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Nutritional strategies to influence adaptations to
training
LAWRENCE L. SPRIET
1
* and MARTIN J. GIBALA
2
1
Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario N1G 2W1 and
2
Department of Kinesiology, McMaster University, Hamilton, Ontario L8S 4K1, Canada
Accepted 7 August 2003
This article highlights new nutritional concerns or practices that may influence the adaptation to training. The
discussion is based on the assumption that the adaptation to repeated bouts of training occurs during recovery
periods and that if one can train harder, the adaptation will be greater. The goal is to maximize with nutrition the
recovery/adaptation that occurs in all rest periods, such that recovery before the next training session is
complete. Four issues have been identified where recent scientific information will force sports nutritionists to
embrace new issues and reassess old issues and, ultimately, alter the nutritional recommendations they give to
athletes. These are: (1) caffeine ingestion; (2) creatine ingestion; (3) the use of intramuscular triacylglycerol
(IMTG) as a fuel during exercise and the nutritional effects on IMTG repletion following exercise; and (4) the
role nutrition may play in regulating the expression of genes during and after exercise training sessions. Recent
findings suggest that low doses of caffeine exert significant ergogenic effects by directly affecting the central
nervous system during exercise. Caffeine can cross the blood–brain barrier and antagonize the effects of
adenosine, resulting in higher concentrations of stimulatory neurotransmitters. These new data strengthen the
case for using low doses of caffeine during training. On the other hand, the data on the role that supplemental
creatine ingestion plays in augmenting the increase in skeletal muscle mass and strength during resistance
training remain equivocal. Some studies are able to demonstrate increases in muscle fibre size with creatine
ingestion and some are not. The final two nutritional topics are new and have not progressed to the point that we
can specifically identify strategies to enhance the adaptation to training. However, it is likely that nutritional
strategies will be needed to replenish the IMTG that is used during endurance exercise. It is not presently clear
whether the IMTG store is chronically reduced when engaging in daily sessions of endurance training or if this
impacts negatively on the ability to train. It is also likely that the increased interest in gene and protein
expression measurements will lead to nutritional strategies to optimize the adaptations that occur in skeletal
muscle during and after exercise training sessions. Research in these areas in the coming years will lead to
strategies designed to improve the adaptive response to training.
Keywords: caffeine, creatine, gene expression, muscle triacylglycerol, nutrition, training.
Introduction
Here, we highlight new nutritional concerns or
practices that may influence the adaptation to
training. We have identified four areas where recent
scientific information will force sports nutritionists to
embrace new issues and reassess old issues and,
potentially, alter some nutritional recommendations
to athletes. These are: (1) caffeine ingestion; (2)
creatine ingestion; (3) the use of intramuscular
triacylglycerol (IMTG) as a fuel during exercise and
the nutritional effects on IMTG repletion following
exercise; and (4) the role that nutrition may play in
determining the expression of genes during and after
exercise training sessions. Recent experiments exam-
ining the effects of caffeine on the central nervous
system during exercise strengthen the case for using
low doses of caffeine during training. O n the other
hand, equivocal data on the role that creatine plays in
augmenting the increase in skeletal muscle mass and
strength during resistance training may do the
opposite. Will the recent surge in information
examining IMTG use during exercise and recovery
after exercise alter our post-exercise nutritional advice
to include a greater emphasis on fat? Lastly, based on
recent measurements of gene expression, are there
nutritional strategies that we should adopt to max-
* Author to whom all correspondence should be addressed.
e-mail: lspriet@uoguelph.ca
Journal of Sports Sciences, 2004, 22, 127–141
Journal of Sports Sciences ISSN 0264-0414 print/ISSN 1466-447X online # 2004 Taylor & Francis Ltd
DOI: 10.1080/0264041031000140608
imize the adap tation that occurs in skeletal muscle
following training sessions?
This article works on the assumption that the
adaptation to repeated bouts of training occurs during
the recovery periods and that if one can train harder, the
adaptation will be greater. This can only be achieved if
recovery before the next training session is complete.
The goal is to use nutrition to maxim ize the recovery/
adaptation that occ urs in the rest periods between
training sessions. It is also clear that characteristics of
each training session (e.g. duration, intensity and
nutritional intake) will also influence the rest period
recovery/adaptati on processes.
Caffeine and the central nervous system
Traditionally, it was believed that the ergogenic effect of
caffeine during endurance exercise was due to a
peripheral mech anism. The finding that caffeine in
high doses of 5–9 mg × kg
71
BM (where BM = body
mass) spared the use of muscle glycogen early in
exercise supported this contention (Essig et al., 1980;
Spriet et al., 1992). However, recen t studies using
9mg× kg
71
BM reported a variable glycogen sparing
response (Chesley et al., 1998) and other studies have
reported no effect on glycogen use when consuming
6mg× kg
71
BM (Grah am et al ., 2000; Laurent et al.,
2000). Of course, there was always the realization that
caffeine m ay also have a central effect during endurance
exercise, but separating the peripheral and central
effects of caffeine in studies with humans is difficult,
as caffeine has the potential to affect many tissues at
once. In addition, the recent finding that caffeine is also
ergogenic during exercise of varying duration and
intensities at doses as low as 3 mg × kg
71
BM (Graham
and Spriet , 1995; Pasman et al., 1995) suggests that its
main effect is on the central nervous system (CNS). At
these low doses, the risk of adverse side-effects is greatly
diminished. Additional peripheral effects of caffeine
acting directly on skeletal muscle (inhi bition of enzymes
or alterations of ion handling) also appear unlikely given
the low plasma caffeine concentrations reported at these
low doses. This suggests that the CNS is sensitive to
lower caffeine doses and responsible for improved
performance during endurance exercise and exercise
of shorter duration.
Caffeine is known to be a CNS stimulant, causing
increased arousal, wakefulness, alertness and vigilance
as well as elevations of mood (Nehlig et al., 1992; Daly,
1993). Caffeine increases brain neurotransmitter con-
centrations, leading to increased locomotor activity and
neuronal firing in animals (Nehlig et al., 1992). It is
generally believed that the effects of caffeine are exerted
via adenosine receptor antagonism. The brain has high
levels of adenosine receptors (Fernstrom and Fern-
strom, 1984; Daly, 1993; Fredholm, 1995) and
adenosine generally decreases the concentration of the
major neurotransmitters, including seroto nin, dopa-
mine, acetylcholine, norepinephrine and glutamate.
This leads to lower motor activity, wakefulness and
vigilance. Caffei ne is an adenosine receptor antagonist
and increases the concentration of these major neuro-
transmitters. However, the exact consequences of these
changes for exercise performance are currently unclear .
