GENERAL ABSTRACT
An emerging hypothesis states that brain catecholamine availability is directly related to
tolerance of prolonged exercise in the heat. The aim of this thesis was to nutritionally alter
the plasma availability of the catecholamine precursors, tyrosine and phenylalanine, to
explore the effect on prolonged exercise (whole body and single limb) in the heat,
including cycling capacity and performance.
In study 1 (Chapter 3) acutely increasing the plasma ratio of tyrosine:large neutral amino
acids (LNAA; the tyrosine ratio) via oral ingestion of 150 mg·kg body mass-1 tyrosine
increased exercise capacity in the heat. The increase was seemingly related to a lower rate
of core temperature (Tcore) increase and/or reduced RPE and thermal sensation late in
exercise. Exercise capacity in the heat was markedly reduced when subjects were acutely
administered a tyrosine and phenylalanine-free amino acid mixture which decreased the
tyrosine ratio (study 2, Chapter 4), and the reduction seemed to be related to a more rapid
attainment of maximal RPE, thermal sensation and heart rate.
Acute tyrosine
supplementation failed to improve simulated time trial performance in the heat (study 3,
Chapter 5). This may be due to a lower physiological and subjective demand inherent in
the self-paced nature of the exercise, in comparison to an exhaustive capacity trial. In
study 4 (Chapter 6), acute tyrosine supplementation had no effect on sustained handgrip
force, heart rate variability, or mood and motivation scores, when subjects were
hyperthermic.
i
The studies within this thesis provide further insight into the aetiology of central fatigue
during prolonged exercise in the heat, and suggest that nutritional catecholamine precursor
availability is involved, in part, in this process. In particular, an increased availability of
nutritional catecholamine precursors, reported for the first time herein, may afford
improved tolerance of prolonged, constant-load, submaximal activity in the heat.
Key words: prolonged exercise, amino acids, hyperthermia, tyrosine, phenylalanine,
catecholamines.
ii
ACKNOWLEDGEMENTS
I would firstly like to thank my supervisors Dr. Rhys Thatcher and Dr. Glen Davison.
Their experience, support and advice have been invaluable throughout the design of each
study within this thesis, data collection and analysis, and during the proof reading of the
final draft.
My deepest gratitude goes to Dr. Manfred Beckmann from the Institute of Biological,
Environmental and Rural Sciences at Aberystwyth University for carrying out the amino
acid analysis for each study within this thesis. I thank him for his patience, particularly
when I hounded him to complete the analysis within a tight timescale. He always came
through with the results despite an extremely heavy workload. I also wish to express
thanks to Ph.D. students within the Department of Sport and Exercise Science at
Aberystwyth University; Arwel Jones, Ffion Curtis and Daniel March, as well as
technician Maggie Pownall. These individuals provided invaluable assistance with subject
monitoring at various times throughout data collection for each of the studies reported
herein. Thanks go to Clive Willson not only for writing the software which was used to
collect the handgrip force data in study 4 (Chapter 7), but also for repairing the climate
chamber at short notice when it was most needed! Clive really is an incredible resource
for technical advice and assistance. I would like to thank all the subjects who generously
donated their time and effort, and with great enthusiasm, to be involved in the studies
reported in this thesis. I wish to express thanks to SHS International, and in particular
iii
Joanna Hill, for the kind donation of the amino acid powders which were used in studies 2
to 4.
I thank Ruth for her unwavering support and patience while I worked unsociable hours for
months on end. Lastly, I would like to dedicate this thesis to my parents, Doris Tumilty
and the late Robert P. Tumilty. My one major regret is that my dad is not alive to witness
the completion of this thesis, but I hope that he would be proud.
iv
PUBLICATIONS
The following publications have arisen to date from some of the work presented in this
thesis:
Journal articles:
Chapter 3:
Tumilty L, Davison G, Beckmann M and Thatcher, R. (2011).
Oral tyrosine
supplementation improves exercise capacity in the heat. European Journal of Applied
Physiology, 111, 2941 – 2950.
