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doi:10.1152/japplphysiol.00625.2001 93:1590-1597, 2002. First published 5 July 2002;J Appl Physiol
Allen C. Parcell
Troy D. Chinevere, Robert D. Sawyer, Andrew R. Creer, Robert K. Conlee and
endurance exercise performance
Effects of l-tyrosine and carbohydrate ingestion on
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Effects of L-tyrosine and carbohydrate ingestion on
endurance exercise performance
TROY D. CHINEVERE, ROBERT D. SAWYER, ANDREW R. CREER,
ROBERT K. CONLEE, AND ALLEN C. PARCELL
Human Performance Research Center, Brigham Young University, Provo, Utah 84602
Received 15 June 2001; accepted in final form 26 June 2002
Chinevere, Troy D., Robert D. Sawyer, Andrew R.
Creer, Robert K. Conlee, and Allen C. Parcell. Effects of
L-tyrosine and carbohydrate ingestion on endurance exercise
performance. J Appl Physiol 93: 1590–1597, 2002. First
published July 5, 2002; 10.1152/japplphysiol.00625.2001.—
To test the effects of tyrosine ingestion with or without
carbohydrate supplementation on endurance performance,
nine competitive cyclists cycled at 70% peak oxygen uptake
for 90 min under four different feeding conditions followed
immediately by a time trial. At 30-min intervals, beginning
60 min before exercise, each subject consumed either 5 ml/kg
body wt of water sweetened with aspartame [placebo (Pla)],
polydextrose (70 g/l) (CHO),
L-tyrosine (25 mg/kg body wt)
(Tyr), or polydextrose (70 g/l) and
L-tyrosine (25 mg/kg body
wt) (CHO⫹Tyr). The experimental trials were given in ran-
dom order and were carried out by using a counterbalanced
double-blind design. No differences were found between
treatments for oxygen uptake, heart rate, or rating of per-
ceived exertion at any time during the 90-min ride. Plasma
tyrosine rose significantly from 60 min before exercise to test
termination (TT) in Tyr (means ⫾ SE) (480 ⫾ 26 mol) and
CHO⫹Tyr (463 ⫾ 34 mol) and was significantly higher in
these groups from 30 min before exercise to TT vs. CHO (90 ⫾
3 mol) and Pla (111 ⫾ 7 mol) (P ⬍ 0.05). Plasma free
tryptophan was higher after 90 min of exercise, 15 min into
the endurance time trial, and at TT in Tyr (10.1 ⫾ 0.9, 10.4 ⫾
0.8, and 12.0 ⫾ 0.9 mol, respectively) and Pla (9.7 ⫾ 0.5,
10.0 ⫾ 0.3, and 11.7 ⫾ 0.5 mol, respectively) vs. CHO (7.8 ⫾
0.5, 8.6 ⫾ 0.5, and 9.3 ⫾ 0.6 mol, respectively) and
CHO⫹Tyr (7.8 ⫾ 0.5, 8.5 ⫾ 0.5, 9.4 ⫾ 0.5 mol, respectively)
(P ⬍ 0.05). The plasma tyrosine-to-free tryptophan ratio was
significantly higher in Tyr and CHO⫹Tyr vs. CHO and Pla
from 30 min before exercise to TT (P ⬍ 0.05). CHO (27.1 ⫾ 0.9
min) and CHO⫹Tyr (26.1 ⫾ 1.1 min) treatments resulted in
a reduced time to complete the endurance time trial com-
pared with Pla (34.4 ⫾ 2.9 min) and Tyr (32.6 ⫾ 3.0 min) (P ⬍
0.05). These findings demonstrate that tyrosine ingestion did
not enhance performance during a cycling time trial after 90
min of steady-state exercise.
central fatigue; cycling; perceived exertion
RECENTLY, IT HAS BEEN HYPOTHESIZED that, during pro-
longed exercise, an increased concentration of brain
serotonin may be an important factor in the onset of
central nervous system fatigue (2–4, 8, 10, 27) and a
high serotonin-to-dopamine ratio results in fatigue
(17). Brain serotonin synthesis depends on the avail-
ability of free tryptophan, its amino acid precursor, and
the activity of the rate-limiting enzyme, tryptophan
hydroxylase (7, 10). Similarly, tyrosine is the amino
acid precursor to dopamine (32). These amino acid
precursors compete for transport across the blood-
brain barrier via the same carrier mechanism (17).
We speculated that, if tyrosine were elevated in the
blood by ingestion during exercise and competed for
transport across the blood brain barrier with trypto-
phan, increased uptake of tyrosine and a decreased
uptake of tryptophan could result in a lower brain
serotonin/dopamine ratio and improved endurance.
