No effect of caffeine on exercise performance in high ambient temperature

Effect of caffeine ingestion on time trial performance in cyclists: a systematic review and meta-analysis

BACKGROUND

Caffeine, widely recognized as an ergogenic aid, has undergone extensive research, demonstrating its effectiveness to enhance endurance performance. However, there remains a significant gap in systematically evaluating its effects on time trial (TT) performance in cyclists.

PURPOSE

This meta-analysis aimed to determine the efficacy of caffeine ingestion to increase cycling TT performance in cyclists and to evaluate the optimal dosage range for maximum effect.

METHODS

A search of four databases was completed on 1 December 2023. The selected studies comprised crossover, placebo-controlled investigations into the effects of caffeine ingestion on cycling TT performance. Completion time (Time) and mean power output (MPO) were used as performance measures for TT. Meta-analyses were performed using a random-effects model to assess the standardized mean differences (SMD) in individual studies.

RESULTS

Fifteen studies met the inclusion criteria for the meta-analyses. Subgroup analysis showed that moderate doses of caffeine intake (4-6 mg/kg) significantly improved cycling performance (SMD Time = -0.55, 95% confidence interval (CI) = -0.84 ~ -0.26, p < 0.01, I2 = 35%; SMD MPO = 0.44, 95% CI = 0.09 ~ 0.79, p < 0.05, I2 = 39%), while the effects of low doses (1-3 mg/kg) of caffeine were not significant (SMD Time = -0.34, 95% CI = -0.84 ~ 0.17, p = 0.19, I2 = 0%; SMD MPO = 0.31, 95% CI = -0.02 ~ 0.65, p = 0.07, I2 = 0%).

CONCLUSION

A moderate dosage (4-6 mg/kg) of caffeine, identified as the optimal dose range, can significantly improve the time trial performance of cyclists, while a low dose (1-3 mg/kg) does not yield improvement. In addition, the improvements in completion time and mean power output resulting from a moderate dose of caffeine are essentially the same in cycling time trails.

Effects of Caffeine-Taurine Co-Ingestion on Endurance Cycling Performance in High Temperature and Humidity Environments

BACKGROUND

Taurine (TAU) and caffeine (CAF), as common ergogenic aids, are known to affect exercise performance; however, the effects of their combined supplementation, particularly in high temperature and humidity environments, have not been studied.

HYPOTHESIS

The combination of TAU and CAF will have a greater effect on endurance cycle performance and improve changes in physiological indicators during exercise compared with TAU or CAF supplementation alone and placebo.

STUDY DESIGN

Single-blind crossover randomized controlled study.

LEVEL OF EVIDENCE

Level 1.

METHODS

Twelve university students majoring in physical education volunteered to receive 4 different supplement ingestions: (1) placebo (maltodextrin), (2) TAU, (3) CAF, (4) TAU + CAF. After a 7-day washout period, participants completed a time to exhaustion (TTE) test in the heat (35°C, 65% relative humidity).

RESULTS

All experimental groups improved TTE compared with the placebo group. Peak and mean power of countermovement jump were significantly higher in the CAF group compared with the placebo group before the exhaustion exercise (P = 0.02, d = 1.2 and P = 0.04, d = 1.1, respectively). Blood lactate was significantly lower after the exhaustion test in the TAU group compared with the CAF (P < 0.01, d = 0.8) and TAU + CAF (P < 0.01, d = 0.7) groups. Core temperature in the TAU group was significantly reduced in the placebo group later in the exhaustion test (P < 0.01, d = 1.9).

CONCLUSION

In high temperature and humidity environments, acute TAU, CAF, and combined supplementation all improved TTE and did not affect recovery from lower limb neuromuscular fatigue compared with placebo, with TAU having the best effect. Combined supplementation failed to exhibit superimposed performance.

CLINICAL RELEVANCE

The results provide suggestions for the effects of TAU, CAF, and their combined intake on exercise performance in high temperature and humidity environments.

Effect of Pre-Exercise Caffeine Intake on Endurance Performance and Core Temperature Regulation During Exercise in the Heat: A Systematic Review with Meta-Analysis

BACKGROUND

Heat is associated with physiological strain and endurance performance (EP) impairments. Studies have investigated the impact of caffeine intake upon EP and core temperature (C_T) in the heat, but results are conflicting. There is a need to systematically determine the impact of pre-exercise caffeine intake in the heat.

