The effect of a taurine-containing drink on performance in 10 endurance-athletes

Summary To determine the effect of a taurine-enriched drink Red Bull on performance, 10 endurance-athletes performed three trials. After 60 min. cycling at approximately 70% VO₂ max, the subjects pedalled to exhaustion on a cycle ergometer. During each exercise, the subjects received 500 ml of a test-drink after 30 min. submaximal cycling: Red Bull without taurine, without glucuronolacton (U1), Red Bull without taurine, without glucuronolacton, without caffeine (U2) and Red Bull original drink containing taurine, glucuronolacton and caffeine (U3).The heart rate level was significantly lower in U3 (p = 0,0031) 15 min. after application. The plasma catecholamines increased slightly from begin of exercise to 15 min. after application of the drinks in all trials but remained on a significantly lower level in U3 (epinephrine (p = 0,0011) and norepinephrine (p = 0,0003). Endurance time was significantly longer with Red Bull original in U3 (p = 0,015). The results of this study show a positive effect of a taurine-containing drink on hormonal responses which leads to a higher performance.     

Effects of red bull energy drink on repeated sprint performance in women athletes

Energy drinks are frequently consumed by athletes prior to competition to improve performance. This study examined the effect of Red Bull™ on repeated sprint performance in women athletes. Fifteen collegiate soccer players participated, with mean age, height, and body mass equal to 19.5 ± 1.1 year, 168.4 ± 5.8 cm, and 63.4 ± 6.1 kg, respectively. After performing a familiarization trial, subjects performed three sets of eight bouts of the modified t test after ingestion of 255 mL of placebo or Red Bull 1 h pre-exercise in a randomized, placebo-controlled crossover design. Throughout testing, sprint time, heart rate (HR), and rating of perceived exertion (RPE) were continuously obtained. Repeated measures analysis of variance was used to examine differences in variables between drink conditions. Across athletes, t test time ranged from 10.4 to 12.7 s. Mean sprint time was similar (p > 0.05) between Red Bull (11.31 ± 0.61 s) and placebo (11.35 ± 0.61 s). HR and RPE increased (p < 0.05) during the bouts, but there was no effect (p > 0.05) of Red Bull on either variable versus placebo. Findings indicate that 255 mL of Red Bull containing 1.3 mg/kg of caffeine and 1 g of taurine does not alter repeated sprint performance, RPE, or HR in women athletes versus placebo. One serving of this energy drink provides no ergogenic benefit for women athletes engaging in sprint-based exercise.     

A combination of caffeine and taurine has no effect on short term memory but induces changes in heart rate and mean arterial blood pressure

Summary. Red Bull energy drink has become extraordinarily popular amongst college students for use as a study aid. We investigated the combined effects of Red Bull’s two active ingredients, caffeine and taurine, on short term memory. Studies on the effects of these two neuromodulators on memory have yielded mixed results, and their combined actions have not yet been investigated. In this double-blind study, college student subjects consumed either caffeine and taurine pills or a placebo and then completed a memory assessment. Heart rate and blood pressure were monitored throughout the testing period. The combination of caffeine and taurine had no effect on short term memory, but did cause a significant decline in heart rate and an increase in mean arterial blood pressure. The heart rate decline may have been caused by pressure-induced bradycardia that was triggered by caffeine ingestion and perhaps enhanced by the actions of taurine.     

The effects of red bull energy drink on human performance and mood

Summary. The effects of Red Bull Energy Drink, which includes taurine, glucuronolactone, and caffeine amongst the ingredients, were examined over 3 studies in a total of 36 volunteers. Assessments included psychomotor performance (reaction time, concentration, memory), subjective alertness and physical endurance. When compared with control drinks, Red Bull Energy Drink significantly (P < 0.05) improved aerobic endurance (maintaining 65–75% max. heart rate) and anaerobic performance (maintaining max. speed) on cycle ergometers. Significant improvements in mental performance included choice reaction time, concentration (number cancellation) and memory (immediate recall), which reflected increased subjective alertness. These consistent and wide ranging improvements in performance are interpreted as reflecting the effects of the combination of ingredients. Received March 20, 2000 Accepted May 25, 2000     