Limited research has been done on the effects of
tyrosine ingestion on exercise endurance. Struder et al.
(30) reported no beneficial effect of tyrosine supple-
mentation when subjects cycled to exhaustion. On the
other hand, Chaouloff et al. (10) reported that high
doses of ␣-methyl-p-tyrosine improved exercise perfor-
mance in rats, and the improved performance corre-
lated with elevated dopamine concentration in the
brain. Some investigations have shown that tyrosine
administration increases dopamine synthesis and con-
centration in the brain (1, 19, 20), whereas others have
shown that tyrosine administration leads to improve-
ment of mood and well-being in human subjects under
stress (5, 24). These results raise the possibility that
tyrosine administration during exercise could offset
feelings of fatigue and lead to improved performance.
The purpose of this study, therefore, was to test the
effects of tyrosine ingestion on endurance under condi-
tions of prolonged exercise. Because carbohydrate in-
gestion has also been shown to reduce the availability
of free tryptophan and to improve endurance for pro-
longed exercise (18), we also determined whether com-
bined tyrosine and carbohydrate supplementation has
a greater beneficial effect on endurance than either one
alone.
METHODS
Nine male competitive cyclists from the local population
participated in this study [25 ⫾ 1 yr; 182 ⫾ 2 cm; 73 ⫾ 2 kg;
peak oxygen consumption (V
˙
O
2 peak
)of4.5⫾ 0.2 l/min,
Address for reprint requests and other correspondence: A. C. Parcell,
Human Performance Research Center, Brigham Young Univ., 120-E
Richards Bldg., Provo, UT 84602 (E-mail: allen_parcell@byu.edu).
The costs of publication of this article were defrayed in part by the
payment of page charges. The article must therefore be hereby
marked ‘‘advertisement’’ in accordance with 18 U.S.C. Section 1734
solely to indicate this fact.
J Appl Physiol 93: 1590–1597, 2002.
First published July 5, 2002; 10.1152/japplphysiol.00625.2001.
8750-7587/02 $5.00 Copyright
©
2002 the American Physiological Society http://www.jap.org1590
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mean ⫾ SE]. The Human Subjects Institutional Review
Board at Brigham Young University approved this study,
and all subjects were informed of the risks, stresses, and
benefits of the investigation before signing an informed con-
sent form.
Pretesting. To estimate submaximal workloads, subjects
performed a continuous, progressive bicycle ergometer (Lode
Excalibur, Lode, Groningen, Netherlands) protocol to deter-
mine V
˙
O
2 peak
. Subjects began cycling at a resistance of 125
W. The resistance was increased by 25 W per min until
volitional exhaustion. Expired volume was determined by a
Fleisch pneumotach, and expired oxygen and carbon dioxide
fractions were analyzed by a mass spectrometer (Marquette).
Oxygen uptake (V
˙
O
2
) and carbon dioxide production were
calculated every 15 s by an on-line computer program (Con-
sentius). The mass spectrometer was calibrated before test-
ing by use of certified medical gases of known concentration.
The heart rate was monitored continuously by radioteleme-
try (Polar Electro, Port Washington, NY), and rating of per-
ceived exertion (RPE, Borg 6–20 scale) was recorded at 1-min
intervals.
Subjects also performed a familiarization trial to acquaint
them with the time trial testing procedures and to minimize
potential learning effects. During the familiarization trial,
subjects pedaled for 90 min at a work rate demanding ⬃70%
of V
˙
O
2 peak
, followed immediately by a time trial performance
test.
Experimental testing. Experimental trials were given in
random order, and the experiment was carried out with the
use of a counterbalanced double-blind design. All trials were
separated by 1 wk. In the evening before each trial, subjects
underwent a 60-min ride at ⬃70% V
˙
O
2 peak
to normalize
muscle glycogen. They then received a meal containing
1,300–1,330 kcal (73% carbohydrate, 13% fat, and 14% pro-
tein). During the 48 h preceding each trial, subjects refrained
from vigorous activity with the exception of the 60-min ride
at ⬃70% V
˙
O
2 peak
on the evening before each trial. A dietary
record was kept, and the subjects were instructed to replicate
food intake before each subsequent trial. Subjects reported to
the laboratory in the morning after an overnight fast.
For blood sampling purposes, a Teflon intravenous cathe-
ter was inserted into a forearm vein under sterile conditions.