OBJECTIVE

To use a meta-analytical approach to determine the effect of pre-exercise caffeine intake on EP and C_T in the heat.

DESIGN

Systematic review with meta-analysis.

DATA SOURCES

Four databases and cross-referencing.

DATA ANALYSIS

Weighted mean effect summaries using robust variance random-effects models for EP and C_T, as well as robust variance meta-regressions to explore confounders.

STUDY SELECTION

Placebo-controlled, randomized studies in adults (≥ 18 years old) with caffeine intake at least 30 min before endurance exercise ≥ 30 min, performed in ambient conditions ≥ 27 °C.

RESULTS

Respectively six and 12 studies examined caffeine's impact on EP and C_T, representing 52 and 205 endurance-trained individuals. On average, 6 mg/kg body mass of caffeine were taken 1 h before exercises of ~ 70 min conducted at 34 °C and 47% relative humidity. Caffeine supplementation non-significantly improved EP by 2.1 ± 0.8% (95% CI - 0.7 to 4.8) and significantly increased the rate of change in C_T by 0.10 ± 0.03 °C/h (95% CI 0.02 to 0.19), compared with the ingestion of a placebo.

CONCLUSION

Caffeine ingestion of 6 mg/kg body mass ~ 1 h before exercise in the heat may provide a worthwhile improvement in EP, is unlikely to be deleterious to EP, and trivially increases the rate of change in C_T.

Can I Have My Coffee and Drink It? A Systematic Review and Meta-analysis to Determine Whether Habitual Caffeine Consumption Affects the Ergogenic Effect of Caffeine

OBJECTIVE

The aim was to quantify the proportion of the literature on caffeine supplementation that reports habitual caffeine consumption, and determine the influence of habitual consumption on the acute exercise response to caffeine supplementation, using a systematic review and meta-analytic approach.

METHODS

Three databases were searched, and articles screened according to inclusion/exclusion criteria. Three-level meta-analyses and meta-regression models were used to investigate the influence of habitual caffeine consumption on caffeine's overall ergogenic effect and within different exercise types (endurance, power, strength), in men and women, and in trained and untrained individuals. Sub-analyses were performed according to the following: acute relative dose (< 3, 3-6, > 6 mg/kg body mass [BM]); whether the acute caffeine dose provided was lower or higher than the mean daily caffeine dose; and the caffeine withdrawal period prior to the intervention (< 24, 24-48, > 48 h).

RESULTS

Sixty caffeine studies included sufficient information on habitual consumption to be included in the meta-analysis. A positive overall effect of caffeine was shown in comparison to placebo (standard mean difference [SMD] = 0.25, 95% confidence interval [CI] 0.20-0.30; p < 0.001) with no influence of relative habitual caffeine consumption (p = 0.59). Subgroup analyses showed a significant ergogenic effect when the caffeine dose was < 3 mg/kg BM (SMD = 0.26, 95% CI 0.12-0.40; p = 0.003) and 3-6 mg/kg BM (SMD = 0.26, 95% CI 0.21-0.32; p < 0.0001), but not > 6 mg/kg BM (SMD = 0.11, 95% CI - 0.07 to 0.30; p = 0.23); when the dose was both higher (SMD = 0.26, 95% CI 0.20-0.31; p < 0.001) and lower (SMD = 0.21, 95% CI 0.06-0.36; p = 0.006) than the habitual caffeine dose; and when withdrawal was < 24 h, 24-48 h, and > 48 h. Caffeine was effective for endurance, power, and strength exercise, with no influence (all p ≥ 0.23) of relative habitual caffeine consumption within exercise types. Habitual caffeine consumption did not modify the ergogenic effect of caffeine in male, female, trained or untrained individuals.

CONCLUSION

Habitual caffeine consumption does not appear to influence the acute ergogenic effect of caffeine.