Diuretic potential of energy drinks

Summary. Recent literature suggests that both caffeine and taurine can induce diuresis and natriuresis in rat and man. Although they act via different cellular mechanisms, their diuretic actions might be additive. This is of considerable interest, as several commercially available energy drinks contain both substances. In this study we examined the possible diuretic effects of caffeine and taurine in a cross-over-design in which 12 healthy male volunteers received each of 4 different test drinks (750 ml of energy drink containing 240 mg caffeine and 3 g taurine, the three other test drinks either lacked caffeine, taurine or both) after restraining from fluids for 12 h. Mixed model analyses demonstrated that urinary output and natriuresis were significantly increased by caffeine (mean differences 243 ml and 27 mmol; both p < 0.001) and that there were no such effects of taurine (mean differences 59 ml and −4 mmol). Additionally, urinary osmolarity at baseline was significantly related to the urinary output (p < 0.001). Urine osmolarity values at baseline and in the 6 h urine collection did not differ significantly between treatments. Taken together, our study demonstrates that diuretic and natriuretic effects of the tested energy drink were largely mediated by caffeine. Taurine played no significant role in the fluid balance in moderately dehydrated healthy young consumers. Consequently, the diuretic potential of energy drinks will not differ significantly from other caffeine containing beverages.     

Effect of taurine and potential interactions with caffeine on cardiovascular function

The major impetus behind the rise in energy drink popularity among adults is their ability to heighten mental alertness, improve physical performance and supply energy. However, accompanying the exponential growth in energy drink usage have been recent case reports and analyses from the National Poison Data System, raising questions regarding the safety of energy drinks. Most of the safety concerns have centered on the effect of energy drinks on cardiovascular and central nervous system function. Although the effects of caffeine excess have been widely studied, little information is available on potential interactions between the other active ingredients of energy drinks and caffeine. One of the active ingredients often mentioned as a candidate for interactions with caffeine is the beta-amino acid, taurine. Although taurine is considered a conditionally essential nutrient for humans and is thought to play a key role in several human diseases, clinical studies evaluating the effects of taurine are limited. However, based on this review regarding possible interactions between caffeine and taurine, we conclude that taurine should neutralize several untoward effects of caffeine excess. In agreement with this conclusion, the European Union’s Scientific Committee on Food published a report in March 2003 summarizing its investigation into potential interactions of the ingredients in energy drinks. At the cardiovascular level, they concluded that “if there are any interactions between caffeine and taurine, taurine might reduce the cardiovascular effects of caffeine.” Although these interactions remain to be further examined in humans, the physiological functions of taurine appear to be inconsistent with the adverse cardiovascular symptoms associated with excessive consumption of caffeine–taurine containing beverages.     

A taurine and caffeine-containing drink stimulates cognitive performance and well-being

Summary. Caffeine- and taurine-containing drinks have been on the European market for about a decade, and research on the individual constituents of these drinks indicates an improvement in cognitive performance resulting from consumption of such drinks. In this double-blind, placebo-controlled study using 10 graduate students, we obtained the P300 components of event-related potential (ERP) waveforms following an auditory oddball paradigm, measured motor reaction time, and applied the d2 test for the assessment of attention. Status of mood was assessed by the “Basler-Befindlichkeitsbogen” questionnaire, a standard test for evaluation of feelings of well-being. Measurements were made at night, prior to and starting one hour after consumption of energy drink ingredients or placebo. At the end of the experiment (midnight), P300 latency and motor reaction time were significantly longer compared with baseline measurements in the placebo group, but were unchanged in the energy drink group. In the test system for evaluating feelings of well-being, total scores, vitality scores and social extrovertedness scores were significantly decreased in the placebo group but not in the energy drink group. The findings clearly indicate that the mixture of three key ingredients of Red Bull^R Energy Drink used in the study (caffeine, taurine, glucuronolactone) have positive effects upon human mental performance and mood. These effects may be mediated by the action of caffeine on purinergic (adenosinergic) receptors and taurine modulation of receptors. As half of the study cohort were non-caffeine users, the described effects cannot be explained in terms of the restoration of plasma caffeine levels to normal following caffeine withdrawal. Received January 5, 2000/Accepted June 5, 2000     

Effect of different protocols of caffeine intake on metabolism and endurance performance.