Two recent studies have re-examined the role the
CNS may play in the ability of caffeine to enhance
exercise performance. Davis et al. (2003) examined the
effects of direct intracerebroventricular injections of
caffeine on the ability of rats to run to exhaustion on a
treadmill. Rats were injected 30 min before running
with either vehicle (placebo), caffeine, an adenosine
receptor agonist (5-N-ethylcarboxam idoadenosine,
NECA), or caffeine and NECA together. Rats were
able to run *80 min in the placebo trial, 120 min after
caffeine injection and only 25 min with NECA (Fig. 1).
When caf feine and NECA were given together, run
time was not different from placebo. Rats were
encouraged to run when needed by gentle hand
prodding and mild electric shock. Fatigue was defined
as the time when the rats would no longer run and
chose to rest on the electric wires despite continual
hand prodding and mild electric shocks for 30 s. When
the study was repeated with peripheral intraperitoneal
injections instead of brain injections, there was no effect
on run performance. The authors concluded that
Fig. 1. Running time for rats running on a treadmill 30 min
after direct intracerebroventricular injections of (1) vehicle
(placebo), (2) caffeine, (3) an adenosine receptor agonist (5-
N-ethylcarboxamidoadenosine, NECA) or (4) caffeine and
NECA together. *Significantly different than vehicle. Adapted
from Davis et al. (2003).
128 Spriet and Gibala
caffeine delayed fatigue through CNS effects in part by
blocking adenosine receptors (Davis et al., 2003).
A second study examined the effects of ingesting flat
cola late in a simulated cycle race. This practice is
common among endurance cyclists and was undertaken
to determine if either the ingestion of extra carbohy-
drate and/or caffeine late in exercise could increase
performance. Eight well-trained cyclists (V
˙
O
2max
,
*71 ml × kg
71
× min
71
) rode for 2 h at 70% maximal
oxygen uptake (V
˙
O
2max
) and then completed a time-
trial in which each cyclist was asked to complete 7 kJ of
work per kilogram of body mass as quickly as possible
(*30 min) on four separate occasions (Cox et al.,
2002). They consumed 5 ml of a 6% sports drink every
20 min for the first hour and then switched to the same
volume of flat cola at 80 and 100 min (and 120 min if
desired). The cola beverage was varied in the four
conditions to include: (a) no caffeine and 6% carbohy-
drate (Control); (b) caffeine (90 mg +) and 11%
carbohydrate (Cola); (c) no caffeine and 11% carbohy-
drate (Extra carbohydrate); and (d) caffeine (90 mg +)
and 6% carbohydrate (Caffeine). Performance times
(min: s) to complete the time-trials were 27:05+0:42
(Control), 26:15+0:43 (Cola), 26:55 + 0:43 (Extra
Cola) and 26:36+0:42 (Caffeine) (Fig. 2). The flat
cola, with the extra carbohydrate and caffeine, sig-
nificantly improved performance time by 50 s. Caffeine
appeared to be the most important ingredient, as
caffeine alone impro ved performance by 29 s (signifi-
cant effect for caffeine compared with no caffeine),
whereas the extra carbohydrate alone improved perfor-
mance by a non-significant 10 s. In the trials with
caffeine ingestion, plasma caffeine increased to low
levels, suggesting that the performance-enha ncing
effects were unlikely to be peripheral and more likely
to be centrally mediated. It appeared that caffeine
reduced the normal progression of fatigue late in
endurance exercise.
These recent studies add strong support to the body
of evidence suggesting that caffeine can improve
performance by directly affecting the CNS. Clearly,
this implies that caffeine in low doses sho uld enhance
the ability to perform in both training and competition.
During training, the ability to train harder should lead
to greater training adaptations.
Does creatine supplementation enhance gains
in muscle size in response to resistance
training?
Over the past decade, no other nutritional supplement
has received as much attention from athletes and
coaches as creatine, an amino acid derivative formed
naturally in the body but also present in relatively small
quantities in the diet. The topic of creatine supple-
mentation and high-intensity exercise performance is
reviewed by Maughan et al. (2004) and will not be
reviewed again here. Rather, our purpose is to review
evidence for and against a potential role for creatine in
enhancing resistance training-induced gains in skeletal
muscle size. It has been consistently reported that
creatine ingestion during heavy resistance training
augments gains in body mass, fat-free mass and
muscular strength (e.g. Earnest et al., 1995; Vanden-
berghe et al., 1997; for a recent review, see Krieder,
2003). These adaptations have often been attributed to
an accelerated rate of muscle protein accretion, but only
recently have investigators teste d this hypothesis
directly by examining changes within skeletal muscle.
The first study to directly examine creatine supple-
mentation in conjunction with heavy resistance training
on skeletal muscle hypertrophy in humans was pub-
lished by Volek and colleagues in 1999. Nineteen
resistance-trained men were randomly assigned in a
double-blind fashion to either a creatine or placebo
(cellulose) group, and then performed whole-body,
periodized heavy resistance training for 12 weeks. The
supplementation consisted of 25 g × day
71
of creatine/
placebo for the first 7 days after basel ine testing,
Fig. 2. Time taken for eight well-trained cyclists to complete
a time-trial. Each cyclist was asked to complete 7 kJ of work
per kilogram of body mass as quickly as possible (*30 min)
after cycling for 2 h at 70% V
˙
O
2max
. The participants
completed the protocol on four separate occasions and
consumed 5 ml of a 6% sports drink every 20 min for the
first hour and then switched to the same volume of flat cola at
80 and 100 min (and 120 min if desired). The cola beverage
was varied in the four conditions to include: (1) no caffeine
and 6% carbohydrate (Control); (2) caffeine (90 mg +) and
11% carbohydrate (Cola); (3) no caffeine and 11% carbohy-
drate (Extra CHO); or (4) caffeine (90 mg +) and 6%
carbohydrate (Caffeine). There was a significant main effect
for caffeine compared with no caffeine. Adapted from Cox et
al. (2002).
129Nutritional strategies and adaptations to training
followed by 5 g × day
71
until the end of the study. After
training, the creatine-supplemented group displayed
significantly greater gains in bench press and squat
strength, as well as large increases in body mass and fat-
free mass. The most notable finding, however, was a
significantly greater increase in Type I, IIa and IIb
muscle fibre cross-sectional area (CSA) in the creatine-
supplemented group compared with placebo. When
expressed as a relative change, the mean increase in
CSA for the fibre types examined was *34% in the
creatine group versus *10% in the placebo group. A
potential limitation to the study, however, was the
unexpected finding that the participants assigned to the
creatine group had *20% smaller muscle fibre areas at
baseline. Thus, while the relative incre ase in CSA was
larger in the creatine group, there were no differences
between conditions in absolute CSA for any fibre type
after training (Fig. 3).