The catheter remained in place during the test trials for
sampling at 60 and 30 min before exercise (PRE60 and
PRE30, respectively). Blood samples were also collected at
the onset of exercise (E0), 30 (E30), 60 (E60) and 90 min
(E90) during exercise, after 15 min (ETT15) into the endur-
ance time trial, and immediately on test termination (TT). At
PRE60, PRE30, E0, E30, E60, and E90, subjects consumed
their randomly assigned drink supplement. The drink sup-
plements consisted of either 5 ml/kg body wt of water sweet-
ened with aspartame [placebo (Pla)], a 5 ml/kg solution with
polydextrose (70 g/l; CHO), a 5 ml/kg body wt solution with
L-tyrosine (25 mg/kg body wt; Tyr), or a 5 ml/kg solution with
polydextrose (70 g/l) and
L-tyrosine (25 mg/kg body wt)
(CHO⫹Tyr). The drinks were matched in color and taste and
delivered to the subjects in opaque water bottles. The drinks
were developed and coded before delivery to the research
center, and the codes were not broken until all of the data
had been analyzed. After the 60-min rest period, the subjects
exercised on the cycle ergometer for 90 min at a work rate
demanding ⬃70% V
˙
O
2 peak
. During the test trials, gas sam
-
ples were taken every 15 min to ensure that ⬃70% V
˙
O
2 peak
was maintained. Heart rate, respiratory exchange ratio
(RER), and RPE (Borg 6–20) were recorded every 15 min.
Table 1 illustrates the experimental protocol during each
trial.
Immediately after the 90-min cycling bout, subjects began
a time trial performance test that required completion of a
predetermined amount of work as rapidly as possible. The
amount of work was equivalent to the amount of work com-
pleted while cycling at 70% V
˙
O
2 peak
for 30 min. To calculate
the total work to be performed during the time trial, a
modification of a formula originally proposed by Jeukendrup
et al. (23) was used
Total amount of work for time trial (J) ⫽ 0.70䡠Wmax䡠1,800
Subjects were aware of the amount of work accumulated but
blinded to the elapsed time. The V
˙
O
2
, heart rate, RER, and
RPE were recorded at 15-min intervals during the time trial.
Elapsed time was recorded at the end of the time trial.
Blood analyses. All blood samples (5 ml) were drawn into a
prechilled EDTA-containing (5 l/ml whole blood) 12 ⫻
75-mm tube and stored in ice water for 10 min and then
centrifuged (Beckman model TJ-6R) at 1,520 g for 10 min at
4°C. A 2-ml plasma aliquot was transferred to a separate
tube and stored frozen (⫺20°C). Plasma lactate and glucose
were analyzed in triplicate by use of an Analox Micro-Stat
GM7 analyzer (Analox Instruments, Lunenburg, MA).
Plasma free fatty acid (FFA) was determined enzymatically
according to the method of Shimizu et al. (28). For separation
of plasma free tryptophan and albumin-bound tryptophan,
the remaining plasma (⬃1 ml) was transferred to a Centri-
free micropartition device (Millipore, Bedford, MA) and cen-
trifuged at 1,500 g at 25°C for 20 min. The ultrafiltrate was
then stored frozen (⫺80°C). Analyses of tyrosine and free
tryptophan were performed directly from the ultrafiltrate by
reverse-phase high-performance liquid chromatography
(Coulochem II, ESA). The chromatogram was equipped with
a Supelcosil LC-18-DB, 150-mm ⫻ 4.6-mm column. Column
Table 1. Time points during experimental procedures when subjects ingested drinks, blood samples were
taken, and V
˙
O
2
, HR, RER, and RPE were measured
Procedure/
Measurement
Time, min
PRE60 PRE30 E0 E15 E30 E45 E60 E75 E90 ETT15 TT
Drink x x x x x x
Blood x x x x x x x x
V
˙
O
2
xxxxxx x x
HR xxxxxx x x
RER xxxxxx x x
RPE xxxxxx x x
V
˙
O
2
, oxygen consumption; HR, heart rate; RER, respiratory exchange ratio; RPE, rating of perceived exertion; PRE60 and PRE30, 60 and
30 min before exercise, respectively; EO, onset of exercise; E15–E90, after 15–90 min of exercise; ETT15, 15 min into the endurance time
trial; TT, test termination.
1591L-TYROSINE INGESTION DURING EXERCISE
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temperature was kept constant at 45°C. Flow rate of the
isocratic mobile phase solution (0.14 M sodium acetate, 4%
acetonitrile, pH 6.4) was kept constant at 1 ml/min. The
ultraviolet detector (Waters 996) was set at 225 nm.
Statistical analysis. Oxygen consumption, heart rate,
RER, RPE, plasma glucose, lactate, FFA, tyrosine, free tryp-
tophan, tyrosine-to-free tryptophan ratio, and time to com-
plete the time trial were analyzed with a two-factor ANOVA
with repeated measures. Differences between means for
treatments at each time were ascertained by examining the
95% confidence intervals. The null hypothesis was rejected
when P ⬍ 0.05. All data are reported as means ⫾ SE.