Competitive athletes completed two studies of 2-h steady-state (SS) cycling at 70% peak O(2) uptake followed by 7 kJ/kg time trial (TT) with carbohydrate (CHO) intake before (2 g/kg) and during (6% CHO drink) exercise. In Study A, 12 subjects received either 6 mg/kg caffeine 1 h preexercise (Precaf), 6 x 1 mg/kg caffeine every 20 min throughout SS (Durcaf), 2 x 5 ml/kg Coca-Cola between 100 and 120 min SS and during TT (Coke), or placebo. Improvements in TT were as follows: Precaf, 3.4% (0.2-6.5%, 95% confidence interval); Durcaf, 3.1% (-0.1-6.5%); and Coke, 3.1% (-0.2-6.2%). In Study B, eight subjects received 3 x 5 ml/kg of different cola drinks during the last 40 min of SS and TT: decaffeinated, 6% CHO (control); caffeinated, 6% CHO; decaffeinated, 11% CHO; and caffeinated, 11% CHO (Coke). Coke enhanced TT by 3.3% (0.8-5.9%), with all trials showing 2.2% TT enhancement (0.5-3.8%; P < 0.05) due to caffeine. Overall, 1) 6 mg/kg caffeine enhanced TT performance independent of timing of intake and 2) replacing sports drink with Coca-Cola during the latter stages of exercise was equally effective in enhancing endurance performance, primarily due to low intake of caffeine (approximately 1.5 mg/kg).     

Readiness potential in different states of physical activation and after ingestion of taurine and/or caffeine containing drinks

Summary. To investigate the influence of taurine and caffeine containing drinks and physical stress on the cortical movement-preparation, the readiness potentials or "Bereitschaftspotentiale" (BPs), preceding voluntary self-placed pedalling movements, were examined after different states of exhaustion on an ergometer. 15 (13 right-handed) healthy men, aged between 22–30, participated in a randomised, cross over, double-blind, placebo controlled study. BPs were averaged out of artefact free EEG-segments from more than 90 triggered events, measured at 17 electrodes of the 10 : 20 system. With increasing effort the BPs were enlarged differently depending on the drink consumed. In placebo trials after exhaustive exercise premovement negative potential curves could be seen even in frontal areas. With caffeine the BPs increased after lower workload, achieving a level, which was reached in the placebo trials only after submaximal physical activation. Furthermore a significant shortening of premovement-brain-potentials in frontal and parietal regions could be seen in the caffeine trials at rest. Taurine admixture seems to inhibit this effects. Received November 2, 1999 / Accepted February 1, 2000     

Beneficial effects of an "energy drink" given to sleepy drivers

Summary. 500 ml of a glucose based "energy" drink versus a control without the active ingredients (caffeine, taurine, glucuronolactone) were given double blind to 11 sleepy participants driving an interactive real-car driving simulator. Lane drifting and a secondary task (reaction time) were measured for two hours post-treatment. The energy drink significantly improved both indices, particularly for the first hour. Received January 2, 2000 / Accepted February 1, 2000     

Compliance of a cobalt chromium coronary stent alloy – the COVIS trial

Aim

Female cardiac transplant recipients' aerobic capacity is 60% lower than sex and age-predicted values. The effect of exercise training on restoring the impaired aerobic endurance and muscle strength in female cardiac transplant recipients is not known. This study examined the effect that aerobic and strength training have on improving aerobic endurance and muscle strength in female cardiac transplant recipients.

Methods

20 female cardiac transplant recipients (51 ± 11 years) participated in this investigation. The subjects performed a baseline six-minute walk test and a leg-press strength test when they were discharged following cardiac transplantation. The subjects then participated in a 12-week exercise program consisting of aerobic and lower extremity strength training. Baseline assessments were repeated following completion of the exercise intervention.

Results

At baseline, the cardiac transplant recipients' aerobic endurance was 50% lower than age-matched predicted values. The training program resulted in a significant increase in aerobic endurance (pre-training: 322 ± 104 m vs. post-training: 501 ± 99 m, p < 0.05) and leg-press strength (pre-training: 48 ± 16 kg. vs. post-training: 78 ± 27 kg, p < 0.05).

Conclusion

Aerobic and strength training are effective interventions that can partially restore the impaired aerobic endurance and strength found in female cardiac transplant recipients.     