Since the first report by Volek et al. (1999), four oth er
studies have described the effects of creatine supple-
mentation on resistive-training-induced changes in
skeletal muscle (Hespel et al., 2001; Stevenson and
Dudley, 2001; Tarnopolsky et al., 2001; Willoughby
and Rosene, 2001). These latter investigations have
produced equivocal findings, with two studies support-
ing the initial work of Volek et al. (1999) (Hespel et al.,
2001; Willoughby and Rosene, 2001) and two studies
concluding that creatine supplementation does not
augment training-induced gains in muscle size (Ste-
venson and Dudley, 2001; Tarnopolsky et al., 2001).
The lack of congruent results may be related in part to
differences in study methodology, inclu ding: the study
population (i.e. resistance-trained or untrained); mode,
frequency and duration of the traini ng intervention;
technique used to assess muscle adaptation; and,
possibly, the type of ‘placebo’ intervention employed.
Hespel and colleagues (2001) examined the effect of
creatine ingestion on the contractile, biochemical and
histochemical properties of human skeletal muscle
during immobilization and rehabilitation. Participants
had their right leg immobilized with a polyester cast for
2 weeks before participa ting in a 10-week rehabilitation
programme that consisted of 4–6 sets of unilateral knee
extension exercises, with 12 repetitions per set at an
intensity of 60% one-repetition maximum (1-RM),
three times a week. The participants were divided into
two groups and throughout the entire period of
immobilization and rehabilitation received either crea-
tine mono hydrate (initial loading dose of 20 g × day
71
,
followed by 5 g × day
71
) or a placebo (not stated). After
immobilization, quadriceps muscle CSA and isometric
knee extension torque were decreased to the same
extent in both conditions when compared with baseline.
However, muscle CSA and peak torque recovered faster
in the creatine-supplemented group than the placebo
group, when assessed after 3 and 10 weeks of the
rehabilitation period. Biopsies obtained from the vastus
lateralis revealed that type I, IIa and IIb muscle fibre
cross-sectional areas after 10 weeks of rehabilitation
when compared with baseline were only elevated in the
creatine-supplemented group. A unique aspect of that
study was that biopsy samples were also examined to
assess, for the first time in humans, the expression of the
myogenic transcription factors MyoD, myogenin , Myf5
and MRF4. There is substantial evidence from experi-
ments with rats to suggest that these factors are involved
in regulating processes intrinsic to muscle cell catabo-
lism and anabolism (e.g. Marsh et al., 1997 ; Adams et
al., 1999). After 10 weeks of rehabilitation, MRF4
protein content was higher in the creat ine-supplemen-
ted group than in the placebo group, whereas myogenin
protein showed the opposite effect, and protein expres-
sion of Myf5 and MyoD remained unchanged. The
authors suggested that changes in specific myogenic
transcription factors induced by oral creatine supple-
mentation might influence the muscle hypertrophy
response during rehabilitative strength training.
In support of the findings of Volek et al. (1999) and
Hespel et al. (2001), Willoughby and Rosene (2001)
concluded that creatine supplementation during
chronic resistance training increased muscle strength
and size, possibly as a result of increased myosin heavy
chain (MHC) synthesis. Untrained young men were
randomly as signed to a control, placebo (6 g × dextrose ×
day
71
) or creatine-supplemented group (6 g × day
71
)
Fig. 3. Cross-sectional area (CSA) of type IIa muscle fibres
in the vastus lateralis of resistance-trained volunteers before
(Pre-TR) and after (Post-TR) a 12-week periodized leg
resistance training programme, during which the participants
were supplemented with either creatine or placebo (cellulose).
The relative increases in CSA for Type I, IIa, IIab and IIb
fibres were higher for the creatine-supplemented group than
the placebo group (mean change of 36 and 10%, respectively).
However, the absolute muscle fibre CSAs were not different
between groups after training, due to the unexpected finding
that the creatine group had 20% smaller muscle CSA than the
placebo group before training. Adapted from Volek et al.
(1999).
130 Spriet and Gibala
in a double-blind fashion. They then performed 12
weeks of lower-body heavy resistance training (leg
press, leg extension, leg curl; three sets of 6–8
repetitions at 85–90% 1-RM, 3 times per week). Needle
biopsy samples from the vastus lateralis revealed that,
following training, myofibrillar protein increased by
58% in the creatine-supplemented group, and this was
significantly greater compared with the relative in-
creases in the placebo and controls groups (12% and
3%, respectively). The relativ e changes in MHC
isoform mRNA for Type I, IIa and IIx were also higher
in the creatine-suppleme nted condition than in the
placebo and control conditions. A subsequent report by
the same aut hors (Willoughby and Rosene, 2003),
based on additional analyses of muscle samples
collected during their original study, concluded that
creatine suppl ementation combined with heavy resis-
tance training increased the mRNA and protein
expression of MRF4 and myogenin. This latter finding
differs from the results of Hespel et al. (2001), although
direct comparisons between studies are hampered by
differences in study design.
In contrast to the studies described above, which
concluded creatine supplementation augments muscle
protein accretion during resistance training in humans,
two well-controlled studies from separate laboratories
have failed to support this hypothesis (Stevenson and
Dudley, 2001; Tarnopolsky et al., 2001). Stevenson and
Dudley (2001) studied 18 resistance-trained individuals
who ingested either creatine or table sugar (initial
loading dose of 20 g × day
71
for 7 days followed by
5g× day
71
thereafter) and then performe d an 8-week
electrostimulation resistive-training programme. Th e
authors’ laboratory previously showed that electrosti-
mulation can induce marked hypertrophy in a few
months without requiring voluntary effort or the
participants to alter their resistance training (Ruther et
al., 1995). The specific electrostimulation protocol
consisted of 3–5 sets of coupled concentric and
eccentric actions that were applied to the left quadriceps
femoris twice weekly while the participants continued
voluntary resistance training on both lower limbs
unsupervised. Quadriceps femoris CSA, assessed using
magnetic resonance imaging, increased after electro-
stimulation by 10% and 12% in the placebo and
creatine-supplemented groups, respectively, with no
significant differences between condition s. A notable
observation was that the CSA of the right quadriceps,
which did not receive electrostimulation but continued
to experience a chronic, unsupervised training stimulus,
increased by 5% in the creatine-supplemented group,
whereas the placebo group showed no change.
Although speculative, one potential explanation for this
finding was that the participants in the creatine-
supplemented group may have performed a larger
overall volume of unsupervised leg training and this
provided a greater cumulative stimulus to the right
(non-electrostimulated) leg compared with the placebo
condition.