RESULTS
Cardiorespiratory responses. During the 90-min ride,
mean V
˙
O
2
(expressed as mean %V
˙
O
2 peak
) across all
time points was found to be 68.5 ⫾ 1.3% for CHO,
68.9 ⫾ 1.6% for CHO⫹Tyr, 70.7 ⫾ 1.6% for Pla, and
70.0 ⫾ 1.3% for Tyr. At ETT15 and TT for CHO⫹Tyr,
V
˙
O
2
was significantly higher vs. any other time within
the same trial, and TT was higher than ETT15 (P ⬍
0.05) (Table 2). For CHO at TT, V
˙
O
2
was higher vs. all
other times within this trial (P ⬍ 0.05). Although
oxygen consumption trended in an upward fashion for
both CHO trials during the time trial compared with
Pla and Tyr, these values were not significantly differ-
ent from one another.
For all trials, RER declined steadily during the 90-
min ride (Table 2). At ETT15, the subjects showed
higher RER values during CHO⫹Tyr vs. Pla and Tyr,
and the RER was significantly higher at TT during
CHO⫹Tyr compared with Tyr and Pla (P ⬍ 0.05). For
the same trial, the RER was higher at TT vs. all other
time points within CHO⫹Tyr (P ⬍ 0.05). At TT during
CHO, the RER was higher compared with Pla (P ⬍
0.05). Within the same trial, the RER was also higher
at TT for CHO compared with E45, E60, E75, E90, and
E105 (P ⬍ 0.05).
Heart rate increased progressively throughout the
90-min ride (Table 2). During the endurance time trial,
heart rate increased abruptly compared with the 90-
min ride at 70% V
˙
O
2 peak
when subjects were fed CHO
and CHO⫹Tyr and was significantly higher at TT
compared with all other times within these trials, re-
spectively (P ⬍ 0.05). CHO and CHO⫹Tyr heart rate
values were higher at TT vs. Pla and Tyr (P ⬍ 0.05).
RPE. A progressive and similar rise in RPE was
observed for the subjects during the 90-min cycling
bout in all trials (Table 2). At TT, the RPE in Tyr was
lower compared with Pla (P ⬍ 0.05).
Blood metabolites. Plasma glucose was significantly
higher at PRE30 and 0 in both CHO and CHO⫹Tyr
(7.02 ⫾ 0.20 and 6.25 ⫾ 0.31 mmol/l, respectively)
compared with the Pla and Tyr trials (4.41 ⫾ 0.15 and
4.38 ⫾ 0.12 mmol/l, respectively) (P ⬍ 0.05) (Fig. 1).
From 0 to E30, plasma glucose declined markedly from
6.01 ⫾ 0.34 to 3.78 ⫾ 0.24 mmol/l in CHO and from
5.97 ⫾ 0.48 to 3.95 ⫾ 0.14 mmol/l in CHO⫹Tyr (P ⬍
0.05). At E60, plasma glucose was significantly higher
in the CHO vs. Pla (5.12 ⫾ 0.30 vs. 4.09 ⫾ 0.12 mmol/l,
respectively) (P ⬍ 0.05). From the onset of exercise to
TT, plasma glucose decreased steadily to 3.12 ⫾ 0.17
mmol/l in Pla and 3.27 ⫾ 0.20 mmol/l in Tyr. These
values were lower at TT vs. CHO (4.96 ⫾ 0.67 mmol/l)
and CHO⫹Tyr (4.72 ⫾ 0.47 mmol/l) (P ⬍ 0.05). No
significant differences were found in plasma glucose
levels between CHO and CHO⫹Tyr at any time during
exercise.
