-rich diet, compared with when a carbohydrate-rich diet is consumed all along.
40
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20
0
7
8
Time (weeks)
Fig. 14.4 Endurance performance to exhaustion
measured on a Krogh bicycle ergometer after 7 weeks’
training on a fat-rich diet ( ) or a carbohydrate-rich
diet ( ) followed by an additional week of training
during which both groups consumed the
carbohydrate-rich diet. *, P < 0.05 compared with the
fat-rich and combined diets, respectively; **, P < 0.05
compared with the fat-rich diet. Adapted from Helge et
al. (1996).
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athlete before an event.
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