Finally, Tarnopolsky and colleagues (2001) provided
a unique perspective on this issue by highlighting that
virtually all studies have compared a creatine-supple-
mented group with a group who received either a non-
isoenergetic or non-isonitrogenous placebo (e.g. cellu-
lose or dextrose). In addition, these authors noted that
few creatine supplementation studies controlled the
timing of supplement ingest ion after exercise. This is an
important consideration, given that the composition
and timing of nutrient delivery can profoundly alter
post-exercise protein metabolism and thus potentially
impact gains in mass, strength and muscle protein
balance (Tipton and Wolfe, 2004). Tarnopolsky et al.
(2001) specifically tested the hypothesis that a post-
exercise creatine–carbohydrate supplement would re-
sult in similar gains in strength and musc le fibre area as
an isoenergetic and isonitrogenous protein–carbohy-
drate supplement. In a double-blind fashion, young
male participants were ran domized to receive either
10 g of creatine + 75 g glucose or 10 g of casein + 75 g
glucose, immediately after each exercise bout during an
8-week whole-body progressive resistance training
programme. Training increased Type I and II muscle
fibre CSA by *20 and *25%, respectively, but there
was no significant differen ce between groups, and gains
in 1-RM strength for each of 16 tested exercises were
similar (Fig. 4). The only difference between conditions
was that the body mass gains were higher in the
creatine-supplemented group. From a practical stand-
point, therefore, the authors concluded that athletes
Fig. 4. Cross-sectional area (CSA) of type II muscle fibres in
the vastus lateralis of untrained volunteers before (Pre-TR)
and after (Post-TR) an 8-week progressive leg resistance
training programme, during which the participants were
provided with a creatine–carbohydrate supplement (Cr-
CHO) or isonitrogenous and isoenergetic protein (casein)–
carbohydrate supplement (PRO-CHO). The relative increases
in CSA for Type I and II fibres were not different between
groups after training. Adapted from Tarnopolsky et al. (2001).
131Nutritional strategies and adaptations to training
who are engaged in sports where a high strength:lean
mass ratio is important may wish to consider a protein–
carbohydrate supplement, whereas athletes who desire a
high absolute mass might consider a creatine–carbohy-
drate supplement.
In summary, there are equivocal data to suggest that
creatine supplementation may enhance resistance train-
ing-induced gains in muscle size, but the potential
mechanisms involved and the potential confounding
influence of nutrient composition/timi ng remain un-
clear. From a theoretical standpoint, the increase in
body water retention that typically accompanies crea-
tine supplementation could alter protein turnover
through changes in cellular hydration status (Haus-
singer et al., 1993), but there is no direct evidence for
this in human muscle following either acute or chronic
creatine ingestion. There are in vitro data which suggest
that creatine administra tion may stimulate muscle
protein synthesis (Ingwall et al., 1972, 1974), although
this is not a universal finding (Fry and Morales, 1980 ).
Vierck et al. (2003) recently reported that treatment
with creatine induced modest differentiation of myo-
genic satellite cells. This is a noteworthy observation
given that adult myofibres are terminally differentiated
and muscle regeneration after injury (e.g. myotrauma
induced by resistance exercise training) appears to be
dependent upon satellite cell activation and prolifera-
tion (Hawke and Garry, 2001). Nonetheless, the human
studies that have been conducted to date have largely
failed to detect any specific effect of creatine ingestion
on skeletal muscle protein turnover. Parise et al . (2001)
reported no effect of short-term creatine loading
(20 g × day
71
for 7 days) on whole-body or mixed-
muscle protein fractional synthetic rate in humans,
although the breakdown and oxidation of some pro-
teins were lower in men, suggesting a possible anticata-
bolic effect of creatine. However, a recent comprehen-
sive study by Louis et al. (2003) concluded that creatine
supplementation (21 g × day
71
for 5 days ) had no effect
on human muscle protein turnover (synthesis or degrad-
ation) at rest in both the post-absorptive and fed states.
Similarly, Phillips et al. (2002) examined the combined
effects of creatine supplementation and chronic res is-
tance training on skeletal muscle protein turnover in
men, and reported no significant difference compared
with a group that recei ved an isoenergetic and
isonitrogenous placebo. Given that creatine supple-
mentation facilitates muscle recovery during repeated
bouts of high-intensity exercise, it has been suggested
that strength athletes who supplement with creatine
may be able to tolerate higher training volumes and
chronically this leads to greater cumulative overload on
the muscles (Krieder, 2003). Whil e this theory appears
plausible, at present there is insufficient evidence to
directly evaluate it. Moreover, one of the few relevant
studies that accurately quantified training volumes
(Volek et al., 1999) reported greater gains in leg muscle
hypertrophy in a creatine-supplemented group, even
though the volume of leg training performed was not
different from that performed by the placebo gro up.
Clearly, additional work is warranted to clarify our
understanding of the potential interactive effect of
creatine supplementation and chronic resistance ex-
ercise training on skeletal muscle hypertrophy and the
potential regulatory mech anisms involved.
Post-exercise nutrition: is fat intake a
concern?
The importance of ingesting carbohydrates in the
minutes and hours after a training bout to maximize
the resynthesis of muscle glycogen is examined by
Burke et al. (2004). Essentially, as soon as one exercise
training session ends, the athlete is in recovery mode
and preparing for the next training session. It seems fair
to conclude that carbohydrate repletion is of paramount
importance, as most training sessions for most sports
will require muscle glycogen as a fuel for exercise.
Of late, concern has been raised about post-exercise
nutrition by a series of experiments examining the use
and replenishment of intramuscular triacylglycerol
(IMTG) during and after exerc ise. It is interesting to
note that skeletal muscle stores a significant amount of
IMTG, enough that the energy equivalent represents
67–100% of the energy stored as muscle glycoge n in
both untrained and trained individuals. Until recently,
there had been considerable controversy about whether
IMTG contributes a significant amount of fuel during
exercise where fat makes a major contribution. How-
ever, recent studies using new techniques for measuring
IMTG and examining the variability of direct assess-
ments of IMTG have largely resolved this controversy.
In sho rt, there appears to be general consensus that
IMTG is an important fuel during prolonged moderate-
intensity exercise (and up to *85% V
˙
O
2max
in well-
trained athletes) and that the fat content of the post-
exercise diet (and, therefore, carbohydrate) influences
the rate that IMTG recovers (van Loon et al., 2003;
Watt et al., 2002b).