Conference contributions
Chapter 3:
Tumilty L., Davison G., Beckmann M. and Thatcher R. (2009). Oral tyrosine
supplementation improves exercise capacity in the heat. Wales Institute of Sport Health
and Exercise Science Annual Conference, Aberystwyth University, July 2009.
v
Tumilty L., Davison G., Beckmann M. and Thatcher R. (2010). Oral tyrosine
supplementation improves exercise capacity in the heat.
Proceedings of The British
Association of Sport and Exercise Sciences Annual Student Conference, Aberystwyth
University, April 2010.
Chapter 4:
Tumilty L., Davison G., Beckmann M. and Thatcher R.
(2011). A tyrosine and
phenylalanine-free amino acid mixture decreases exercise capacity in a warm environment.
Proceedings of The British Association of Sport and Exercise Sciences Annual Student
Conference, P15O, University of Chester, April 2011.
Tumilty L., Davison G., Beckmann M. and Thatcher R. (2011). Acute tyrosine and
phenylalanine depletion decreases exercise capacity in a warm environment. Proceedings
of The British Association of Sport and Exercise Sciences Annual Conference, 1545, Free
Communications 24, September, 2011.
vi
CONTENTS
General Abstract
i
Acknowledgements
iii
Publications
v
Contents
vii
List of Tables
xi
List of Figures
xii
List of Abbreviations
xv
Chapter 1 – Literature Review
1
1.1 Introduction
1
1.1 Central fatigue: a brief historical perspective
6
1.2 Catecholamines and prolonged exercise in the heat
15
1.3 Brain monoamine synthesis
19
1.3.1 Catecholamine synthesis
20
1.3.2 Control of catecholamine synthesis
23
1.3.3 Serotonin synthesis
25
1.3.4 Control of serotonin synthesis
27
1.4 The blood-brain barrier and amino acid transport
27
1.5 Tyrosine supplementation studies
31
1.5.1 Physiological effects of tyrosine supplementation in the rat
31
1.5.2 Behavioural effects of tyrosine supplementation in the rat
40
1.5.3 Tyrosine supplementation in humans
44
1.5.4 Cognitive function and mood effects of tyrosine supplementation
47
1.5.5 Tyrosine supplementation and depression
60
1.5.6 Tyrosine supplementation and exercise performance
61
1.6 Acute tyrosine and phenylalanine depletion studies
1.6.1 Physiological effects of acute tyrosine and phenylalanine
65
70
depletion in animals
1.6.2 Acute tyrosine and phenylalanine depletion: effect on human
89
brain neurotransmission
vii
1.6.3 Behavioural effects of tyrosine and phenylalanine depletion
92
1.6.4 Effects on mood and affect
92
1.6.5 Cognition and psychomotor tasks
95
1.7 Summary
Chapter 2 – General Methods
98
101
2.1 Subjects and ethical approval
101
2.2 Habituation trials
102
2.3 Cycle ergometer
102
2.4 Drinks preparation
102
2.5 Determination of peak oxygen uptake ( V O2peak) and constant-load
102
submaximal exercise power output
2.6 Heat stress induction
104
2.7 Physiological measurement methods
104
2.7.1 Temperature monitoring
104
2.7.2 Urine volume and osmolality
106
2.7.3 Heart rate
106
2.7.4 Saliva flow rate
106
2.7.5 Air temperature, relative humidity and wind speed measurement
107
2.7.6 Subjective ratings
108
2.7.7 Familiarisation and main trials expired gas analysis
109
2.7.8 Experimental procedures
109
2.8 Analytical methods
110
2.8.1 Blood collection and treatment
111
2.8.2 Plasma amino acid measurement
111
2.8.3 Blood glucose, lactate, haemoglobin and haematocrit
112
2.9 General statistical methods
Chapter 3 – Study 1: The effect of acute tyrosine supplementation on exercise
113
114
capacity in the heat
3.1 Introduction
116
3.2 Pilot work
118
viii
3.3 Methods
121
3.4 Results
124
3.5 Discussion
136
Chapter 4 – Study 2: The effect of acute tyrosine and phenylalanine depletion and
145
exercise capacity in the heat
4.1 Introduction
147
4.2 Pilot work
149
4.3 Methods
152
4.4 Results
156
4.5 Discussion
169
Chapter 5 – Study 3: Acute tyrosine supplementation and exercise performance in
178
the heat
5.1 Introduction
180
5.2 Methods
182
5.3 Results
187
5.4 Discussion
201
Chapter 6 – Study 4: The effect of acute tyrosine supplementation on muscle force
209
production in hyperthermic subjects.