Nutritional optimization for female elite football players-topical review

Women's football is an intermittent sport characterized by frequent intense actions throughout the match. The high number of matches with limited recovery time played across a long competitive season underlines the importance of nutritional strategies to meet these large physical demands. In order to maximize sport performance and maintain good health, energy intake must be optimal. However, a considerable proportion of female elite football players does not have sufficient energy intake to match the energy expenditure, resulting in low energy availability that might have detrimental physiologic consequences and impair performance. Carbohydrates appear to be the primary fuel covering the total energy supply during match-play, and female elite football players should aim to consume sufficient carbohydrates to meet the requirements of their training program and to optimize the replenishment of muscle glycogen stores between training bouts and matches. However, several macro- and micronutrients are important for ensuring sufficient energy and nutrients for performance optimization and for overall health status in female elite football players. The inadequacy of macro-and micronutrients in the diet of these athletes may impair performance and training adaptations, and increase the risk of health disorders, compromising the player's professional career. In this topical review, we present knowledge and relevant nutritional recommendations for elite female football players for the benefit of sports nutritionists, dietitians, sports scientists, healthcare specialists, and applied researchers. We focus on dietary intake and cover the most pertinent topics in sports nutrition for the relevant physical demands in female elite football players as follows: energy intake, macronutrient and micronutrient requirements and optimal composition of the everyday diet, nutritional and hydration strategies to optimize performance and recovery, potential ergogenic effects of authorized relevant supplements, and future research considerations.

The Effect of Dietary Supplements on Endurance Exercise Performance and Core Temperature in Hot Environments: A Meta-analysis and Meta-regression

BACKGROUND

The ergogenic effects of dietary supplements on endurance exercise performance are well-established; however, their efficacy in hot environmental conditions has not been systematically evaluated.

OBJECTIVES

(1) To meta-analyse studies investigating the effects of selected dietary supplements on endurance performance and core temperature responses in the heat. Supplements were included if they were deemed to: (a) have a strong evidence base for 'directly' improving thermoneutral endurance performance, based on current position statements, or (b) have a proposed mechanism of action that related to modifiable factors associated with thermal balance. (2) To conduct meta-regressions to evaluate the moderating effect of selected variables on endurance performance and core temperature responses in the heat following dietary supplementation.

METHODS

A search was performed using various databases in May 2020. After screening, 25 peer-reviewed articles were identified for inclusion, across three separate meta-analyses: (1) exercise performance; (2) end core temperature; (3) submaximal core temperature. The moderating effect of several variables were assessed via sub-analysis and meta-regression.

RESULTS

Overall, dietary supplementation had a trivial significant positive effect on exercise performance (Hedges' g = 0.18, 95% CI 0.007-0.352, P = 0.042), a trivial non-significant positive effect on submaximal core temperature (Hedges' g = 0.18, 95% CI - 0.021 to 0.379, P = 0.080) and a small non-significant positive effect on end core temperature (Hedges' g = 0.20, 95% CI - 0.041 to 0.439, P = 0.104) in the heat. There was a non-significant effect of individual supplements on exercise performance (P = 0.973) and submaximal core temperature (P = 0.599). However, end core temperature was significantly affected by supplement type (P = 0.003), which was attributable to caffeine's large significant positive effect (n = 8; Hedges' g = 0.82, 95% CI 0.433-1.202, P < 0.001) and taurine's medium significant negative effect (n = 1; Hedges' g = - 0.96, 95% CI - 1.855 to - 0.069, P = 0.035).

CONCLUSION

Supplements such as caffeine and nitrates do not enhance endurance performance in the heat, with caffeine also increasing core temperature responses. Some amino acids might offer the greatest performance benefits in the heat. Exercising in the heat negatively affected the efficacy of many dietary supplements, indicating that further research is needed and current guidelines for performance in hot environments likely require revision.

Caffeine metabolites are associated with different forms of caffeine supplementation and with perceived exertion during endurance exercise

This investigation compared the urine caffeine metabolites produced from different forms of caffeine supplementation given to runners 15 minutes before a series of 5-km running trials. Fourteen amateur competitive runners completed a series of self-paced outdoor time trials following ingestion of placebo or one of three alternate forms of caffeine supplement. Trials were randomized in a crossover design with equivalent doses of caffeine (4.0 mg.kg-1) administered 15 minutes before each trial via chewing gum, a novel dissolvable mouth strip or tablet. Runners produced a urine sample following each caffeinated trial that was tested for caffeine and its metabolites by high-performance liquid chromatography. The tablet form of caffeine produced a lower (p = 0.04) urinary ratio of the metabolite paraxanthine to caffeine compared with either gum or strip. Independently of caffeine delivery mode, subjects who metabolized a higher proportion of caffeine to paraxanthine recorded a lower (p = 0.01) perceived exertion. We demonstrate that oral swallowed caffeine administered 15 minutes before 5-km running is less metabolized compared with caffeinated products designed to be chewed or dissolved in the mouth. We suggest the metabolism of caffeine to paraxanthine has an inverse relationship with perceived exertion independently of caffeine delivery mode.

