Download Legal nutritional supplements during a sporting event

© The Authors Journal compilation © 2008 Biochemical Society
4
Legal nutritional supplements
during a sporting event
Ceri Nicholas1
Department of Sport and Exercise Science, University of Chester,
Parkgate Road, Chester CH1 4BJ, U.K.
Abstract
Nutrition significantly influences sports performance; however, the efficacy
of any nutritional supplement or strategy should be carefully considered
in relation to the event and the sex, training and nutritional status of the
participant. The causes of fatigue, mechanism of action, safety and legality
of the supplement, together with the scientific evidence from studies with
an appropriate experimental design, should all be taken into account before
incorporating into the training and/or competition diet. The efficacy of
ingesting nutritional supplements immediately before and/or during endurance
exercise (duration 45–180 min) is reviewed in this chapter. The ingestion
of CES (carbohydrate–electrolyte solutions) have been shown to improve
both exercise capacity and performance, either due to the maintenance of
euglycaemia throughout exercise or the sparing of muscle glycogen early
on in exercise. The addition of caffeine to CES may improve endurance
performance as a consequence of a reduced perception of effort. Research
suggests that the addition of protein to CES may only be effective when
a suboptimal amount of CHO (carbohydrate) is ingested during exercise
(<60 g of CHO·h−1); however, recovery of performance may be enhanced
due to a reduction in subsequent muscle soreness and the promotion of
muscle protein synthesis after exercise. The findings from studies investigating
the effects of ingesting MCTs (medium-chain triacylglycerols) and BCAAs
1To
whom correspondence should be addressed (email c.nicholas@chester.
ac.uk).
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Essays in Biochemistry volume 44 2008
(branched-chain amino acids), either on their own or in combination with
CES, on endurance performance have been equivocal and therefore would
not be recommended. Any nutritional strategy should be practised in training
before being used during a competition.
Introduction
Ever since humans began participating in sports competitions it has been
believed that physical performance and nutrition are linked. Subsequent
research has shown that not only are ingested nutrients metabolized to provide
the ATP for muscle contraction, but altering the macronutrient intake of an
individual can significantly affect performance [1]. However, such findings
cannot be applied to all events under every circumstance. For any supplement
or nutritional strategy to be effective during a sporting event (which includes
individual and team sports), there are several factors which the athlete should
consider before taking the supplement or using the strategy. These are: (i)
what are the causes of fatigue during the event and what is the rationale for
the efficacy of the strategy or supplement?; (ii) what is the evidence for an
ergogenic effect?; (iii) was this evidence obtained from studies which included
appropriate standards of experimental design?; (iv) how do the findings relate
to the event in terms of mode, distance, intensity, characteristics of subjects?;
(v) is it safe?; and (vi) is it legal?
For the purpose of this chapter, only supplementation immediately before
and/or during prolonged exercise that can be sustained for 45–180 min will
be considered as it is generally accepted that supplementation during exercise
of less than 45 min has not demonstrated any ergogenic effects [2]. Prolonged
exercise may be performed continuously (as during cycling or running events)
or intermittently (such as during the multiple sprint sports of soccer, rugby
and hockey). Several supplements have been reported to improve performance when ingested in the days before or the hours before exercise [3–5], but
it is supplementation during the event itself that is the focus of this chapter. In
the first instance, energy metabolism and causes of fatigue will be considered.
Then it is the aim to highlight the rationale for ingestion of several nutrients
and supplements during prolonged exercise and provide a summary of their
effects on performance (see Table 1). The ingestion of CES (carbohydrate–
electrolyte solutions) alone during exercise and when combined with caffeine,
protein, BCAAs (branched-chain amino acids) and MCTs (medium-chain
triacylglycerols) will be discussed.
Energy metabolism and fatigue during prolonged exercise
The level of skill and the muscles’ capacity to perform work will determine
the ability of each athlete to perform any given task [6]. When the upper
limits have been reached, fatigue occurs and this is generally defined as
the failure to maintain an expected or required force or power output [7].
