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Journal of Human Kinetics volume 72/2020, 69-78 DOI: 10.2478/hukin-2019-0099 69
Section II ‐ Exercise Physiology & Sports Medicine
1 - Department of Movement/Sports Sciences, Faculty of Physical Education, Yarmouk University, Irbid, Jordan. Shafiq Irshidat St.
2 - Department of Basic Medical Sciences, Faculty of Medicine, Yarmouk University, Irbid, Jordan.
3 - Department of Clinical Sciences, Faculty of Medicine, Yarmouk University, Irbid, Jordan.
Authors submitted their contribution to the article to the editorial board.
Accepted for printing in the Journal of Human Kinetics vol. 72/2020 in April 2020.
Effects of Oral Branched‐Chain Amino Acids (BCAAs) Intake
on Muscular and Central Fatigue During an Incremental Exercise
by
Mohammad Fayiz AbuMoh'd1, Laila Matalqah2, Zainalabidden Al-Abdulla3
The aim of this study was to investigate the effects of oral branched-chain amino acids (BCAAs) intake on
muscular (creatine kinase and myoglobin) and central (serotonin) fatigue during an incremental exercise protocol and
to determine the time to exhaustion. Sixteen male long-distance runners (25.7 ± 2.0 yrs) performed two trials, 14 days
apart. Using a double-blind, placebo-controlled, randomised crossover design, participants ingested either 20 g of
BCAAs (BCAA trial) or a placebo 1 hour prior to performing an incremental exercise session on a treadmill. The
starting speed was 8 km/h and this was increased by 1 km/h every 5 minutes until volitional exhaustion. Blood analysis
indicated that plasma levels of serotonin were lower in the BCAA trial (259.3 ± 13.5 ng/ml) than the placebo trial
(289.1 ± 14.5 ng/ml) (p < 0.05). There was a similar pattern of results for free fatty acid (p < 0.05). The creatine kinase
level was higher in the BCAA trial (346.1 ± 33.7 U/L) than the placebo trial (307.3 ± 30.2 U/L). No significant
difference between trials was observed regarding the level of myoglobin (p = 0.139). Time to exhaustion was longer in
the BCAA trial (50.4 ± 2.3 min) than the placebo trial (46.6 ± 3.2 min). In conclusion, oral intake of 20 g of BCAAs 1
hour prior to an incremental treadmill exercise session increased time to exhaustion, probably due to the reduction in
serotonin concentration. As myoglobin levels were within the normal range in both trials, we conclude that the
participants did not reach muscular fatigue.
Key words: Serotonin, free fatty acid, endurance exercise, myoglobin.
Introduction
Muscle fatigue is the main limiting factor
for physical performance during prolonged
exercise and is characterised by impaired
excitation-contraction coupling (Marshall et al.,
2014; Presland et al., 2005). Several studies have
reported on metabolic and neuromuscular factors
that cause fatigue during prolonged exercise in
athletes (Presland et al., 2005; Skurvydas et al.,
2011). However, fatigue can occur in the absence
of underlying mechanisms. It has been suggested
that depletion of muscle glycogen and
hypoglycaemia reduce muscle function during
long-distance running (Bailey et al., 1993). Fatigue
mechanisms may also include muscle metabolite
accumulation (Bingham et al., 2017; Skurvydas et
al., 2011), production of reactive oxygen species
(ROS) (Nielson et al., 2008), and altered motor
unit recruitment patterns (Bingham et al., 2017).
Additionally, it is well documented that leakage
of intracellular enzymes into plasma that are
associated with decreased metabolic stability,
such as creatine kinase (CK) and myoglobin could
affect performance (Koo et al., 2014; Skurvydas et
al., 2011). However, cessation of prolonged
exercise occurs not because of muscular fatigue,
but due to central fatigue (Eichelberger and
Bilodeau, 2007; Skurvydas et al., 2011).
Central fatigue has been described as a
reduction in muscle force due to reduced central
drive (Buhot et al., 2000; Millet et al., 2003) and is
70 Effects of oral branched-chain amino acids (BCAAs) intake...
Journal of Human Kinetics - volume 72/2020 http://www.johk.pl
associated with increased plasma levels of the
neurotransmitter serotonin (Marshall et al., 2014;
McLellan, 2013). Serotonin (5-hydroxytryptamine,
5-HT) is distributed throughout the brain (Buhot
et al., 2000) and has been shown to modulate
functions mediated by the cerebral cortex (Gendle
and Golding, 2010). Synthesis of 5-HT is
dependent on plasma levels of free tryptophan
(TRP), which is the precursor of 5-HT (Gendle and
Golding, 2010; McLellan, 2013). Increases in the
level of 5-HT in the brain are dependent on the
ratio of free TRP to large neutral amino acids
(LNAAs), such as branched-chain amino acids
(BCAAs) (Gendle and Golding, 2010; McLellan,
2013; Nemet and Eliakim, 2007).