Cardiac adaptation to endurance exercise in rats

Endurance exercise is widely assumed to improve cardiac function in humans. This project has determined cardiac function following endurance exercise for 6 (n = 30) or 12 (n = 25) weeks in male Wistar rats (8 weeks old). The exercise protocol was 30 min/day at 0.8 km/h for 5 days/week with an endurance test on the 6th day by running at 1.2 km/h until exhaustion. Exercise endurance increased by 318% after 6 weeks and 609% after 12 weeks. Heart weight/kg body weight increased by 10.2% after 6 weeks and 24.1% after 12 weeks. Echocardiography after 12 weeks showed increases in left ventricular internal diameter in diastole (6.39 ± 0.32 to 7.90 ± 0.17 mm), systolic volume (49 ± 7 to 83 ± 11 μl) and cardiac output (75 ± 3 to 107 ± 8 ml/min) but not left wall thickness in diastole (1.74 ± 0.07 to 1.80 ± 0.06 mm). Isolated Langendorff hearts from trained rats displayed decreased left ventricular myocardial stiffness (22 ± 1.1 to 19.1 ± 0.3) and reduced purine efflux during pacing-induced workload increases. 31P-NMR spectroscopy in isolated hearts from trained rats showed decreased PCr and PCr/ATP ratios with increased creatine, AMP and ADP concentrations. Thus, this endurance exercise protocol resulted in physiological hypertrophy while maintaining or improving cardiac function. (Mol Cell Biochem 251: 51–59, 2003)     

Fructose ingestion acutely elevates blood pressure in healthy young humans

Overconsumption of fructose, particularly in the form of soft drinks, is increasingly recognized as a public health concern. The acute cardiovascular responses to ingesting fructose have not, however, been well-studied in humans. In this randomized crossover study, we compared cardiovascular autonomic regulation after ingesting water and drinks containing either glucose or fructose in 15 healthy volunteers (aged 21-33 yr). The total volume of each drink was 500 ml, and the sugar content 60 g. For 30 min before and 2 h after each drink, we recorded beat-to-beat heart rate, arterial blood pressure, and cardiac output. Energy expenditure was determined on a minute-by-minute basis. Ingesting the fructose drink significantly increased blood pressure, heart rate, and cardiac output but not total peripheral resistance. Glucose ingestion resulted in a significantly greater increase in cardiac output than fructose but no change in blood pressure and a concomitant decrease in total peripheral resistance. Ingesting glucose and fructose, but not water, significantly increased blood pressure variability and decreased cardiovagal baroreflex sensitivity. Energy expenditure increased by a similar amount after glucose and fructose ingestion, but fructose elicited a significantly greater increase in respiratory quotient. These results show that ingestion of glucose and fructose drinks is characterized by specific hemodynamic responses. In particular, fructose ingestion elicits an increase in blood pressure that is probably mediated by an increase in cardiac output without compensatory peripheral vasodilatation.     

Post-exercise rehydration in man: effects of electrolyte addition to ingested fluids

This study examined the effects on water balance of adding electrolytes to fluids ingested after exercise-induced dehydration. Eight healthy male volunteers were dehydrated by approximately 2% of body mass by intermittent cycle exercise. Over a 30-min period after exercise, subjects ingested one of the four test drinks of a volume equivalent to their body mass loss. Drink A was a 90 mmol·l^–1 glucose solution; drink B contained 60 mmol·l^–1 sodium chloride; drink C contained 25 mmol·l^–1 potassium chloride; drink D contained 90 mmol·l^–1 glucose, 60 mmol·l^–1 sodium chloride and 25 mmol·l^–1 potassium chloride. Treatment order was randomised. Blood and urine samples were obtained at intervals throughout the study; subjects remained fasted throughout. Plasma volume increased to the same extent after the rehydration period on all treatments. Serum electrolyte (Na⁺, K⁺ and Cl^–) concentrations fell initially after rehydration before returning to their pre-exercise levels. Cumulative urine output was greater after ingestion of drink A than after ingestion of any of the other drinks. On the morning following the trial, subjects were in greater net negative fluid balance [mean (SEM);P<0.02] on trial A [745 (130) ml] than on trials B [405 (51) ml], C [467 (87) ml] or D [407 (34) ml]. There were no differences at any time between the three electrolyte-containing solutions in urine output or net fluid balance. One hour after the end of the rehydration period, urine osmolality had fallen, with a significant treatment effect (P=0.016); urine osmolality was lowest after ingestion of drink A. On the morning after the test, subjects were in greater net negative sodium balance (P<0.001) after trials A and C than after trials B and D. Negative potassium balance was greater (P<0.001) after trials A and B than after C and D. Chloride balance was positive after drink D and a smaller negative balance (P<0.001) was observed after drink B than after A and C. These results suggest that although the measured blood parameters were similar for all trials, better whole body water and electrolyte balance resulted from the ingestion of electrolyte-containing drinks. There appeared, however, to be no additive effect of including both sodium and potassium under the conditions of this experiment.     