6.1 Introduction
210
6.2 Methods
212
6.3 Results
219
6.4 Discussion
229
Chapter 7 – General Discussion
237
7.1 Effect of supplementation and acute depletion on the tyrosine ratio
238
7.2 Catecholamine precursor availability and thermoregulatory responses
251
7.3 Central of peripheral effect?
255
7.4 Summary and conclusions
263
7.5 Future directions
264
References
266
Appendix 1
307
Appendix 2
309
ix
Appendix 3
313
Appendix 4
316
x
LIST OF TABLES
Summary of exercise studies with pharmacological and nutritional
manipulation of central neurotransmitters in humans.
11
Summary of physiological and behavioural effects from studies with
tyrosine administration in animals.
32
Summary of studies examining the effect of tyrosine supplementation on
cognition, behaviour and exercise in humans.
48
Composition of amino acid mixtures used to acutely deplete plasma
tyrosine and phenylalanine in humans.
68
Summary of findings from studies with acute tyrosine and phenylalanine
depletion in animals.
71
Summary of results from studies with acute tyrosine and phenylalanine
depletion in humans.
79
Area under the curve for plasma amino acids derived from GC-MS
analysis for tyrosine (TYR) and placebo (PLA) trials.
127
Changes in plasma volume, blood glucose, and blood lactate in tyrosine
and placebo trials.
128
Table 3.3
Estimated substrate oxidation rates during exercise.
134
Table 4.1
Plasma amino acid concentrations in TYR-free and BAL trials.
157
Table 4.2
Blood glucose and blood lactate concentrations at rest, pre-exercise and
exhaustion in TYR-free and BAL trials.
162
Table 5.2
Plasma amino acid concentrations in TYR and Whey trials.
190
Table 6.1
Measures of heart rate variability during 0 -20 min, 0 -5 min, 5-10 min,
10-15 min and 15-20 min of TT, in TYR and Whey trials.
225
Table 1.1
Table 1.2
Table 1.3
Table 1.4
Table 1.5
Table 1.6
Table 3.1
Table 3.2
xi
LIST OF FIGURES
Illustration of the changes that occur from rest to prolonged exercise on
plasma concentrations of large neutral amino acids (LNAA), tryptophan
(TRP) and free fatty acids (FFA).
10
Key steps in the synthesis of dopamine (DA) and noradrenaline (NA)
within catecholamine neurons.
22
Figure 1.3
Key steps in the synthesis of serotonin (5-HT) within serotonin neurons.
26
Figure 1.4
The relationship between food intake, plasma amino acids, and brain
synthesis of catecholamines and serotonin. LNAA, large neutral amino
acids.
30
Plasma tyrosine ratio in three subjects following ingestion of three doses
of tyrosine; 150, 200 and 250 mg·kg body mass-1.
119
Mean (± SD) and individual exercise times to exhaustion in tyrosine
(TYR) and placebo (PLA) conditions.
124
Plasma tyrosine concentration, tryptophan ratio and tyrosine ratio in
tyrosine and placebo trials.
126
Figure 3.4
Heart rate responses to exercise in tyrosine and placebo trials.
130
Figure 3.5
Mean weighted skin temperature and core temperature responses to
exercise in tyrosine and placebo trials.
131
Ratings of thermal sensation and perceived exertion in tyrosine and
placebo trials.
132
Plasma tyrosine ratio following ingestion of a balanced mixture
containing nine amino acids, an alternative balanced mixture containing
six amino acids, the same complete mixture minus tyrosine and
phenylalanine and the same alternative reduced quantity mixture minus
tyrosine and phenylalanine.
151
Mean (± SD) and individual exercise times to exhaustion following
ingestion of a tyrosine and phenylalanine free amino acid mixture (TYRfree) and a balanced amino acid mixture (BAL).