BCAAs (leucine, isoleucine and valine),
which form one-third of total muscle protein
(Mero, 1999), are the only amino acids
metabolised in skeletal muscle (Koo et al., 2014).
Oral BCAAs ingestion is commonly used by
athletes (Choi et al., 2013; Wisnik et al., 2011) as a
nutritional supplement (Burke, 2001; Nemet and
Eliakim, 2007) to reduce central fatigue by
lowering 5-HT activity (Choi et al., 2013) and
increasing dopamine synthesis and release (Bailey
et al., 1993), to trigger neural signals (Zheng et al.,
2013), and to enhance physical performance (Choi
et al., 2013). Several studies have indicated that
taking BCAAs before and during prolonged
exercise can alleviate central fatigue (Bailey et al.,
1993; Burke, 2001; Nemet and Eliakim, 2007) and
muscular fatigue (Areces et al., 2015; Bailey et al.,
1993; Nemet and Eliakim, 2007), but another
study found that BCAAs administration had no
effect on performance (Nemet and Eliakim, 2007).
Some of the studies into effects of BCAAs on
serotonergic pathway response were not
conducted on humans (Choi et al., 2013; Forrest et
al., 2004; Jakeman et al., 1994). Choi and
colleagues (2013) concluded that in rats oral
BCAAs supplements reduced brain levels of
serotonin and catecholamine during exercise.
Despite this body of research, it remains unclear
whether central or muscular fatigue is the main
cause of exhaustion during prolonged exercise.
Hence, the aim of this study was to examine
whether, in long-distance runners, oral intake of
BCAAs would reduce muscular (CK and
myoglobin) or central (serotonin) fatigue during
an incremental exercise protocol when compared
with a placebo and to determine time to
exhaustion under both conditions. We
hypothesised that oral BCAAs intake would
attenuate serotonin levels during exercise and
thus enhance performance.
Methods
Participants
Sixteen male long-distance runners (age:
25.7 ± 2.0 years; body height: 172.6 ± 4.5 cm; body
mass: 64.7 ± 4.5 kg; BMI: 21.0 ± 1.3 kg/m², resting
heart rate: 61.2 ± 2.2 bpm, VO2 max = 50.4 ± 7.2
ml/kg/min) who were members of the Jordan
Military Sports Federation (population size = 21)
participated in this study. They were healthy and
free of any medical problems. Participants were
asked to abstain from energy drinks and all
ergogenic substances throughout the study.
Volunteer athletes provided written consent prior
to participation. The participants were instructed
to maintain their regular training schedule, but to
abstain from intense exercise for 72 hours prior to
each trial. The study was approved by the internal
Health Research Committee at the Yarmouk
University.
Experimental design
This study was a randomised, double-
blind trial with a crossover design. The protocol
consisted of two trials: a BCAA (supplement) trial
and a placebo trial, 14 days apart. Both trials
started in the early morning (8:30 AM) to avoid
effects of a circadian rhythm, as it is well
established that athletic performance is influenced
by the circadian rhythm (Russo et al., 2015).
Similarly, physical performance is associated with
a biological rhythm, which influences both the
mental and physical energy that an athlete needs
to perform at a given level (Chtourou et al., 2012).
It is, therefore, difficult to compare results
obtained at different times of the day. Three days
prior to the commencement of the study, blood
samples (10 ml) were collected from the
antecubital median vein at 8:30 AM, after an
overnight fast (approximately 9 hours) to
determine baseline levels of the variables under
investigation. The results are shown in Table 1.
During this preliminary session, demographic
information about participants was collected and
participants were familiarised with the treadmill.
Procedures
One hour before starting the exercise trial,
the athletes ingested an oral BCAAs supplement
by Mohammad AbuMoh'd et al. 71
© Editorial Committee of Journal of Human Kinetics
or a placebo. After 45 minutes, the athletes were
asked to warm up for 5 minutes on a treadmill at
a speed of their choice (not exceeding the starting
speed of the test protocol) and to stretch the lower
limbs for 4-5 min. After 5 minutes of rest, the
incremental exercise protocol started. At first, the
athlete ran at 8 km/h on a treadmill (Techno-Gym,
Lifefitness-6322; USA). Every 5 minutes the speed
was increased by 1 km/h until volitional
exhaustion was reached. Athletes were asked to
raise their hand to indicate exhaustion so that
distance and time to exhaustion could be recorded
instantly. Blood samples were taken from all
athletes immediately after the completion of each
trial.