Fatigue can be physiological or psychological, and in physiological terms,
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Millard-Stafford et al. [41]
Increased CHO
Faster absorption of CHO
decreased RPE
No changes
No changes
Van Zyl et al. [68]
Jeukendrup et al. [66]
Goedecke et al. [71]
CES+MCTs
Decrement in performance
No effect
Improved 40 km TT
No effect
rates, increased ammonia
Increased fat oxidation
Madsen et al. [61]
f-TRP/BCAA ratio
Increased fat oxidation
Improved TT performance
post-exercise CK
No changes in oxidation
Increased endurance capacity
More work performed
Increased endurance capacity
Performance improved
Increased endurance capacity [18]
Performance improved
Increased endurance capacity [17]
Effect on performance/capacity
Increased endurance capacity
No changes but lower
post-exercise CK
No changes, but lower
Decreased RPE
No changes
Decreased plasma
Ready et al. [51]
Saunders et al. [49]
Alterations in insulin
Enhanced recovery
Cureton et al. [43]
Ivy et al. [48]
oxidation
Glycogen sparing [23]
Not measured
Tsintzas et al. [18,23]
Kovacs et al. [39]
glycogen
Increased fat oxidation
No changes in metabolism,
CHO oxidation
Glycogen sparing [22]
Nicholas et al. [17,22]
euglycaemia
Sparing of muscle
Effect on metabolism
Increased glucose and
Study
Coggan and Coyle [16]
Rationale
Maintenance of
CES+BCAAs
CES+protein
CES+caffeine
Supplement
CES
Table 1. Rationale for nutritional supplementation during exercise and effects on performance
CK, creatine kinase; RPE, rate of perceived exertion; TT, time trial.
C. Nicholas
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fatigue is considered to be either central or peripheral [8]. As a consequence
of fatigue, the athlete must slow down or stop, and co-ordination and skill
performance is also affected [6]. During prolonged exercise there are many
physiological
factors which determine performance, which include a high
.
V o2max, a large fractional utilization of aerobic capacity, a high capacity to
use fat as fuel and the ability to regulate body temperature [6]. The ability
to supply energy by aerobic metabolism is vital during prolonged exercise.
Anaerobic metabolism plays only a minor role at the start of exercise, or when
there is a change in pace or during a sprint finish. However, despite the oxygen
deficit observed at the start of exercise, highly trained endurance athletes
can achieve a steady state of oxygen consumption within 1 min of exercise
beginning, and, thereafter, CHO (carbohydrate) and fat oxidation become the
principal means of resynthesizing ATP [6] (see Figure 1). Exercise intensity
predominantly determines the metabolic response to exercise, though duration
is also an important factor [10]. Energy demand increases with the intensity of
exercise and there is a concomitant increase in CHO metabolism from both
plasma glucose and muscle glycogen [10] (see Figure 2). As CHO stored as
muscle and liver glycogen are limited, with 100 g and 300 g of glycogen stored
in the liver and muscles respectively [6], the availability of CHO is a potential
limitation to performance during prolonged endurance and intermittent
high-intensity exercise and thus CHO depletion is the
. main cause of fatigue
following 1–3 h of continuous exercise at 60–80% Vo2max [11]. Indeed, since
the 1960s, it has been well-documented that there is a strong relationship
Fats
Carbohydrates
Proteins
Glucose/glycogen
Amino acids
Triacylglycerols
Fatty acids + glycerol
Glycolysis
O2
Transamination
deamination
Alanine
E-oxidation
Glycine
Pyruvate
Ammonia
Urea
Lactate
Oxaloacetate
CO2
Acetyl-CoA
Citrate
TCA
cycle
NAD+
NADH H+
Acetoacetate
Glutamate
Electron transport chain
O2
H2O
Figure 1. Summary of the main pathways of energy metabolism using CHOs, fats
and protein as energy sources
TCA, tricarboxylic acid; taken from Gleeson, M. (2000) Biochemistry of exercise. In Nutrition in
Sport (R.J. Maughan, ed.). pp. 17–38, Blackwell Science, Oxford, used with permission.