Many laboratories have measured net IMTG use
with direct biochemical analysis of needle biop sy
samples taken from the vastus lateralis muscles of
men and reported that IMTG was not an impor tant fuel
during exercise lasting 90–120 min at a power output of
50–65% V
˙
O
2max
. Others have reported net IMTG use
during exercise in men and women (see Watt et al.,
2002b, for a review). A major criticism of this work was
that the needle muscle biopsy samples were contami-
nated by the presence of adipose tissue triacylglycerol,
132 Spriet and Gibala
making any estimation of IMTG inaccurate. The fact
that the between-biopsy (3 biopsies) variability of this
measurement was about *20–26% in a group of
untrained and active individuals supported this conten-
tion (Wendling et al., 1996). Interestingly, the within-
biopsy variability was low (*6%) and in the same range
reported for other fuels and metabolites measured in
human muscle biopsies. At the same time, almost all of
the studies that have estimated the use of IMTG during
exercise by measuring whole-body respiratory exchange
ratio (RER) and exogenous free fatty acids reported a
significant use of IMTG during prolo nged exercise. In
addition, many studies employing histochemical IMTG
staining techniques reported that IMTG was reduced
after endurance exercise (Watt et al., 2002b).
Recently, a new technique that uses
1
H-magnetic
resonance spectroscopy (MRS) to distinguish between
intra- and extramuscular triacylglycerol has been used
to examine this issue (Szczepaniak et al., 1999). Again,
the studies using this technique have for the most part
reported a net IMTG use during endurance exercise in
a variety of upper and lower leg muscles (see Watt et al.,
2002b). In addition, Watt et al. (2002a) re-investigated
the issue of IMTG use during exercise when measured
biochemically by taking double biopsy samples
throughout exercise in a group of well-trained cyclists.
They reported that the between-biopsy variability was
lower in trained (*12%) than untrained (*24%)
cyclists and that this allowed for the detection of
significant decreases in IMTG content during 2 h of
cycling at 57% V
˙
O
2max
. Therefore, the authors argued
in a recent review (Watt et al., 2002b) that much of the
controversy regarding IMTG use during exercise in the
studies employing biochemical analyses of muscle
biopsy samples is a function of two things: (1) there is
significant variability between muscle biopsy samples in
human skeletal muscle, although this is less in trained
individuals; (2) because of the high energy density of
fat, the amount of IMTG used during 90–120 min of
cycling at 50–65% V
˙
O
2max
is not large and can be less
than the between-biopsy variability in untrained/active
individuals. Another point that is apparent from these
recent studies is that adipose tissue contamination of
the biochemical estimates of IMTG is not present or is
minimal, as the measured values are in the same range
as or lower than the IMTG values reported using the
1
H-MRS technique. A final point is that the few studies
that have examin ed this issue in well-trained females all
suggest that IMTG is a significant fuel source during
endurance exercise. Interestingly, this includes one
study that used muscle biopsies and biochemical IMTG
determination (Steffensen et al., 2002), one that
deduced IMTG use from RER and plasma free fatty
acid oxidation estimate s (Romijn et al., 2000) and one
that used the
1
H-MRS technique (Larson-Meyer et al.,
2002). In summary, it would appear that there is
general consensus that IMTG is a significant source of
fuel during moderate aerobic exercise in active and
trained individuals. This may extend to intense aerobic
power outputs (*85% V
˙
O
2max
) in well-trained ind ivi-
duals. However, there still exists controversy over the
accuracy of the various methods used to estimate
IMTG use during exercise and, therefore, the magni-
tude of the IMTG contribution to total fuel use.
The practical reality of using IMTG during exercise
training sessions is that it will need to be replenished
during the recovery period between workouts. Although
it is unclear at present whether beginning an exercise
session with an IMTG store that is less than normal wi ll
actually limit the ability to exercise or train, it is clear
that an inability to replenish this store over repeated
training sessions could lead to such a situation. The
remainder of this section will highlight the recent work
that has examined the issue of IMTG repletion after
exercise.
Starling et al. (199 7) reported that the vastus lateralis
IMTG content (biochemical measurement) was higher
1 day after exercise when participants ingested a high fat
diet (68% of energy) rather than a very low fat diet
(5%). The participants first completed 120 min of
exercise at 65% V
˙
O
2max
, then consumed one of the
diets for 12 h and fasted another 12 h. The IMTG
concentration increased in the 24 h after exercise on the
high fat diet (from 32.8 to 44.7 mmol × kg
71
dry mass)
but did not recover when on the low fat diet (from 30.9
to 27.5 mmol × kg
71
dry mass). However, the high fat
diet was also a low carbohydrate diet and the repletion
of glycogen was impaired and performance during a
subsequent self-paced cycling time-trial decreased
(time to complete 1600 kJ; high fat = 139 min, low
fat = 117 min). Two other studies measured recovery
IMTG after exhaustive cycling using muscle biopsies
and biochemical techniques. In both cases, the well-
trained athletes consumed a diet high in carbohydrate
(65–70% carbohydrate, 20% fat, 10–15% protein) in
the first 18–30 h after a prolonged and exhaustive
exercise bout. Neither study found evidence of an
increase in IMTG after exercise. Kiens and Richter
(1998) reported significantly lower IMTG concentra-
tions at 3, 6, 18 and 30 h with a low point of 20% below
the immediate post-exercise value 18 h after exercise.
On the other hand, Kimber et al. (2003) reported no
significant change in IMTG after 3, 6 and 18 h of
recovery compared with the immediate post-exercise
value. Additional studies have specifically examined the
effects of two or more diets on IMTG use and recovery.
Coyle et al. (2001) examined the effect of 7-day diets
that contained 32, 22 or 2% of energy intake from fat on
IMTG and glycogen stores and subsequent fuel
utilization during 2 h of exercise at 67% V
˙
O
2max
in
133Nutritional strategies and adaptations to training
well-trained cyclists. The changes in carbohydrate
consumption were reciprocal with fat and energy intake
from protein constant at 10%, such th at all diets were
eucaloric. Participants exe rcised for 2 h at 67% V
˙
O
2max
in the morning on all days of the 7-day diets except the
first day. The pre-exercise IMTG store was unaffected
by a reduction in fat intake from 32 to 22%, but was
decreased by *20% following the 2% fat diet. Muscle
glycogen increased from a low of *530 mmol × kg
71
dry mass after consuming 32% fat to *700 and
825 mmol × kg
71
dry mass after the 22 and 2% fat
diets, respectively. During exercise, there were no
differences in estimated fuel use between the 32 and
22% fat diets, but the 2% fat diet decreased fat and
increased carbohydrate oxidation rates compared with
the 22% fat diet (RER, 0.908 vs 0.876). The amount of
plasma free fatty acid oxidation was not decreased in the
2% fat condition, implying that the reduction in whole-
body fat oxidation was solely due to a reduction in free
fatty acid oxidation from IMTG. The finding that
IMTG was well maintained even during the very low fat
intake diet led the authors to suggest that fat may be
synthesized from carbohydrate in such a low fat/high
carbohydrate conditi on.