Supplementation protocol
For the BCCA trial, athletes ingested 20 g
of BCAAs dissolved in 400 ml of water and 200 ml
of strawberry juice one hour prior to the start of
the incremental exercise protocol. Each gram of
BCAAs supplement contained 300 mg of valine,
250 mg of leucine, and 100 mg of isoleucine. For
the placebo trial, participants ingested a mixture
of 400 ml of water and 200 ml of strawberry juice
one hour prior to the test. Athletes ingested both
drinks while sitting in the laboratory.
Blood sample analysis
The blood samples were centrifuged for
biochemical analysis. Plasma was stored in a plain
tube, centrifuged at 3500 rpm for 10 min and a
serotonin level was analysed using (Cobas, 6000,
Roche, Germany). Serum levels of free fatty acid
(FFA) were analysed with RIA (Elecsys, 2010,
Switzerland), serum CK using (Cobas, C 111,
Roche, Germany) and serum myoglobin with
(Immunlyte, 210, USA). The assays of FFA, CK,
and myoglobin were stored in plain tubes and
centrifuged at 5000 rpm for 5 min.
Statistical analysis
The Statistical Package for the Social
Sciences Software (SPSS) version 18.0 was used
for all analyses. A paired sample t-test was used
to assess between-trial differences in biochemical
responses and time to exhaustion. Data are
presented as Mean ± SD. The significance level
was set at p < 0.05.
Results
Time to exhaustion
Figure 1 illustrates that pre-exercise
intake of BCAAs (50.4 ± 2.3 min) increased time to
exhaustion by 3.818 min compared to the placebo
(46.6 ± 3.2 min), with an effect size of 1.3 (t = -
4.134, p = 0.001). The average maximal speed was
19 km/h in the BCAA trial and 18 km/h in the
placebo trial. The normality of test was 0.9 for
both the placebo and the BCAA trial.
Furthermore, the 95% confidence interval of the
difference was -5.7 at the lower bound and -1.8 at
the upper bound.
Table1
Baseline values of biochemical variables
Variables Value
Mean ± SD
Plasma serotonin (ng/ml) 165.796 ± 13.840
Serum free fatty acid (mmol/l) 0.353 ± .0589
Serum creatine kinase (U/L) 95.31 ± 22.387
Serum myoglobin (ng/ml) 37.25 ± 6.952
72 Effects of oral branched-chain amino acids (BCAAs) intake...
Journal of Human Kinetics - volume 72/2020 http://www.johk.pl
Figure 1
Time to exhaustion in the BCAA and placebo trials. *Significantly different
from the placebo trial (p = 0.001).
Figure 2
Plasma serotonin in the BCAA and placebo trials. *Significantly different
from the placebo trial (p = 0.001).
by Mohammad AbuMoh'd et al. 73
© Editorial Committee of Journal of Human Kinetics
Figure 3
Serum free fatty acid in the BCAA and placebo trials. *Significantly
different from the placebo trial (p = 0.020).
Figure 4
Serum creatine kinase in the BCAA and placebo trials.*Significantly
different from the BCAA trial (p = 0.001).
74 Effects of oral branched-chain amino acids (BCAAs) intake...
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Figure 5
Serum myoglobin in the BCAA and placebo trials. Serum myoglobin
is not significantly different between trials (p = 0.139).
Biochemical variables
Figures 2–5 illustrate biochemical variables in
both trials. Analysis revealed that oral BCAAs
intake decreased plasma serotonin levels
compared to the placebo (t = -6.7, p = 0.001), with
an effect size of 2.1 (BCCA: 259.3 ± 13.5 ng/ml;
placebo: 289.1 ± 14.5 ng/ml) (Figure 2). The serum
FFA level was lower in the BCAA trial (0.6 ± 0.08
mmol/l) than in the placebo trial (0.697 ± 0.080
mmol/L; t = -2.591, p = 0.020), with an effect size of
0.9 (Figure 3). The serum CK level was slightly
higher after the BCCA trial (346.1 ± 33.7 U/L) than
the placebo trial (307.3 ± 30.2 U/L; t = 3.8, p =
0.001), with an effect size of 1.2 (Figure 4).