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300
250
Muscle glycogen
Muscle triacylglycerol
Plasma NEFA
Plasma glucose
cal/kg/min
200
150
100
50
0
25
65
˜
% of VO2max
85
Figure 2. Maximal contribution to energy expenditure derived from glucose and
NEFA taken from blood and minimal contribution of muscle triacylglycerols and glycogen stores after 30 min of exercise, expressed as function of exercise intensity
Taken from Romijn, J.A., Coyle, E.F., Sidossis, L.S., Gastaldelli, A., Horowitz, J.F., Endert, E. and
Wolfe, R.R. (1993) Regulation of endogenous fat and carbohydrate metabolism in relation to
exercise intensity and duration. Am. J. Physiol. 265, E380–E391, used with permission.
between pre-exercise muscle glycogen content and endurance performance,
which may be continuous [12] or intermittent [13]. Thus it is not surprising
that over the last 40 years, researchers have investigated different strategies to
delay fatigue by either increasing carbohydrate availability or by decreasing
the utilization of endogenous CHO during exercise.
Ingestion of CES
The ingestion of CES has the twin aim of providing the exercising individual
with both fluid to replace the losses incurred by sweating, and fuel to
supplement the body’s limited CHO stores [14]. Regular fluid ingestion during
prolonged exercise is important and it has been demonstrated that endurance
exercise performance is impaired when dehydration exceeds approx. 2% of
pre-exercise body mass (or even less in hot weather) [15]. There is a plethora of
evidence which suggests that ingesting CES during exercise improves exercise
capacity [16–18] and performance [19,20] compared with water or other
placebo beverages. The mechanism for this ergogenic effect is probably due to
the maintenance of euglycaemia, ensuring a constant supply of glucose to the
brain and a high rate of exogenous CHO oxidation later during exercise [16]
(see Figure 3), or the sparing of muscle glycogen early on in exercise [22,23].
At this stage, it is important to distinguish between endurance ‘capacity’
and ‘performance’ as research studies under controlled laboratory conditions
have tended to use either measurement as the dependent variable. Endurance
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A
6
5.5
Plasma glucose (mM)
5
4.5
4
*
*
*
3.5
3
*
*
*
Placebo
CHO
2.5
2
1.5
1
0
1
2
3
4
3
4
Exercise time (h)
B
2.4
CHO oxidation (g/min)
2.2
2
1.8
*
1.6
Placebo
CHO
1.4
*
1.2
1
0
1
2
Exercise time (h)
Figure 3. Plasma glucose
(A) and rates of CHO oxidation (B) during exercise
.
to fatigue at 70–74% V O2peak with ingestion of either a placebo or CHO solution
every 20 min
Values are means+S.E.M. (n = 7). *P<0.05, difference from CHO. Taken from Hargreaves, M.
(2000) Carbohydrate replacement during exercise. In Nutrition in Sport (R.J. Maughan, ed.),
pp.112–118, Blackwell Science, Oxford, used with permission.
capacity measures the time to exhaustion at a fixed work rate, whereas endurance performance measures the time taken to complete a fixed distance or
workload, or is a measure of the amount of work done or distance covered in
a predetermined time. The measurement of endurance capacity can be a valid
approach, but results should be viewed with some caution as the reliability of
some protocols has been called into question [24], as has the ecological validity
[25]. In addition, it is difficult to extrapolate the results obtained from measures of capacity to a race situation. Large differences in performance can be
demonstrated; however, in a simulated race situation, the advantage would be
reduced to no more than a few percent, and in real competition it would probably be even less.
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For optimal hydration and CHO ingestion, it is recommended that an
athlete drinks 600–1400 ml·h−1 of a 4–8% CES in small doses, every 15 min
during exercise [26]. Such a strategy ensures optimal delivery of CHO and
a maximum CHO oxidation rate of 1.0–1.2 g·min−1 [27]. Improvements in
endurance exercise performance and capacity are generally seen when CHO is
ingested at a rate of 30–60 g·h−1 and when the exercise duration is 1 h or more
[17,19,23]. Indeed, the ingestion of more than 60 g·h−1 of single CHOs (such
as glucose, sucrose or glucose polymers) does not result in higher rates of
exogenous CHO oxidation [27]. Exogenous CHO availability does not tend
to limit performance of less than 1 h duration, as it has been recently shown
that a 40 km cycling time trial performance (which lasted approximately 1 h)
was improved when subjects had a 6% CHO solution mouth rinse compared
with a placebo mouth rinse [28]. The authors proposed that the mechanism for
improved performance was that the CHO solution stimulated CHO receptors
in the mouth, resulting in an increase in motivation.