Surprisingly, the small decrement in IMTG and
increase in muscle glycogen contents following the low
fat diet led to increased glycogen oxidation and reduced
IMTG oxidation while not affecting the amount of
extramuscular fuel that was oxidized. It is not known
how the changes in initial fuel stores predisposed the
muscle to these changes in fuel use. It could be argued
that, at an intensity of 67% V
˙
O
2max
, it doesn’t matter
whether the athlete consumes a diet of 2–22% fat, 10%
protein and the balance carbohydrate, as fuel reliance
simply follows dietary intake. However, what happens
to the IMTG store over time with repeated bouts of
training and a low fat diet?
Subsequent studies have specifically examined the
time-course of IMTG repletion after a single bout of
prolonged exercise. Decombaz et al . (2001) examined
the effect of post-exercise diets containing either 14.5 or
55.5% of the total energy intake fr om fat on IMTG and
glycogen repletion in the tibialis anterior of six
untrained men and six trained male runners at 9 and
30 h post-exercise. The protein contribution was
constant at 14% and carbohydrate was reciprocal with
fat such that diets were eucaloric. The exercise
consisted of 2 h of running or walking uphill on a
treadmill at *50% V
˙
O
2max
. The resting IMTG and
glycogen contents were higher in the muscles of trained
participants. The IMT G content decreased by about
22–26% in both groups but the absolute decrease was
almost double in the trained group (Fig. 5). Whole-
body RER averaged *0.89 and *0.93 in the trained
and untrained groups, respectively, and IMTG was
estimated to contribute *15% and *17% of whole-
body fat metabolism, respectively. Repletion of IMTG
on the high fat diet was apparent at 9 h of r ecovery and
reached values *30% higher than pre-exercise at 30 h
of recovery in both groups. Recovery on the low fat diet
produced no recovery in IMTG during the initial 9 h,
with values reaching *80% and *95% of that pre-
exercise at 30 h in the trained and untrained groups
(Fig. 5). Glycogen use during exercise was 35–43% of
the resting store in both groups. Glycogen repletion was
faster on the low fat (high carbohydrate, *70%) diet,
reaching pre-exercise values by 9 h. No further changes
occurred in the trained group but the untrained
participants supercompensated glycogen stores to
155% of pre-exercise by 30 h. In contrast, on the high
fat (low carbohydrate, *30%) diet, muscle glycogen
was not replenished to pre-exercise values until 30 h.
The authors concluded that, given the dynamic nature
of the IMTG store and the potential for it to spare
muscle glycogen, ‘it would be wise to try and optimize
[IMTG] storage before competition, at the same time
ensuring that glycogen storage is not compromised’
(Decombaz et al., 2001). However, this statement
seems premature given that there is no evidence to
demonstrate that IMTG is ever limiting for an athlete
during training or competition. Also, there is no
information determining the importance of IMTG
during the higher exercise intens ities that are employed
during training and compe titions. Exercise at power
outputs of *50% would have little relevance to a
population of trained athletes.
Larson-Meyer et al. (2002) also examined this issue
by measuring the influence of recovery diet on soleus
muscle IMTG repletion after a 2-h treadmill run at
*67% V
˙
O
2max
in seven well-trained recreational
female runners. Post-exercise diets contained either
10 or 35% of the total energy intake from fat, 15% from
protein and, therefore, 75 or 50% from carbohydrate.
Soleus IMTG was measured before, immediately after
and 22 h (*1 day) and 70 h (*3 days) after the
exercise bout. Participants were allowed to run for
45 min after the IMTG measurement on the first
recovery day and again on the second recovery day.
Exercise decreased IMTG content by *25%, and the
higher fat diet repleted IMTG by 22 h and to a value
*22% higher than pre-exercise by 70 h. In contrast,
the low fat diet did not allow complete repletion of
IMTG, reaching *88% of pre-exercise levels, even
after 70 h. Absolute IMTG numbers were not re-
ported, only changes relative to the bone marrow peak
1
H-MRS signal. The authors noted that the conse-
quences of compromised IMTG stores on the low fat
recovery diet need to be investigated during heavy
training and performance (Larson-Meyer et al., 2002).
They also stated that, ‘the current study provides
134 Spriet and Gibala
evidence that diets too low in fat are probably not ideal
for endurance athletes’.
Johnson et al. (2003) recently examined the effects
of a strenuous exercise bout, followed by a recovery
diet with varying fat contents for 2 days, on vastus
lateralis IMTG concentrations and the abilit y to
complete a subsequent cycling time-trial (*3h) in
six highly trained male cyclists. The high fat/low
carbohydrate diet contained 56% fat, 6% carbohydrate
and 37% of total energy from protein; the low fat/high
carbohydrate diet contained 19% fat, 63% carbo-
hydrate and 17% protein. Unfortunately, the diets were
not isoenergetic and the participants consumed *33%
more energy on the low fat/high carbohydrate diet.
Calculated IMTG was *10.7 and 7.0 mmol × kg
71
after the high and low fat recovery diets, respectively.
Muscle glycogen was very low after the high fat/low
carbohydrate diet and high after the low fat/high
carbohydrate diet. During the subsequent time-trial
rides, performance time was worse (*200 vs
178 min), exercise intensity was lower (66 vs 71%
V
˙
O
2max
) and RER was lower ( * 0.80 vs 0.90) after the
high fat compared with the low fat diet. Plasma free
fatty acids were higher throughout exercise after the
high fat diet and the parti cipants received a carbohy-
drate drink at timed intervals during exercise in both
trials. The IMTG concentration decreased by about
55–65% in both time-trial rides, but the absolute
amount of IMTG used was much higher in the high
than in the low fat trial (6.63 vs 3.63 mmol × kg
71
wet
weight). The authors concluded that near-depletion of
IMTG was evident in some of the athletes during
prolonged strenuous cycling, regardless of the pre-
diet. If the depletion of IMTG was prevalent in the
Type I fibres, it remains to be se en whether this could
have a negative effect on training or competition
performance.