Myoglobin levels were similar after both trials
(BCCA: 69.6 ± 7.9 ng/ml; placebo: 74.0 ± 11.1
ng/ml; t=-1.562, p = 0.139), with an effect size of 0.4
(Figure 5). The normality of the test in these
variables was 1.0, 0.8, 0.07, and 0.8 for serotonin,
FFA, CK, and myoglobin, respectively.
Furthermore, the 95% confidence interval of the
difference was -39.274 at the lower bound and -
20.335 at the upper bound, -0.132 at the lower and
-0.012 at the upper, 17.554 at the lower and 59.946
at the upper, and -10.343 at the lower and 1.593 at
the upper bound for serotonin, FFA, CK, and
myoglobin, respectively.
Discussion
The purpose of this study was to
investigate the effect of oral intake of BCAAs on
biomarkers of muscular and central fatigue and to
determine time to exhaustion, in long-distance
runners, during an incremental treadmill exercise
test. We hypothesised that the intake of BCAAs
supplement would alter serotonin concentration
during exercise and that this would enhance
performance.
Time to exhaustion
We found that the time to exhaustion was
increased by pre-exercise administration of
BCAAs, which could be due to the role of BCAAs
by Mohammad AbuMoh'd et al. 75
© Editorial Committee of Journal of Human Kinetics
in sustaining prolonged physical effort (Burke,
2001; Nemet and Eliakim, 2007). This result is
consistent with the study of Mittleman et al.
(1999), which found that in a sample of 13 trained
subjects, ingestion of 9.4 g or 15.8 g of BCCAs
(women and men, respectively), 1 hour prior to
performance of a cycling exercise protocol
increased time to exhaustion (BCAA trial: 153
min; placebo: 137 min). The present study used a
different mode of exercise, namely treadmill
running. Another study (Stepto et al., 2011)
showed that LNAAs enhanced agility
performance by 5% and time to exhaustion by 3%
in a sample of 15 sub-elite male soccer players.
They examined the effects of 43 g of the LNAA
supplement and a mixture containing 45.3 g of
protein on fatigue, motor skills, and mental
performance. They concluded that changes in
TRP:LNAAs ratio contributed to enhanced
performance.
Improvements in time to exhaustion have
been attributed to the effect of BCAAs on
psychomotor performance (Burke, 2001). Our
findings are consistent with those of Wisnik et al.
(2011), who examined the effects of ingestion of 7
g of BCAAs in soccer players. The BCAAs were
administered 1 hour prior to a multiple-choice
reaction time test during treadmill exercise
(running-walking speed varied from 0 to 6.4 ms to
simulate a soccer game). The results showed that
reaction time was 10% lower for a BCAA trial
compared to a placebo trial (p < 0.05), but the
authors were unable to suggest a mechanism by
which the effect of BCAAs on psychomotor
performance could be linked to arousal. It has,
however, been suggested that BCAAs
supplementation reduces muscle soreness, which
delays the perception of fatigue (Slater and
Phillips, 2011), which could explain why BCAA
administration increases time to volitional
exhaustion.
Biochemical variables
The main finding of the study was that
the plasma serotonin concentration following
exercise to exhaustion was lower in the BCAA
trial compared to the placebo trial. This
neuropharmacological result could be due to
attenuation of the elevation in a TRP:BCAAs ratio
by administration of BCCAs (Burke, 2001; Nemet
and Eliakim, 2007). It is well documented that
prolonged physical exercise increases serotonin
levels (Steinberg et al., 1998) rather than
intermittent exercise (Eichelberger and Bilodeau,
2007). This is mainly due to slower oxidation of
BCAAs during the recovery period (Nemet and
Eliakim, 2007) and subsequently, it prevents an
increase of plasma TRP concentration (Steinberg
et al., 1998). However, in our study the plasma
serotonin level was higher in both trials compared
to baseline value (165.7 ± 13.8 ng/ml; Table 1) and
normal range (68–232 ng/ml), indicating that
during exercise the participants reached the point
of ‘central fatigue’, due to the serotonergic system.
The athletes in our study may have experienced
an increase in serotonin concentration that
minimised the CNS drive (Abbiss and Laursen,
2005) and had an adverse effect on excitation-
contraction coupling in the later stages of the
placebo trial. We suggest that central fatigue was
lower in the BCCA trial, due to administration of
BCAAs.