It should be pointed out that not all studies have reported an improvement
in performance and capacity with CHO feedings during exercise [29–31]. The
reason for these different results is not clear, and may be partly explained by
differences in the type and amount of CHO given, by the different exercise
models used, and by differences in the training status and nutritional status of
the subjects studied. However, it should be noted that no study has reported
an adverse effect of CHO ingestion (with or without electrolytes) during exercise on performance.
Which CES is best?
There is general agreement in the literature that sports drinks which have
CHO concentrations above 6–10% do not produce additional performance
benefits [16,32]. The ideal composition of a sports drink will vary, depending
upon environmental conditions (temperature and humidity), the intensity and
duration of exercise and on the physiological and biochemical characteristics of
the individual athlete [33]. When the ambient temperature is warm, and when
performance during prolonged events may be limited by dehydration, it is vital
to replace fluid and, possibly, the electrolytes lost in sweat during exercise. In
this situation, where fluid replacement is the first priority, the CHO content
of the drinks should be low. Availability of ingested fluids depends on the rates
of gastric emptying and intestinal absorption (for more information, see [33]).
Where performance may be limited by CHO availability, then the concentration
should be increased to 6–10% CHO. Increasing the CHO content of the drinks
will increase the amount of fuel which can be supplied, but will tend to decrease
the rate at which water can be made available [14] (see Figure 4).
More recently, researchers have investigated whether the co-ingestion of
multiple types of CHO, other macronutrients (such as protein) or supplements
(for example caffeine, BCAAs, MCTs) further increase performance, either by
increasing the rate of exogenous CHO oxidation compared with single CHOs,
or by a different mechanism.
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A
180
0.4% glucose
5% glucose
10% glucose
20% glucose
Volume emptied (ml)
160
140
120
100
80
60
40
20
0
0
10
20
30
40
50
60
40
50
60
Time (min)
80
B
0.4% glucose
5% glucose
10% glucose
20% glucose
Glucose emptied (g)
70
60
50
40
30
20
10
0
0
10
20
30
Time (min)
Figure 4. Effects of glucose concentration on the rate of gastric emptying in six
healthy male subjects at rest
(A) Volume of solution emptied after ingestion of 200 ml of flavoured water (0.4% glucose) or
solutions containing 5%, 10% or 20% glucose. (B) Amount of glucose emptied from the stomach
when these solutions are consumed. Although the volume emptied is generally greatest for the
lowest glucose concentrations, the amount of glucose emptied tends to increase as the glucose
concentration increases. Taken from Maughan, R.J. (1992) Fluid and electrolyte loss and replacement in exercise In Foods, Nutrition and Sports Performance (C. Williams & J.T. Devlin, eds). pp.
147–178, Routledge, London, used with permission.
Ingestion of multiple CHOs
An increased rate of CHO oxidation (approx. 1.7 g·min−1) was reported in
a study when glucose, fructose and sucrose were ingested simultaneously at
high rates (2.4 g·min−1) compared with the ingestion of an isocaloric amount of
glucose (approx. 1.2 g·min−1) [34]. Other researchers that have combined the
ingestion of glucose, sucrose and maltose or glucose (and glucose polymers)
and fructose have found similar results, in a thermo-neutral environment
[35–37], and in the heat [38]. This increase in the rate of exogenous CHO is
probably due to the fact that the intestinal absorption of sucrose and fructose
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is somewhat different from that of glucose [34]. However, whether this
increase in exogenous CHO oxidation and resultant decrease in endogenous
CHO oxidation translates into an increase in endurance performance is yet to
be established.
Caffeine and CHO ingestion
It has been established that in order to improve performance during endurance
exercise, the primary strategy is to limit the depletion of muscle glycogen.