Novel insights on caffeine supplementation, CYP1A2 genotype, physiological responses and exercise performance

Caffeine is a popular ergogenic aid due to its primary physiological effects that occur through antagonism of adenosine receptors in the central nervous system. This leads to a cascade of physiological reactions which increases focus and volition, and reduces perception of effort and pain, contributing to improved exercise performance. Substantial variability in the physiological and performance response to acute caffeine consumption is apparent, and a growing number of studies are implicating a single-nucleotide polymorphism in the CYP1A2 gene, responsible for caffeine metabolism, as a key factor that influences the acute responses to caffeine ingestion. However, existing literature regarding the influence of this polymorphism on the ergogenic effects of caffeine is controversial. Fast caffeine metabolisers (AA homozygotes) appear most likely to benefit from caffeine supplementation, although over half of studies showed no differences in the responses to caffeine between CYP1A2 genotypes, while others even showed either a possible advantage or disadvantage for C-allele carriers. Contrasting data are limited by weak study designs and small samples sizes, which did not allow separation of C-allele carriers into their sub-groups (AC and CC), and insufficient mechanistic evidence to elucidate findings. Mixed results prevent practical recommendations based upon genotype while genetic testing for CYP1A2 is also currently unwarranted. More mechanistic and applied research is required to elucidate how the CYP1A2 polymorphism might alter caffeine's ergogenic effect and the magnitude thereof, and whether CYP1A2 genotyping prior to caffeine supplementation is necessary.

International society of sports nutrition position stand: caffeine and exercise performance

Following critical evaluation of the available literature to date, The International Society of Sports Nutrition (ISSN) position regarding caffeine intake is as follows: 1. Supplementation with caffeine has been shown to acutely enhance various aspects of exercise performance in many but not all studies. Small to moderate benefits of caffeine use include, but are not limited to: muscular endurance, movement velocity and muscular strength, sprinting, jumping, and throwing performance, as well as a wide range of aerobic and anaerobic sport-specific actions. 2. Aerobic endurance appears to be the form of exercise with the most consistent moderate-to-large benefits from caffeine use, although the magnitude of its effects differs between individuals. 3. Caffeine has consistently been shown to improve exercise performance when consumed in doses of 3-6 mg/kg body mass. Minimal effective doses of caffeine currently remain unclear but they may be as low as 2 mg/kg body mass. Very high doses of caffeine (e.g. 9 mg/kg) are associated with a high incidence of side-effects and do not seem to be required to elicit an ergogenic effect. 4. The most commonly used timing of caffeine supplementation is 60 min pre-exercise. Optimal timing of caffeine ingestion likely depends on the source of caffeine. For example, as compared to caffeine capsules, caffeine chewing gums may require a shorter waiting time from consumption to the start of the exercise session. 5. Caffeine appears to improve physical performance in both trained and untrained individuals. 6. Inter-individual differences in sport and exercise performance as well as adverse effects on sleep or feelings of anxiety following caffeine ingestion may be attributed to genetic variation associated with caffeine metabolism, and physical and psychological response. Other factors such as habitual caffeine intake also may play a role in between-individual response variation. 7. Caffeine has been shown to be ergogenic for cognitive function, including attention and vigilance, in most individuals. 8. Caffeine may improve cognitive and physical performance in some individuals under conditions of sleep deprivation. 9. The use of caffeine in conjunction with endurance exercise in the heat and at altitude is well supported when dosages range from 3 to 6 mg/kg and 4-6 mg/kg, respectively. 10. Alternative sources of caffeine such as caffeinated chewing gum, mouth rinses, energy gels and chews have been shown to improve performance, primarily in aerobic exercise. 11. Energy drinks and pre-workout supplements containing caffeine have been demonstrated to enhance both anaerobic and aerobic performance.