Not surprisingly, there was a decrease in
plasma FFA in the BCAA trial compared to the
placebo trial. This result may be explained by the
role of BCAAs in facilitating delivery of energy to
the working skeletal muscles (Burke, 2001; Nemet
and Eliakim, 2007) and attenuating lipolysis
during incremental treadmill exercise. In
biochemical terms, muscular fatigue during
prolonged physical exercise has been attributed to
an increase in plasma FFA levels (Bailey et al.,
1993). This could explain why time to exhaustion
was lower in the placebo trial. This finding agrees
with that of Wisnik et al. (2011), who reported
that FFA concentration during the first half of a
soccer game was higher in a placebo trial than in a
BCAA trial. Many researchers have demonstrated
that exercising on a bicycle ergometer produces
much more plasma FFAs levels post-exercise in
endurance athletes than untrained individuals
(Bailey et al., 1993; Men'shikov, 2004; Nemet and
Eliakim, 2007; Presland et al., 2005; Romijn et al.,
1993). However, some of these studies did not
include BCAA supplementation. Additionally,
lipid oxidation is an important source of energy
during prolonged physical exercise in athletes
(Men'shikov, 2004; Tarnopolsky, 2004) and the
body’s ability to use fatty acids is increased
during endurance exercise (Tarnopolsky, 2004).
However, amino acid oxidation by skeletal
muscles can provide up to 6% of total energy
needed during prolonged endurance exercise
76 Effects of oral branched-chain amino acids (BCAAs) intake...
Journal of Human Kinetics - volume 72/2020 http://www.johk.pl
(Phillips et al., 1993), and more after BCAA
supplementation. This may explain why FFA
levels were higher in the placebo trial than the
BCAA trial.
There was an increase in CK in the BCAA
trial compared to the placebo trial. This result
could have occurred because sustained skeletal
muscle contractions (Areces et al., 2015) took
place over a longer time and at a higher speed in
the BCAA trial. Several studies have reported that
prolonged exercise elevates levels of CK,
myoglobin and lipoprotein (Nosaka and
Clarkson, 1992; Smith et al., 2004) and that these
increases are not necessarily linked to muscle
damage (Smith et al., 2004). Importantly, if this
elevation lasts for several days, its magnitude is
linked to muscle damage-induced muscular
fatigue (Nosaka and Clarkson, 1992). However, in
this study, we measured CK levels only at the
point of exhaustion. Additionally, serum CK
values in both trials of this study may not have
been related to rhabdomyolysis or muscle
damage-induced muscular fatigue. This
possibility is supported by the data on myoglobin
levels, which were within the normal range in
both trials (28–76 ng/ml). We suggest, therefore,
that our participants did not reach the “values of
CK-induced” muscular fatigue.
Limitations
This study did not measure other
hormones associated with central fatigue, such as
prolactin and tryptophan, or regulators of the
basal metabolic rate (BMR) that influence energy
balance include thyroxine and triiodothyronine.
Conclusion
In conclusion, oral intake of BCAAs had a
positive effect on long-distance runners’
performance on an incremental treadmill test.
This suggests that ingestion of 20 g of BCAAs
dissolved in 400 ml of water with 200 ml of
strawberry juice 1 hour prior to an incremental
exercise session increases time to exhaustion,
probably due to the reduction in plasma serotonin
concentration, which delays onset of central
fatigue. Our results showed that time to
exhaustion was increased by oral intake of
BCAAs, which have an important role in energy
expenditure during exercise, as well as
attenuating the exercise-induced increase in the
TRP:BCAAs ratio and thus preventing high
plasma serotonin levels.
Practical implications
Ingestion of an oral BCAAs supplement 1
hour prior to an incremental exercise session
increases BCAAs oxidation and thus reduces
plasma levels of FFAs and serotonin, which
delays central fatigue in long-distance runners.
Furthermore, ingestion of 20 g of a BCAAs
supplement 1 hour prior to an incremental
endurance exercise session improves performance
by reducing serotonergic activity.
Supplementation with 20 g of BCAAs increases
time to exhaustion because it results in
maintenance of the pre-exercise TRP: LNAAs
ratio.
Acknowledgements
This study was partially supported by the Faculty of Scientific Research and Graduate Studies in
Yarmouk University, Jordan. The authors thank Dr. Nabeel Al-Mushasha for advice on statistical analysis
and the athletes for their participation and effort throughout the study.
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Corresponding author:
Mohammad Fayiz AbuMoh'd
Department of Movement/Sports Sciences, Faculty of Physical Education,
Yarmouk University, Irbid, Jordan. Shafiq Irshidat St.
Tel: +962799913157,
ORCID: 0000-0001-6969-4643
E-mail: famohammad@yu.edu.jo