Lastly, van Loon et al. (2003) had nine endurance-
trained athletes cycle for 3 h at 55% V
˙
O
2max
on two
occasions followed by either a diet high in fat (39% fat,
49% carbohydrate, 14% protein) or low in fat (24% fat,
62% carbohydrate and 14% protein) for 3 days. Vastus
lateralis IMTG decreased by *21% after the 3-h cycle
and had repleted towards normal in the high fat diet at
24 h and reached pre-exercise values at 48 h. In the low
fat trial, IMTG was not sign ificantly repleted even after
48 h. In addition, quantitative fluorescence microscopy
of the muscle following 48 h of recovery and ingestion
of the high fat diet revealed higher IMTG content in
Type 1 musc le fibres than with the low fat diet (2.1 vs
1.4% area staining). These results are surprising given
that the so-called ‘low fat diet’ contained 24% of the
total energy intake as fat.
In summary, it is clear that high fat intakes (35–57%
of energy) in the recovery periods following a prolonged
exercise bout will replete IMTG stores quicker than low
fat intakes (10–24% ). The amount of ingested fat
required for IMTG repletion has been estimated to be
*2g× kg
71
BM × day
71
(Decombaz, 2003). However,
a high fat intake may compromise the ability to replete
muscle glycogen and impair performance. One might
expect that diets with moderate fat intake (about 20–
30% fat) would be adequate for IMTG replenishment,
but the findings with intakes of 19 and 24% do not
support this contention. However, the 19% fat diet in
the study of Johnson et al. (2003) was confounded by a
low energy intake and although van Loon et al. (2003)
reported no repletion in 48 h with a 24% fat recovery
Fig. 5. Effects of recovery diet on intramuscular triacylglycerol (IMCL) content in the tibialis anterior of six trained male runners
(T) and six untrained men (UT). Arrows indicate exercise that consisted of 2 h of running/walking uphill on a treadmill at *50%
V
˙
O
2max
. The participants then consumed eucaloric diets containing either 55.5% (a) or 14.5% (b) of the total caloric intake from
fat (14% protein, balance carbohydrate) for the next 30 h.
135Nutritional strategies and adaptations to training
diet, the exercise-induced decrease in IMTG was only
of the order of *20%. It must be borne in mind that
there is considerable discussion in these papers regard-
ing the best way to express the IMTG data – against a
muscle water or creatine reference or a bone marrow
reference (for discussion, see Larson-Meyer et al.,
2002). Given the possibility for water shifts during
exercise and the variability in the creatine measurement
in some studies (Johnson et al., 2003), it is possible that
20% changes in IMTG content are near the detectable
limit of the
1
H-MRS technique. However, van Loon et
al. (2003) reported IMTG repeatability of the order of
5%. Recovery of IMTG was complete after only 22 h
when a 35% fat diet was consumed after exercise.
Additional studies examining fat intake after exercise in
the range of 20–30% are needed to clarify this issue. It
may be that when 2 days are available for recovery, a
diet that provides sufficient energy and includes about
2 g fat × kg
71
and 6–10 g carbohydrate × kg
71
will be
optimal. If prolonged exercise occurs nearly every day,
is a chronically low IMTG cont ent simply a reality?
This would leave the endurance athlete in a carbohy-
drate dependent state where their post-exercise diet
would concentrate on adequate carbohyd rate and
protein intake. Decombaz (2003) has suggested that
carbohydrate content should be high and fat content
low in the initial 6–8 h of recovery and then fat can be
added in the form of regular meals.
Nutrient–gene interactions
Exercise physiologists have been interested in the
ability of skeletal muscle to adapt to repeated bouts
of exercise since this field of study began. Under-
standing of the basic mechanisms that regulate these
changes has progressed tremendously in the last 10
years with the increasing use of powerful molecular
and cellular tools (Booth, 1998; Hargreaves and
Cameron-Smith, 2002). Numerous investigations have
now reported that exercise upregulates the expression
of several genes that encode for skeletal m uscle
proteins that play a role in meeting the demands of
exercise (Kraniou et al., 2000; Pilegaard et al., 2000;
Tunstall et al., 2002; Nordsborg et al., 2003; see
Hargreaves and Cameron-Smith, 2002, for a review).
It has been suggested that the cellular adaptations to
exercise training may be due to the cumulative effects
of the transient increases in gene transcription that
occur during and after repeated exercise bouts
(Williams and Neufer, 1996; Hargreaves and Camer-
on-Smith, 2002). In this section, we are interested in
particular in the recent work that has been done that
links nutrition to the adaptations that occur in human
skeletal muscle during training. Selected examples of
the consequences of short-term dietary manipulations
on gene expression in human skeletal muscle are
discussed below.
Peters et al. (2001) examined the effects of consum-
ing an isoenergetic high fat diet (73% fat, 5%
carbohydrate, 22% protein) following a standardized
pre-diet (30% fat, 50% carbohydrate, 21% protein) on
pyruvate dehydrogenase kinase (PDK) isoform mRNA
and protein content and PDK activity in human skeletal
muscle. Six active males had muscle biopsies taken
from the vastus lateralis before the high fat diet (day 0)
and in the morning 1, 2 and 3 days after being on the
high fat diet. One confounding factor in this study was
that the participants were not allowed to exercise as they
normally would have during the 3 days of the high fat
diet. The resting RER decreased progressively from
*0.8 on day 0 to *0.7 on day 3. Ketone body and free
fatty acid (0.38–0.83 mmol × l
71
) concentrations in-
creased progressively during the high fat diet while
fasting insulin decreased by 50%. The PDK activity
increased after only 1 day on the high fat diet and
continued to increase in a linear fashion while on this
diet (Fig. 6). Concentrations of both mRNA and
protein PDK 4 isoform increased dramatically after 1
day on the high fat diet and remained high with no
further increases on days 2 or 3, while PDK 2
concentrations were unchanged (Fig. 7). The linear
increase in PDK activity in the face of a constant PDK 4
protein concentration during high fat ingestion, sug-
gested that either another PDK isoform (i.e. PDK 1 or
3 – there are only four) contributed to the increase in
PDK activity or there was an increase in PDK-specific
activity. Given that it has been shown that the
Fig. 6. Pyruvate dehydrogenase kinase (PDK) activity in the
vastus lateralis of men following a normal diet (time 0) and
following 1, 2 and 3 days on a high fat/low carbohydrate diet
(73% fat, 5% carbohydrate, 22% protein).
a
Significantly
different from time 0.
b
Significantly different from ‘a’ alone.
136 Spriet and Gibala
abundance of PDK 1 and 3 are very low in human
skeletal muscle and do not respond to 40 h of fasting
(Spriet et al., 2002), it is likely that the latter explanation
is correct. There are existing data for rodent skeletal
muscle that demonstrate the abi lity of the PDH
complex to increase the binding of newly formed
PDK 4 protein or existing PDK 2 protein, thereby
accounting for the increased PDK-specific activity. This
is an impressive adaptive quality for an enzyme that
plays such an important role in the oxidation of
carbohydrate both at rest and during exercise.