Caffeine and Exercise Performance: Possible Directions for Definitive Findings

Caffeine is one of the most studied supplements in the world. Studies correlate its use to increased exercise performance in endurance activities, as well as its possible ergogenic effects for both intermittent and strength activities. Recent findings show that caffeine may increase or decrease exercise performance. These antagonist responses may occur even when using the same dosage and for individuals with the same characteristics, making it challenging to explain caffeine's impact and applicability. This review article provides an analytic look at studies involving the use of caffeine for human physical performance, and addresses factors that could influence the ergogenic effects of caffeine on different proposed activities. These factors subdivide into caffeine effects, daily habits, physiological factors, and genetic factors. Each variable has been focused on by discussions to research related to caffeine. A better understanding and control of these variables should be considered in future research into personalized nutritional strategies.

No significant effect of caffeine on five kilometer running performance after muscle damage

Caffeine has documented hypoalgesic effects during exercise. However, there is a lack of research focusing on caffeine's potential analgesic effects to ameliorate delayed onset muscle soreness. A placebo controlled randomized cross-over trial was carried out to determine if 5 mg/kg of body weight (mg/kgBW) of caffeine attenuates muscle pain and improves 5 k running performance following delayed onset muscle soreness. Prior to participating, eleven runners (9 male; 2 female; age, 24.5 ± 6.3 years; height, 173.6 ± 7.8 cm; body mass, 66.3 ± 7.5 kg; BMI, 23.18 kg/m² ± 1.6; VO_2max 61.0 ± 6.1 ml/kg/min^-1), were asked to discontinue supplement use for 72 hours and abstain from caffeine consumption for 48 hours. Participants performed a 30-minute downhill run on a treadmill set at -10% grade at 70% VO_2max to induce delayed onset of muscle soreness. Participants then returned 48 hours after to complete a 5 k time trial run where they consumed either 5 mg/kgBW of caffeine or a placebo. Rate of perceived exertion and heart rate were taken every two minutes during the trial. There was no detectable statistical difference between 5 k performance between caffeine (1074.9 ± 119.7 sec) or placebo (1053.8 ± 86.8 sec) (p = .41). Algometer readings were similar between both treatments for muscle soreness in the rectus femoris (p = .791) and the vastus medialis oblique (p = .371). Muscle soreness ratings were found to be greater in the caffeine condition compared to the placebo condition (p = .030). There was no effect of treatment on rating of perceived exertion between conditions (p = .574). The present study suggests that caffeine is not effective at reducing muscle soreness, rating of perceived exertion, or improving running performance in a time trial in the presence of muscle soreness.

Low caffeine dose improves intermittent sprint performance in hot and humid environments

While the effects of caffeine have been evaluated in relation to endurance exercise, few studies have assessed the ergogenic effects of low caffeine doses on intermittent exercise performance in hot and humid environments. Thus, we aimed to determine the effects of low-dose caffeine supplementation on intermittent exercise performance under these conditions. Eight male soccer players (age, 19.9 ± 0.3 years; height, 173.7 ± 6.3 cm; body mass, 65.1 ± 5.5 kg; V˙O₂max, 50.0 ± 3.1 mL ⋅ kg^-1⋅ min^-1) participated in this double-blind, randomized, cross-over study. Caffeine was orally administered at 60 min before exercise (dosage, 3 mg ⋅ kg^-1). The participants completed a 90-min intermittent sprint cycling protocol under two conditions (after receiving caffeine and placebo) at 32 °C and at 70% relative humidity. A significant improvement in the total amount of work was observed in the caffeine condition compared to the placebo condition (155.0 ± 15.8 vs 150.8 ± 14.5 kJ, respectively; p < 0.05, d = 0.28). In contrast, the rectal temperature measured at the end of exercise showed no significant difference between the conditions (38.9 ± 0.4 °C and 38.7 ± 0.5 °C in the caffeine and placebo conditions, respectively; p > 0.05, d = 0.57). Other thermal responses, such as the mean skin temperature, heart rate, or sweat volume, were not significantly different between these conditions. These results suggested that a low caffeine dose improved the intermittent sprint performance and the reasons could be explained by the fact that a low caffeine dose ingestion did not affect the thermoregulatory responses compared to the placebo condition and, thus, did not attenuate its ergogenic effect on exercise in hot and humid environments.