Metabolic, catecholamine, and endurance responses to caffeine during intense exercise

Jackman, M., P. Wendling, D. Friars, and T. E. Graham.Metabolic, catecholamine, and endurance responses to caffeine during intense exercise. J. Appl.Physiol. 81(4): 1658-1663, 1996.This studyexamined the possible effects of caffeine ingestion on muscle metabolism and endurance during brief intense exercise. We tested 14 subjects after they ingested placebo or caffeine (6 mg/kg) with anexercise protocol in which they cycled for 2 min, rested 6 min, cycled2 min, rested 6 min, and then cycled to voluntary exhaustion. In eachexercise the intensity required the subject's maximalO₂ consumption. Eight subjects hadmuscle and venous blood samples taken before and after each exerciseperiod. The caffeine ingestion resulted in a significant increase inendurance (4.12 ± 0.36 and 4.93 ± 0.60 min for placebo andcaffeine, respectively) and resulted in a significant increase inplasma epinephrine concentration throughout the protocol but not innorepinephrine concentration. During the first two exercise bouts, thepower and work output were not different; blood lactate concentrationswere not affected significantly by caffeine ingestion, but during theexercise bouts muscle lactate concentration was significantly increasedby caffeine. The net decrease in muscle glycogen was not differentbetween treatments at any point in the protocol, and even at the time of fatigue there was at least 50% of the original glycogenconcentration remaining. The data demonstrated that caffeine ingestioncan be an effective ergogenic aid for exercise that is as brief as4-6 min. However, the mechanism is not associated with muscleglycogen sparing. It is possible that caffeine is exerting actionsdirectly on the active muscle and/or the neural processes thatare involved in the activity.

     

Effect of different carbohydrate drinks on whole body carbohydrate storage after exhaustive exercise.

Seven untrained male subjects participated in a double-blind, crossover study conducted to determine the efficacy of different carbohydrate drinks in promoting carbohydrate storage in the whole body and skeletal muscle during recovery from exhaustive exercise. The postabsorptive subjects first completed an exercise protocol designed to deplete muscle fibers of glycogen, then consumed 330 ml of one of three carbohydrate drinks (18.5% glucose polymer, 18.5% sucrose, or 12% sucrose; wt/vol) and also received a primed constant infusion of [1-(13)C]glucose for 2 h. Nonoxidative glucose disposal (3.51 +/- 0.28, 18.5% glucose polymer; 2.96 +/- 0.32, 18.5% sucrose; 2.97 +/- 0.16, 12% sucrose; all mmol. kg(-1). h(-1)) and storage of muscle glycogen (5.31 +/- 1.11, 18.5% glucose polymer; 4.07 +/- 1.05, 18.5% sucrose; 3.45 +/- 0.85, 12% sucrose; all mmol. kg wet wt(-1). h(-1); P < 0.05) were greater after consumption of the glucose polymer drink than after either sucrose drink. The results suggest that the consumption of a glucose polymer drink (containing 61 g carbohydrate) promotes a more rapid storage of carbohydrate in the whole body, skeletal muscle in particular, than an isoenergetic sucrose drink.     

Prolonged decrease in cardiac volumes after maximal upright bicycle exercise

Sequential exercise-gated cardiac blood pool scintigrams provide a noninvasive technique for evaluating the effect of therapeutic interventions on cardiac volumes and function only if both exercise periods are equivalent in the absence of an intervention. To assess whether they are indeed equivalent, 14 healthy subjects underwent gated blood pool scintigraphy during two maximal upright exercise periods separated by 60 min without changing position. Although resting cardiac output and blood pressure returned to base-line values 60 min after the first exercise period, mean resting heart rate was markedly higher (89.4 +/- 2.7 vs. 66.5 +/- 2.5 beats/min, P less than 0.001) and upright cardiac volumes lower [39.1 +/- 4.9 vs. 56.3 +/- 6.0 ml, P less than 0.001, for end-systolic volume (ESV) and 112.6 +/- 8.0 vs. 144.9 +/- 9.0 ml, P less than 0.001, for end-diastolic volume (EDV)] than before the first exercise period. These differences persisted during low levels of the subsequent exercise but not at high and maximum work loads. Cardiac volumes and heart rate 60 min after an identical exercise protocol in a second group of 22 subjects who received propranolol, 0.15 mg/kg iv, after their initial exercise, however, were the same as those preexercise. Thus higher sympathetic tone may be responsible for the persistently higher heart rate and decreased cardiac volumes after exercise, and the assumption that cardiac volumes and function are similar during two closely spaced sequential exercise studies is not always valid.     