Table 2. V
˙
O
2
, HR, RER, and RPE during 90 min of cycling followed immediately by a time trial
Variable
Time, min
E15 E30 E45 E60 E75 E90 ETT15 TT
V
˙
O
2
l/min
CHO 2.94⫾ 0.10 3.05⫾ 0.12 3.03⫾ 0.12 3.07⫾ 0.10 3.09⫾ 0.10 3.17⫾ 0.10 3.36⫾ 0.22 3.73⫾ 0.19
c,e
CHO⫹Tyr 2.98⫾ 0.13 3.06⫾ 0.14 3.08⫾ 0.14 3.10⫾ 0.13 3.13⫾ 0.12 3.15⫾ 0.16 3.67⫾ 0.14
d,e
4.04⫾ 0.13
d,e
Tyr 3.03⫾ 0.10 3.09⫾ 0.11 3.09⫾ 0.11 3.17⫾ 0.10 3.19⫾ 0.10 3.22⫾ 0.10 3.10⫾ 0.24 3.44⫾ 0.26
Pla 3.07⫾ 0.14 3.12⫾ 0.14 3.08⫾ 0.13 3.16⫾ 0.14 3.24⫾ 0.14 3.28⫾ 0.12 3.30⫾ 0.27 3.35⫾ 0.40
HR, beats/min
CHO 157⫾ 4 161⫾ 4 165⫾ 4 164 165⫾ 4 166⫾ 5 171⫾ 3 181⫾ 4
b,e
CHO⫹Tyr 154⫾ 3 159⫾ 3 160⫾ 2 162⫾ 3 161⫾ 2 163⫾ 2 173⫾ 4 183⫾ 4
b,e
Tyr 154⫾3 159⫾4 161⫾3 162⫾4 164⫾4 166⫾4 162⫾ 6 170⫾6
Pla 156⫾ 5 159⫾ 5 161⫾ 4 163⫾ 4 165⫾ 4 166⫾ 4 167⫾ 4 161⫾ 8
RER
CHO 0.89⫾ 0.01 0.88⫾ 0.01
a
0.87⫾ 0.01
a
0.86⫾ 0.01 0.86⫾ 0.01 0.86⫾ 0.01 0.86⫾ 0.01 0.90⫾ 0.01
c,f
CHO⫹Tyr 0.89⫾ 0.01 0.89⫾ 0.01
b
0.88⫾ 0.01
b
0.87⫾ 0.01
c
0.87⫾ 0.01
c
0.87⫾ 0.01
c
0.89⫾ 0.01
d
0.93⫾ 0.01
b,e
Tyr 0.87⫾ 0.01 0.86⫾ 0.01 0.85⫾ 0.01 0.86⫾ 0.01 0.86⫾ 0.01 0.86⫾ 0.01 0.85⫾ 0.01 0.87⫾ 0.02
Pla 0.87⫾ 0.01 0.87⫾ 0.01 0.86⫾ 0.01 0.85⫾ 0.01 0.85⫾ 0.01 0.85⫾ 0.01 0.85⫾ 0.01 0.86⫾ 0.02
RPE
CHO 12.2⫾ 0.6 13.2⫾ 0.6 13.6⫾ 0.5 14.4⫾ 0.6 15.1⫾ 0.6 15.3⫾ 0.8 17.1⫾ 0.6 18.3⫾ 0.3
CHO⫹Tyr 11.8⫾ 0.8 13.2⫾ 0.6 13.6⫾ 0.6 14.2⫾ 0.8 14.9⫾ 0.8 15.1⫾ 0.8 16.7⫾ 0.6 18.6⫾ 0.3
Tyr 12.4⫾ 0.5 13.2⫾ 0.6 14.1⫾ 0.6 14.7⫾ 0.8 15.3⫾ 0.9 15.9⫾ 1.0 16.4⫾ 0.6 18.1⫾ 0.5
c
Pla 12.3⫾ 0.4 13.4⫾ 0.4 13.4⫾ 0.4 14.0⫾ 0.5 14.8⫾ 0.4 15.9⫾ 0.6 17.4⫾ 0.5 19.3⫾ 0.3
Values are means ⫾ SE. CHO, polydextrose; Tyr, L-tyrosine; Pla, placebo.
a
Significantly different from Tyr (P ⬍ 0.05);
b
significantly
different from Tyr and Pla (P ⬍ 0.05);
c
significantly different from Pla (P ⬍ 0.05);
d
significantly different from CHO, Tyr, and Pla (P⬍ 0.05);
e
significantly different from all other time points within the same trial (P ⬍ 0.05);
f
significantly different from E45, E60, E75, and E90 within
the same trial (P ⬍ 0.05).
1592 L-TYROSINE INGESTION DURING EXERCISE
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Blood lactate concentration rose from 0 to E30 in all
groups and was maintained at near constant levels
from E30 to E90 (Fig. 2). At TT, the value for
CHO⫹Tyr (7.47 ⫾ 0.76 mmol/l) was higher than CHO
(5.74 ⫾ 0.96 mmol/l), Pla (4.21 ⫾ 0.87 mmol/l), and Tyr
(5.35 ⫾ 0.88 mmol/l). At ETT15 and TT for CHO⫹Tyr,
blood lactate levels were higher compared with all
other times during this trial (P ⬍ 0.05). Blood lactate
was higher at TT in CHO vs. LA (P ⬍ 0.05). Signifi-
cantly higher lactate levels were observed at ETT15
and TT during CHO, Tyr, and Pla vs. baseline levels
and E30, E60, and E90 (P ⬍ 0.05).