The PDH complex quickly responds to a lack of
carbohydrate availability and/or increased fat oxidation
by upregulating PDK activity, which drives more of the
PDH enzyme into the inactive form and ultimately
decreases skeletal muscle carbohydrate oxidation and
spares the small store of carbohydrate in the body.
Experiments that have artificially elevated plasma free
fatty acid for 4–5 h have reported very large increases in
PDK 4 mRNA levels, underscoring how rapidly fuel
availability can upregulate PDK gene expression and
decrease carbohydrate oxidation in skeletal muscle (R.
J. Tunstall, unpublished observations). We also recently
examined the response of 4 h of exercise at *55%
V
˙
O
2max
on PDK activity, as it has been shown that
carbohydrate oxidation and PDH activity decrease as
exercise is prolonged beyond 2–3 h (Watt et al., 2002a).
Muscle biopsies were taken at rest, and at 10 min and
4 h of exercise. Carbohydrate oxidation and PDH
activity (at 4 h) decreased and plasma free fatty acid
concentration and fat oxidation increased during
exercise as expected. The PDK activity was unchanged
at 10 min but doubled at 4 h with no changes in PDK 2
and 4 protein levels (Peters et al ., 2003). Again, but this
time in an exercise context , PDK increased and
appeared to account for the decreasing PDH activity
without an increase in total PDK protein. The
participants cons umed only water during this trial, so
it is not known whether supplemental carbohydrate
ingestion could reverse these changes both at the gene
expression and enzyme activity levels.
Cameron-Smith et al. (2003) examined the effects of
either a high carbohydrate diet (70–75% carbohydrate,
515% fat) or an isoenergetic high fat diet (465% fat,
529% carbohydrate) for 5 days on the expression of
genes encoding proteins for fatty acid transport and
beta-oxidation in human skeletal muscle. The partici-
pants were 14 well-trained cyclists and triathletes
(V
˙
O
2max
=67 ml× kg
71
× min
71
) who continued to train
daily during the 5-day diet interventions. Plasma free
fatty acids were significantly higher after 5 days on the
high fat diet compared with baseline and after the high
carbohydrate diet (*0.85 vs 0.39–0.41 mmol × l
71
).
Carbohydrate oxidation was reduced and fat oxidation
was increased during 20 min of cycling at *70%
V
˙
O
2max
following the high fat diet compared with the
high carbohydrate diet. The mRNA concentrations of
carnitine palmitoyltransferase I, uncoupling protein 3
and the plasma membrane fatty acid binding protein
(FABP
pm
) measured in the muscle s of six of the
participants were unaffected by either diet, although
there was large variability in the data. However, there
were sign ificant increases in the fatty acid transporter
(FAT/CD36) and beta-hydroxyacyl-CoA dehydrogen-
ase mRNA values following the high fat die t compared
with the high carbohydrate diet and baseline values.
FAT/CD36 protein content was al so increased in the
muscles of eight participants following the high fat diet,
although FABP
pm
was unchanged. These data provide
strong support for the idea that increased dietary fat
intake can increase the mRNA content of genes that are
necessary for the uptake and oxidation of free fatty
acids. This muscle adaptation seems entirely appro -
priate in the face of reduced availability of carbohydrate
and increased availability of plasma free fatty acids. It is
also noteworthy that these changes occurred in very
well-trained athletes, who presumably have maximized
the ability of their muscles to oxidize fat through years
Fig. 7. Pyruvate dehydrogenase kinase 2 (a) and 4 (b) isoform protein content in the vastus lateralis of men following a normal
diet (time 0) and following 1, 2 and 3 days on a high fat/low carbohydrate diet (73% fat, 5% carbohydrate, 22% protein).
a
Significantly different from time 0.
137Nutritional strategies and adaptations to training
of endurance training. It is also important to again
stress that increases in mRNA are not always predictive
of increases in protein or measures of functional activity
(as discussed above). There are many additional steps
and points of regulation between increased mRNA
contents and increased protein synthesis rates. This fact
points to the need for simultaneous measurements of
mRNA and proteins and functional measures including
substrate transport and enzyme activities.
The mechanisms by which the adaptations to a high
fat/low carbohydrate diet are mediated appear to be
related to free fatty acid activation of the family of
peroxisome proliferator activator receptors (PPARs)
(Duplus et al., 2000; Jump and Clarke, 1999) and/or an
insulin effect consequent to the decreased carbohydrate
availability. However, it has been pointed out that it is
unlikely that PPAR-mediated processes are the only
mechanism by which free fatty acids can ind uce gene
expression (Duplus and Forest, 2002). Pilegaard et al.
(2002) have also recently suggested that low concentra-
tions of muscle glycogen may enhance the mRNA
content of some genes involved in exercise metabolism.
They manipulated pre-exercise muscle glycogen with a
combination of exercise and diet and found that the
PDK 4 and UCP 3 genes were upregulated to a greater
extent in response to exercise. It is not known whether
these exaggerated responses translate into greater
protein contents or higher functional activities. One
would predict that signals responsive both to increased
fat availability and decreased carbohydrate availability
would work in concert to determine the exact responses
in gene expression in skeletal muscle.
In summary, it is clear that nutrition can alter gene
expression both at rest and in combination with
exercise. However, it is important to note that the
dietary manipulations dis cussed above were for the
most part drastic and unlikely to be used by athletes
actively engaging in training and competition. It
remains to be seen whether smaller changes in dietary
carbohydrate and fat content will have effects on gene
expression independent of, or in combination with, the
training-induced changes.
Summary
In this review, we have attempted to highlight four areas
where recent scientific information has forced us to
reassess old issues and embrace new ones that may lead
to altered nutritional recommendations for athletes. We
revisited the issues of supplementing with caffeine and
creatine, supplements that are already commonly used
by athletes. Recent findings suggest that low doses of
caffeine exert a major ergogenic effect on the central
nervous system during exercise and strengthen the case
for using low doses of caffeine during training. On the
other hand, the data on the role that creatine plays in
augmenting the increase in skeletal muscle mass and
strength during resistance training remain equivocal and
further study is required. The final two topics were
nutritional issues that are new and have not progressed
to the point that we can specifically identify strategies to
enhance the adaptation to training. However, it is likely
that nutritional strategies will be needed to replenish
intramuscular fat stores in individuals engaging in
chronic endurance training. It is also likely that the
increased interest in gene and protein expression
measurements will lead to nutritional strategies that will
optimize the adaptations that occur in skeletal muscle
during and after exercise training sessions.
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