This can be achieved either by increasing CHO availability during exercise by
nutritional strategies, or by decreasing CHO utilization during exercise. The
latter can occur as a consequence of an increased contribution of fatty acid
oxidation to overall energy expenditure, and therefore spare the body’s limited
CHO supply. Many strategies pre-exercise have been used (supplementation
with caffeine or carnitine, high fat diets, intravenous infusion of triacylglycerol
and heparin), which are not the focus of this chapter. Caffeine ingestion
pre-exercise has been reported to have an ergogenic effect during prolonged,
submaximal exercise and these studies and the underlying mechanisms are
reported elsewhere in this book (see Chapter 8). However, it has also been
documented that when caffeine was co-ingested with a CES beverage before
and during exercise, performance during a 1 h cycling time trial was improved
compared with a CES alone [39]. However, no metabolic or subjective
measurements were taken in this study, and therefore no explanation could
be deduced for the improvements in performance. It was suggested in a
subsequent study by Jacobson et al. [40] that the co-ingestion of caffeine with
CHO may improve time trial performance due to a decrease in subjective
effort, rather than by enhancing substrate availability. A more recent study
[41] also suggested that a reduced perception of effort contributed to improved
performance with the ingestion of CES plus caffeine during exercise in the
heat, despite no differences in substrate utilization. Yeo et al. [42]
found
.
that exogenous CHO oxidation during a 2 h cycle ride at 64% V o2max was
increased when caffeine was co-ingested with glucose, possibly due to an
increase in intestinal absorption. It has been suggested that any increase in the
rate of exogenous CHO oxidation would benefit the athlete as the reliance
on endogenous CHO stores is reduced [27]. However, performance was
not measured in this study, so any ergogenic effect can only be speculated.
Performance was measured in a study by Cureton et al. [43] who found that
CES plus caffeine ingestion resulted in more work being produced during
a 15 min performance
ride completed after a 2 h cycle alternating between
.
60% and 75% Vo2max. Perceived rate of exertion was lower and consequently
participants were able to maintain a higher rate of energy expenditure.
Not all studies have reported an ergogenic effect of CHO plus caffeine
ingestion compared with a CES alone [40,44,45]. Cox et al. [44] looked at the
effects of different protocols of caffeine intake on metabolism and performance and reported that regardless of whether caffeine was ingested before or at
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regular intervals
during exercise, time trial performance following 2 h cycling
.
at 70% Vo2peak was enhanced. A caffeine plus CES consumed during the latter
stages of exercise was equally as effective as a CES in enhancing time trial
performance.
Protein and CHO ingestion
The nutritional strategies of protein and CHO ingestion post-exercise are
well-documented elsewhere (for reviews, the reader is directed to [46,47]).
Recently it has been suggested that adding small amounts of protein (typically
20% of total energy) to a CHO beverage ingested during exercise may be more
beneficial than traditional CHO-only beverages [48,49]. It has been reported
that CHO and protein beverages increased performance time to fatigue in
trained cyclists by ⱖ29% compared with CHO ingestion alone [48,49]. The
explanation for an improved performance may be that participants in the CHO
plus protein trials received more energy than in the CHO-only trial [49].
Some researchers have suggested that the addition of protein to a CHO beverage is only effective when suboptimal amounts of CHO are ingested during
exercise [47,25]. Further to this, Saunders et al. [49] have proposed two possible
mechanisms which may otherwise explain this ergogenic effect. The protein in
the CHO plus protein beverage may facilitate faster absorption of the CHO,
and alterations in insulin stimulation may have contributed to an increased
exercise capacity. However, these mechanisms are questionable, as Ivy et al.
[48] observed higher insulin in CHO plus protein compared with water, but
there was no difference compared with the CHO-only beverage. In terms of the
observations that muscle damage and soreness may be reduced, Bloomer and
Goldfarb [50] have suggested that the ingestion of CHO plus protein may attenuate secondary or delayed onset damage, rather than decreasing the mechanical
damage initially sustained from exercise. CHO plus protein may increase protein concentrations outside the cell, which may increase protein synthesis and
repair during exercise. Further investigation is clearly warranted.