The pattern for the plasma FFA response is shown in
Fig. 3. In general, consumption of carbohydrate sup-
pressed the levels of FFA compared with the noncar-
bohydrate trials over the duration of the exercise test.
In subjects who ingested tyrosine, plasma tyrosine
concentration rose significantly from baseline values
throughout exercise (Fig. 4). From PRE30 to TT,
plasma tyrosine levels were significantly increased in
Tyr and CHO⫹Tyr vs. Pla and CHO (P ⬍ 0.05).
Plasma free tryptophan levels declined from PRE60
to E30 in all groups. Compared with baseline values,
plasma free tryptophan was significantly lower at E30
in CHO and CHO⫹Tyr (P ⬍ 0.05) (Fig. 5). From E60 to
TT, plasma free tryptophan levels rose in all groups. At
each time point from E90 to TT, Pla and Tyr values
were significantly higher than those for CHO and
CHO⫹Tyr (P ⬍ 0.05). No significant differences were
observed in plasma free tryptophan levels between
CHO and CHO⫹Tyr.
The correlation between FFA and tryptophan was
significant for Tyr (r ⫽ 0.72, P ⬍ 0.05), Pla (r ⫽ 0.74),
Fig. 1. Plasma glucose concentrations during the four trials. TT, test
termination; PRE60 and PRE30, 60 and 30 min before exercise,
respectively; E0, onset of exercise; E30, E60, and E90, 30, 60, and 90
min during exercise, respectively; ETT15, 15 min into the endurance
time trial.
a
polydextrose (CHO) different from placebo (Pla), L-ty
-
rosine (Tyr), and CHO⫹Tyr (P ⬍ 0.05);
b
CHO⫹Tyr different from Pla
and Tyr (P ⬍ 0.05);
c
CHO different from Pla and Tyr (P ⬍ 0.05);
d
CHO different from Pla (P ⬍ 0.05).
Fig. 2. Blood lactate concentrations during the four trials.
a
CHO⫹
Tyr different from Pla, Tyr, and CHO (P ⬍ 0.05);
b
CHO different
from Pla (P ⬍ 0.05);
c
CHO⫹Tyr different from all other times within
this trial (P ⬍ 0.05);
d
CHO different from PRE60, PRE30, E0, E30,
E60, and E90 within this trial (P ⬍ 0.05);
e
Tyr different from PRE60,
PRE30, E0, E30, E60, and E90 within this trial (P ⬍ 0.05);
f
Pla
different from PRE60, PRE30, E0, E30, E60, and E90 within this
trial (P ⬍ 0.05);
g
Tyr different from PRE60, PRE30, E0, E30, and E60
within this trial (P ⬍ 0.05).
Fig. 3. Plasma free fatty acid concentrations during the 4 trials.
a
Pla
different from CHO and CHO⫹Tyr (P ⬍ 0.05);
b
Tyr different from CHO
and CHO⫹Tyr (P ⬍ 0.05);
c
CHO different from Pla and Tyr (P ⬍ 0.05).
1593L-TYROSINE INGESTION DURING EXERCISE
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and all treatments taken together (r ⫽ 0.70, P ⬍
0.001). The highest correlation was seen with the Pla
treatment. The plasma tyrosine-to-free tryptophan ra-
tio (Fig. 6) increased dramatically from PRE60 to TT in
Tyr and CHO⫹Tyr. From PRE30, the plasma tyrosine-
to-free tryptophan ratio was significantly higher for
Tyr and CHO⫹Tyr vs. CHO and Pla (P ⬍ 0.05).
Endurance time trial. Subjects fed CHO completed
the endurance time trial in 27.17 ⫾ 0.92 min. Those fed
CHO⫹Tyr completed it in 26.11 ⫾ 1.01 min. These
times were significantly lower than performance times
for Pla (34.44 ⫾ 2.89 min) and Tyr (32.64 ⫾ 3.05 min)
(P ⬍ 0.05). For the entire F-test, the regular statistical
power was observed to be 0.76. For CHO⫹Tyr, six of
nine subjects completed the time trial faster than all
other trials, and eight of nine subjects finished faster
vs. Tyr and Pla. For the CHO trial, three of nine
completed the time trial faster vs. all other trials, six of
nine subjects finished faster than Tyr, and eight of nine
subjects finished faster than Pla. Six of nine subjects
finished the time trial faster during Tyr compared with
the Pla trial. No significant differences were found in
time to complete the endurance time trial for CHO vs.