Effects of alcohol mixed with energy drink and alcohol alone on subjective intoxication

Recent studies suggest that the combination of caffeine-containing drinks together with alcohol might reduce the subjective feelings of alcohol intoxication—the so-called “masking effect”. In this study, we aimed to review the effects of alcohol in combination with caffeine or energy drink with special focus on the “masking effect”. Fifty-two healthy male volunteers were analysed concerning breath alcohol concentration and subjective sensations of intoxication using a 18 item Visual Analogue Scale in a randomised, double-blinded, controlled, four treatments cross-over trial after consumption of (A) placebo, (B) alcohol (vodka 37.5 % at a dose of 46.5 g ethanol), (C) alcohol in combination with caffeine at a dose of 80 mg (equivalent to one 250 ml can of energy drink) and (D) alcohol in combination with energy drink at a dose of 250 ml (one can). Primary variables were headache, weakness, salivation and motor coordination. Out of four primary variables, weakness and motor coordination showed a statistically significant difference between alcohol and non-alcohol group, out of 14 secondary variables, five more variables (dizziness, alterations in sight, alterations in walking, agitation and alterations in speech) also showed significant differences due mainly to contrasts with the non-alcohol group. In none of these end points, could a statistically significant effect be found for the additional ingestion of energy drink or caffeine on the subjective feelings of alcohol intoxication. This within-subjects study does not confirm the presence of a “masking effect” when combining caffeine or energy drink with alcohol.     

Fluid balance in exercise dehydration and rehydration with different glucose-electrolyte drinks

After exercise dehydration (3% of body weight) the restoration of water and electrolyte balance was followed in 6 male subjects. During a 2 h rest period after exercise, a drink of one of four solutions was given as 9 X 300 ml portions at 15 min intervals: control (C-drink), high potassium (K-drink), high sodium (Na-drink) or high sugar (S-drink). An exercise test (submaximal and supramaximal work) was performed before dehydration and after rehydration. Dehydration reduced plasma volume by 16%, a process reversed on resting even before fluid ingestion began, due to release of water accumulated in the muscles during exercise. After 2 h rehydration, plasma volume was above the initial resting value with all 4 drinks. The final plasma volumes after the Na-drink (+14%) and C-drink (+9%) were significantly higher than after the K- and S-drinks. The Na-drink favoured filling of the extracellular compartment, whereas the K- and S-drinks favoured intracellular rehydration. In spite of the higher than normal plasma volume after rehydration, mean heart rate during the submaximal test was 10 bpm higher after rest and rehydration than in the initial test, and was not different between the drinks. The amount of work which could be performed in the supramaximal test (105% VO2max) was 20% less after exercise dehydration and subsequent rest and rehydration than before. This reduction was similar for all drinks, and may be due to a decreased muscle glycogen content (70% of initial) at the time of the second test.     

Effect of caffeine on metabolism, exercise endurance, and catecholamine responses after withdrawal

In this studythe effects of acute caffeine ingestion on exercise performance,hormonal (epinephrine, norepinephrine, insulin), and metabolic (freefatty acids, glycerol, glucose, lactate, expired gases) parametersduring short-term withdrawal from dietary caffeine were investigated.Recreational athletes who were habitual caffeine users(n = 6) (maximum oxygen uptake 54.5 ± 3.3 ml · kg¹ · min¹and daily caffeine intake 761.3 ± 11.8 mg/day) were tested under conditions of no withdrawal and 2-day and 4-day withdrawal from dietarycaffeine. There were seven trials in total with a minimum of 10 daysbetween trials. On the day of the exercise trial, subjects ingestedeither dextrose placebo or 6 mg/kg caffeine in capsule form 1 h beforecycle ergometry to exhaustion at 80-85% of maximum oxygen uptake.Test substances were assigned in a random, double-blind manner. A finalplacebo control trial completed the experiment. There was nosignificant difference in any measured parameters among days ofwithdrawal after ingestion of placebo. At exhaustion in the 2- and4-day withdrawal trials, there were significant increases in plasmanorepinephrine in response to caffeine ingestion. Caffeine-inducedincreases in serum free fatty acids occurred after 4 days and only atrest. Subjects responded to caffeine with increases in plasmaepinephrine (P < 0.05) atexhaustion and prolonged exercise time in all caffeine trials comparedwith placebo, regardless of withdrawal from caffeine. It is concluded that increased endurance is unrelated to hormonal or metabolic changesand that it is not related to prior caffeine habituation inrecreational athletes.