However, before an athlete concludes that it is the addition of protein
which results in performance benefits for their event, it is important to establish whether the rate of CHO ingestion is optimal for the event and whether
the performance measure mimics the competition. Indeed, many of the performance benefits reported in the literature have been during exercise tasks
to fatigue [48,49] or the rate of CHO ingestion has been less than optimal
(47 g·h−1 [48]; 37 g·h−1 [49]). Van Essen and Gibala [25] did not find any
improvement in a 80 km time trial performance when 2% protein was added
to a 6% CHO drink when the ingestion rate of CHO was 60 g·h−1. It is not
only the performance benefits during one exercise bout that are important,
but also the ability of the athlete to recover between events or training. It has
also been observed that the addition of protein to CES reduces post-exercise
muscle damage [49,51,52], and enhances muscle glycogen repletion [53] compared with CHO-only beverages. Koopman et al. [54] observed that during
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.
6 h of exhaustive exercise at 50% Vo2max, the combined ingestion of protein
and CHO improved net protein balance during prolonged moderate intensity
exercise and post-exercise recovery in trained athletes. Protein balance was
negative when only CHO was ingested, which may be linked to the observed
increase in post-exercise muscle damage. Thus as it has been previously
reported that there is no additional benefit to be gained from consuming a CES
greater than 6–10% concentration, the addition of extra energy in the form of
protein to CHO beverages may be important from a practical point of view if
it produces additional performance or recovery benefits.
Ingestion of BCAAs
It has been proposed that fatigue during prolonged exercise may have a
central as well as a physical basis. It has been suggested that an increased
concentration of brain serotonin (5-hydroxytryptamine) may contribute to
CNS (central nervous system) fatigue during prolonged exercise (the original
central fatigue hypothesis; [55]). Brain serotonin synthesis is dependent on
the availability of its amino acid precursor, free tryptophan (termed f-TRP)
and the activity of the rate-limiting enzyme tryptophan hydroxylase [56].
Both BCAAs and f-TRP compete for entry to the brain on the same amino
acid carrier. In the brain, tryptophan is converted into serotonin, and an
increase in this neurotransmitter influences behaviour, such as sleep and mood,
which may affect perceived exertion during exercise. Thus any increase in the
plasma fatty acid level [tryptophan, like fatty acids, is bound to albumin in the
plasma, so increasing NEFA (non-esterified fatty acid) displaces tryptophan
from albumin increasing its availability for uptake] or decrease in plasma
concentration of BCAAs results in an increased entry of tryptophan into the
brain, and thus central fatigue [55] (see Figure 5).
Consequently, researchers have attempted to manipulate the f-TRP/
BCAA ratio and subsequent central serotonergic activity during exercise. This
has been achieved essentially by two main nutritional strategies which involve
supplementation with BCAAs and/or CHOs before and/or during exercise.
Previous studies have shown that ingesting BCAAs before prolonged
exercise may result in performance benefits [57], but ingestion of BCAAs
alone during exercise [58–60] or combined with CHO [61] do not result in
improvements to performance. This is possibly due to the fact that any positive
effects of an increased concentration of BCAAs on brain serotonin activity, is
offset by the negative effects of increased ammonia concentration in the muscle
and brain [62]. However, CHO feedings alone during prolonged exercise do
decrease the f-TRP/BCAA ratio and result in improved endurance performance, but it is difficult to separate the central effects from the well-established
peripheral mechanisms [62]. Further to the original central fatigue theory, a
more recent hypothesis is that a high ratio of serotonin-to-dopamine results in
feelings of tiredness and lethargy and hence an earlier onset of fatigue, whereas
a low ratio may improve performance as a consequence of increased motivation
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TRP
5-
HT
5-HT
T
5-H
TRP
TR
B
r
a
i
n
P
BCAA
Al TRP
(A)
BCAA
C
a
p
i
l
l
a
r
y
f-TRP
BCAA
Al NEFA
Al
TR
P
NEFA
NEFA
TRP
TRP
5-HT -HT
5
5HT
T
5-H
5-HT
TRP TRP
TRP
TRP TRP
TRP
BCA
Al NEFA
(B)
BCAA
TRP
A
Al
TRP
Al NEFA
NEFA
f-TRP
BCAA
NEFA
NEFA
NEFA
NEFA
B
r
a
i
n
C
a
p
i
l
l
a
r
y
Figure 5. The primary components of the central fatigue hypothesis
(A) At rest, plasma concentration of BCAA, NEFA and TRP [bound and unbound to albumin
(Al)] and their proposed effects on transport of TRP across the blood–brain barrier for the synthesis of serotonin (5-HT) in serotonergic neurones. (B) Reflects the increases in NEFA, f-TRP
and f-TRP/BCAA that occur during prolonged exercise. The resulting increase in brain serotonin
synthesis can cause central fatigue. Taken from Davis, J.M. (2000) Nutrition, neurotransmitters
and central nervous system fatigue. In Nutrition in Sport (R.J. Maughan, ed.), pp. 171–183,
Blackwell Science, Oxford, used with permission.