CHO⫹Tyr or Pla vs. Tyr. For specific comparisons
between CHO vs. CHO⫹Tyr and Pla vs. Tyr, a SD of
1.5 would be required to detect differences at a statis-
tical power value of ⬃0.82 with nine subjects per group
(25). The resultant effect size (ES) for CHO⫹Tyr com-
pared with CHO is calculated as 0.37 with an esti-
mated statistical power value at 0.11 (P ⫽ 0.05). For
Tyr vs. Pla, the ES is calculated as 0.21, yielding a
statistical power value of ⬃0.08 (P ⫽ 0.05) (25). Despite
no significant differences observed between Tyr and
Pla and between CHO and CHO⫹Tyr, the ES and low
statistical power suggest that caution be exercised
when making conclusions about these latter compari-
sons.
DISCUSSION
The purpose of this study was to determine whether
repeated doses of L-tyrosine, either with or without
carbohydrate feedings, would improve cycling time
trial performance after 90 min of submaximal cycling.
The results showed that tyrosine ingestion, either
alone or with carbohydrates, did not improve perfor-
Fig. 4. Plasma tyrosine concentrations during the 4 trials.
a
CHO
⫹Tyr different from CHO and Pla (P ⬍ 0.05);
b
Tyr different from
CHO and Pla (P ⬍ 0.05).
Fig. 5. Plasma free tryptophan concentrations during the 4 trials.
a
Tyr different from CHO and CHO⫹Tyr (P ⬍ 0.05);
b
Pla different
from CHO and CHO⫹Tyr (P ⬍ 0.05).
Fig. 6. Tyrosine-to-free tryptophan ratios during the 4 trials.
a
Tyr
different from CHO and Pla (P ⬍ 0.05);
b
CHO⫹Tyr different from
CHO and Pla (P ⬍ 0.05).
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mance. The results did confirm that carbohydrate in-
gestion every 30 min during steady-state exercise sig-
nificantly improved time trial performance. We
conclude that under the experimental conditions of this
study tyrosine was not ergogenic.
Only one other study has investigated the effects of
tyrosine ingestion on exercise performance in humans
(30). In a study by Struder et al. (30), subjects ingested
10gof
L-tyrosine 15 min before and 60 min after
beginning an exercise bout corresponding to an inten-
sity of 2 mmol/l blood lactate. After 90 min of exercise,
peak plasma tyrosine levels rose from ⬃90 to ⬃260
mol/l and then declined slightly to ⬃240 mol/l at
exhaustion (150 ⫾ 42 min of exercise). Their results
showed no effect of tyrosine on exercise time to exhaus-
tion nor on the outcomes of tests of mental performance
and self-perception performed immediately after the
exercise to exhaustion. In their subjects who consumed
tyrosine, they found high plasma prolactin levels that
would suggest reduced dopamine levels in the brain
(30). Animal studies have shown that tyrosine doses of
20 mg/kg resulted in marked increases in dopamine
synthesis, but doses of 50 mg/kg resulted in dopamine
levels less than baseline (1). It is possible that the
doses used by Struder et al. were too high and led to an
inhibition of dopamine synthesis rather than a stimu-
lus. The doses used in the present study were about
half of those used by Struder and associates but yielded
plasma concentrations twice as high as those reported
by them despite having similar baseline values as in
this study (⬃80–100 mol/l). This might reflect the
differences in mode and timing of administration and
measurement between the two studies. Nevertheless,
the lack of effect of tyrosine in our study may also be
the result of excess tyrosine in the ingestate. Perhaps,
in contrast to carbohydrate feeding, the continuous
ingestion of tyrosine over time, as imposed under the
present design, exceeds the amount that might be
beneficial. Because we could not measure brain concen-
trations of any substances, all of this is conjecture, but
the possibility is intriguing. The present results lay the
foundation for further investigation.
It was our aim in the present study to increase the
plasma tyrosine-to-free tryptophan ratio in the blood of
our subjects, because a high tyrosine-to-free trypto-
phan ratio may favor tyrosine uptake into the brain
and could subsequently augment brain dopamine syn-
thesis and reduce serotonin synthesis and potentially
enhance performance. The results show that plasma
tyrosine levels increased approximately fivefold
whereas plasma free tryptophan increased 19% and
decreased ⬃13% in Tyr and CHO⫹Tyr at TT compared
with baseline values in these groups, respectively. The
plasma tyrosine-to-free tryptophan ratio at TT was
43.4 in Tyr and 50.0 in CHO⫹Tyr compared with
baseline values of ⬃10.0. Although these increases in
the plasma tyrosine-to-free tryptophan ratio may have
resulted in reduced free tryptophan uptake and in-
creased tyrosine uptake into the brain, they did not
lead to significantly enhanced performance.