156
Plasma tyrosine plus phenylalanine concentration, tyrosine ratio,
tryptophan concentration and tryptophan ratio in TYR-free and BAL
trials.
160
Figure 4.4
Plasma volume changes in TYR-free and BAL trials.
163
Figure 4.5
Heart rate responses to exercise in TYR-free and BAL trials.
164
Figure 1.1
Figure 1.2
Figure 3.1
Figure 3.2
Figure 3.3
Figure 3.6
Figure 4.1
Figure 4.2
Figure 4.3
xii
Core temperature and mean weighted skin temperature responses to
exercise in TYR-free and BAL trials.
165
Ratings of perceived exertion and thermal sensation in TYR-free and
BAL trials.
167
Figure 4.8
Saliva flow rate in TYR-free and BAL trials.
168
Figure 5.1
Mean (± SD) and individual times to complete cycling time trial in TYR
and Whey trials
187
Figure 5.2
Power output during a cycling time trial in TYR and Whey trials.
188
Figure 5.3
Changes in plasma tyrosine concentration, tyrosine ratio, plasma
tryptophan concentration and tryptophan ratio in response to ingestion of
TYR or Whey.
191
Blood glucose concentration, blood lactate concentration and plasma
volume changes following 60 min constant-load submaximal cycling and
cycling time trial in TYR and Whey trials.
192
Heart rate responses during 1 h rest, 60 min constant-load submaximal
exercise and cycling time trial in TYR and Whey trials.
195
Core temperature, mean weighted skin temperature and body heat content
responses during 1 h rest, 60 min of constant-load submaximal exercise
and cycling time trial, in TYR and Whey trials
197
RPE and thermal sensation ratings during 60 min of constant-load
submaximal cycling and cycling time trial in TYR and Whey trials.
198
Figure 4.6
Figure 4.7
Figure 5.4
Figure 5.5
Figure 5.6
Figure 5.7
Figure 5.8
Estimated rates of fat and carbohydrate oxidation, and respiratory
exchange ratio at 30 min and 50 min of constant-load submaximal cycling
in TYR and Whey trials.
200
Figure 6.1
Custom-setup chair used for measurement of handgrip force.
218
Figure 6.2
Effect of exercise on sub-scale scores for energetic arousal, hedonic tone,
anger/frustration, success motivation, intrinsic motivation, and tense
arousal of the UWIST mood adjective checklist and success motivation
and intrinsic motivation scales, at Pre-exercise and 53 min of constantload submaximal exercise in TYR and Whey trials.
220
Force produced during individual bouts of three 3-s maximal handgrip
contractions at baseline, following 60 min of constant-load submaximal
cycling exercise and a cycling time trial in TYR and Whey trials.
221
Mean force produced during 1-min sustained handgrip contractions at
baseline, following 60 min of constant-load submaximal cycling exercise
and following a cycling time trial in TYR and Whey trials.
222
Figure 6.3
Figure 6.4
xiii
Figure 6.5
Figure 6.6
Figure 6.7
Figure 7.1
Figure 7.2
Handgrip force during 1-min sustained contraction over consecutive 5 s
intervals at baseline, after 60 min cycling and after cycling time trial in
TYR and Whey trials.
223
Power spectrum analysis of R-R variability during 0 – 20 min of TT, and
consecutive 5 min intervals from 0 – 20 min during cycling time trial in
TYR and Whey trials.
226
Saliva flow rate at rest, immediately before 60 min constant-load
submaximal exercise and at end of a cycling time trial in TYR and Whey
trials.
228
Pooled data for resting plasma tyrosine ratio and percentage change in
tyrosine ratio from rest, 1 h after ingestion of 150 mg·kg body mass-1
tyrosine [Study1 (n = 8) and Study 3 (n = 7)] or an amino acid mixture
devoid of tyrosine and phenylalanine (Study 2, n = 8).
240
Relationship between the difference in the percentage in the tyrosine ratio
from rest to pre-exercise, between experimental and placebo trials, and
the percentage difference in exercise time induced by the experimental
drink. Pooled data from experimental trials in study 1 and study 2 (n =
14).
249
xiv