and arousal [56]. Since tyrosine is the amino acid precursor to dopamine, it has
been proposed that an elevation in blood tyrosine during exercise would result
in both a decreased uptake of tryptophan in the brain and therefore a lower
synthesis of serotonin and therefore also a lower brain serotonin/dopamine
ratio, which may improve endurance performance [63]. However, no improvement in performance was demonstrated during a cycling time trial when tyrosine was added to a CHO solution [63].
Ingestion of MCTs and CHO
Another possible nutritional strategy to increase the oxidation of fat and spare
the body’s limited CHO store, and thereby improve performance, is to ingest
MCTs with CHO during prolonged exercise. Dietary fat supplementation
during exercise is not recommended, as not only is there a plentiful supply of
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endogenous fat, but the digestion and absorption of long-chain triacylglycerols
may take 3–4 h. However, MCTs, unlike long-chain triacylglycerols, are
rapidly absorbed, entering the liver via the portal vein and are therefore rapidly
oxidized both at rest and during exercise [64,65]. Although theoretically
plausible, the ingestion of MCTs alone have generally not produced an
improvement in performance, but have had a negative effect, probably due
to the gastrointestinal discomfort experienced by many subjects [66]. The
addition of MCTs to a CHO beverage increases the oxidation of MCTs during
exercise, and therefore they may serve as an additional fuel source for the
exercising muscle, without compromising CHO oxidation and gastrointestinal
comfort [67]. Van Zyl et al. [68] was one of the first studies to show that MCTs
co-ingested with CHO significantly improved a 40 km time trial performance
following 2 h of submaximal exercise, due to an increase in fat oxidation and
therefore a reduced reliance on CHO oxidation and subsequent sparing of
muscle glycogen. However, other subsequent studies have reported either no
difference in performance when MCTs were co-ingested with CHO following
2 h of submaximal exercise [66,69] and a 100 km time trial performance [70],
or a decrement in performance [71]. In addition, MCT ingestion had no effect
on CHO or fat oxidation rates. The lack of improvement, or decrement in
performance, during both endurance (<3 h) and ultra-endurance (>4 h) may
be due to the adverse gastrointestinal symptoms (nausea, vomiting, stomach
cramps, bloating and diarrhoea) associated with ingestion of MCT in large
amounts (~85 g) [66,69,71].
Conclusion
In conclusion, there is a plethora of research documenting the benefits on
endurance capacity and performance of ingesting CES during sporting events.
Supplementation with either BCAAs or MCTs, either on their own or in
addition to CES, do not appear to have an ergogenic effect. More equivocal is
the further advantage of ingesting caffeine or protein in addition to CES during
the event on performance and the efficacy of ingesting protein plus CES on
muscle soreness and the recovery of performance. Further research is clearly
warranted in this area.
Summary
•
•
Athletes must carefully consider the available evidence before deciding
whether a particular supplement or strategy during their event will be
effective.
Factors such as environmental conditions, the type and amount of supplement given, the training and nutritional status of participants, and
whether the performance measure mimics the competition should be
taken into account.
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A 6–8% CES ingested at regular intervals during an endurance event
that is continuous or intermittent in nature appears to be the optimal
nutritional strategy when ingested at the rate of 60 g of CHO·h−1. The
addition of protein to the CES may be beneficial to performance if suboptimal amounts of CHO are ingested, and may reduce post-exercise
muscle damage and soreness.
It is vital that any proposed nutritional strategy is practised during
training, before using it in competition.
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