Some observations in the present results do suggest
a possible enhancing effect of tyrosine. The V
˙
O
2
, RER,
and blood lactate reached significantly higher levels at
TT in CHO⫹Tyr than in CHO alone. In addition,
higher V
˙
O
2
and RER data were seen during CHO⫹Tyr
at ETT15 with respect to all other trials and compared
with previous times within the same trial, suggesting
that subjects were exercising harder during this time
period. Even though these metabolic responses did not
result in a significant improvement on the time trial,
they may suggest that had the time trial been designed
differently, perhaps longer in length, the tyrosine in
conjunction with carbohydrate may have resulted in
improved performance. This suggestion is derived from
the report of Banderet and Lieberman (5), who ob-
served enhanced performance in numerous mood, cog-
nitive, reaction time, and vigilance measures while
subjects were exposed to 4.5 h of extreme environmen-
tal conditions. They suggested that decrements in per-
formance resulting from central catecholamine deple-
tion during prolonged exposure to stress could be
attenuated by tyrosine ingestion. Further support for
this supposition is found in our RPE data of Table 2.
The fact that no differences were found in RPE for
CHO⫹Tyr vs. CHO, even though the metabolic data
indicate that subjects in CHO⫹Tyr were working
harder during the time trial, suggests that enhanced
central dopaminergic activity may have occurred and
nullified the perception of fatigue. That this did not
translate into improved performance time may once
again be a reflection of the design. On the other hand,
all of these observations taken together could mean
that tyrosine ingestion somehow invoked an ergolytic
response. For example, the higher V
˙
O
2
and RER in
CHO⫹Tyr compared with Tyr with no difference in
RPE and performance time might suggest that the
abundance of plasma tyrosine might have resulted in
reduced metabolic efficiency. Future studies must be
designed to test these possibilities.
In the present study, the feeding of glucose within 60
min before exercise resulted in a significant elevation
of blood glucose at the onset of exercise but led to a
precipitous drop in blood glucose during the first 30
min of the exercise bout. Costill and co-workers (14)
observed a similar response and attributed the decline
of blood glucose to the combined effects of exercise and
an elevated insulin concentration. In their study, en-
durance was reduced as a result of preexercise con-
sumption of glucose. In contrast, in our study, blood
glucose levels were restored in both CHO groups after
E60 because of repeated carbohydrate consumption,
and improvements were seen in time trial perfor-
mance. The improved performance seen in this study
concurs with previous studies that found that carbohy-
drate ingestion during exercise improves performance
(13, 15, 21, 22, 26). The improvement seen in the CHO
groups is likely due to the maintenance of blood glucose
homeostasis allowing a constant supply of glucose for
oxidation in the working muscle (12). In addition, one
could also speculate that the mechanism for a carbo-
hydrate-induced increase in performance might also be
1595
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related to the indirect effect on the brain resulting from
glucose alteration of tryptophan. For instance, ele-
vated blood glucose levels resulting from carbohydrate
ingestion during exercise have been shown to suppress
FFA levels (11, 26). This is clearly the case in the
present study in which both CHO groups had lower
FFA levels compared with the non-CHO groups (Fig.
3). Suppressed FFA levels have been shown to reduce
plasma free tryptophan, because FFA and tryptophan
compete for binding to albumin (16). The lower the
FFA, the less tryptophan that would come off the
albumin and the lower would be the level of free tryp-
tophan. This response is also clearly demonstrated in
Fig. 5, which shows that in the two CHO groups that
have less FFA there is a reduction in free tryptophan.
Previous research suggests that fatigue after pro-
longed exercise is associated with elevations of free
tryptophan and serotonin in various regions of the
brain and cerebrospinal fluid (6, 8–10) resulting from
high concentrations of plasma free tryptophan. It is
possible that, in the present study, the feeding of glu-
cose resulted in a reduction of free tryptophan and a
concomitant reduction of the tryptophan and serotonin
in the brain and an improvement in performance. This
suggestion is in harmony with that of Davis et al. (18)
who observed similar effects of glucose on tryptophan
during prolonged exercise.
In summary, this study was designed to test whether
repeated ingestions of tyrosine during prolonged exer-
cise could improve performance of human subjects dur-
ing an endurance time trial. Despite evidence that
tyrosine may have promoted some beneficial central
effects that reduced perception of fatigue, it had no
significant effect on performance time during the time
trial. Carbohydrate feedings resulted in enhanced per-
formance as observed in previous studies. The meta-
bolic data suggest that the beneficial effects of carbo-
hydrate ingestion may be related as much to its
indirect effect on reducing central fatigue as it is to its
well known peripheral effects on substrate metabo-
lism.
The authors thank Dong Ho Han for excellent technical assis-
tance.
This work was supported in part by the Gatorade Sports Science
Institute and Natures Sunshine.
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