Effect of Caffeine on Endurance Performance in Athletes May Depend on HTR2A and CYP1A2 Genotypes

Guest, NS, Corey, P, Tyrrell, PN, and El-Sohemy, A. Effect of caffeine on endurance performance in athletes may depend on HTR2A and CYP1A2 genotypes. J Strength Cond Res XX(X): 000-000, 2020-This investigation determined whether variation in the HTR2A (serotonin receptor) gene modifies the ergogenic effects of caffeine on endurance and further modifies performance by the CYP1A2 genotype. Male athletes (n = 100; 25 ± 4 years) completed 10-km cycling time trials under 3 conditions as follows: 0, 2, or 4 mg of caffeine per kg body mass. Using a randomized, double-blinded, placebo-controlled design, data were analyzed using analysis of covariance to compare changes in cycling time between placebo (0 mg·kg) and each caffeine dose and adjusted for the placebo trial and order of treatment. A significance of ρ ≤ 0.05 was used. Subjects were genotyped for HTR2A (rs6313) and CYP1A2 (rs762551). A significant caffeine-HTR2A interaction (p = 0.003) was observed; however, after adjustment for placebo trials, the interaction was no longer significant (p = 0.37). Because of the strong caffeine-CYP1A2 interaction (p < 0.0001) previously reported in these subjects, where the 4-mg dose resulted in divergent effects (slower and faster) on the 10-km cycling time, we conducted a simplified model to examine these same factors by the HTR2A genotype. The post hoc analysis excluded HTR2A CT heterozygotes and 2-mg·kg caffeine trials. Among CYP1A2 fast metabolizers alone, a significant difference (1.7 minutes; p = 0.006) was observed when comparing (4- vs. 0-mg·kg caffeine trials) between the HTR2A CC (n = 16; 2.4 minutes) and TT (n = 7; 0.7 minutes) genotypes. Our results show that 4-mg·kg caffeine improves performance in individuals with the HTR2A CC genotype but only in those who are also CYP1A2 AA fast metabolizers. This study was registered with clinicaltrials.gov (NCT02109783).

Effects of caffeine on neuromuscular function in a non-fatigued state and during fatiguing exercise

NEW FINDINGS

What is the central question of the study? What are the effects of caffeine on neuromuscular function in a non-fatigued state and during fatiguing exercise? What is the main finding and its importance? In a non-fatigued state, caffeine decreased the duration of the silent period evoked by transcranial magnetic stimulation. Caffeine-induced reduction of inhibitory mechanisms in the central nervous system before exercise was associated with an increased performance. Individuals who benefit from caffeine ingestion may experience lower perception of effort during exercise and an accelerated recovery of M-wave amplitude postfatigue. This study elucidates the mechanisms of action of caffeine and demonstrates that inter-individual variability of its effects on neuromuscular function is a fruitful area for further work.

ABSTRACT

Caffeine enhances exercise performance, but its mechanisms of action remain unclear. In this study, we investigated its effects on neuromuscular function in a non-fatigued state and during fatiguing exercise. Eighteen men participated in this randomized, double-blind, placebo-controlled crossover trial. Baseline measures included plantarflexion force, drop jump, squat jump, voluntary activation of triceps surae muscle, soleus muscle contractile properties, M-wave, α-motoneuron excitability (H-reflex), corticospinal excitability, short-interval intracortical inhibition, intracortical facilitation, silent period evoked by transcranial magnetic stimulation (SP) and plasma potassium and caffeine concentrations. Immediately after baseline testing, participants ingested caffeine (6 mg·kg^-1 ) or placebo. After a 1-h rest, baseline measures were repeated, followed by a fatiguing stretch-shortening cycle exercise (sets of 40 bilateral rebound jumps on a sledge apparatus) until task failure. Neuromuscular testing was carried out throughout the fatigue protocol and afterwards. Caffeine enhanced drop jump height (by 4.2%) and decreased the SP (by 12.6%) in a non-fatigued state. A caffeine-related decrease in SP and short-interval intracortical inhibition before the fatiguing activity was associated with an increased time to task failure. The participants who benefitted from an improved performance on the caffeine day reported a significantly lower sense of effort during exercise and had an accelerated postexercise recovery of M-wave amplitude. Caffeine modulates inhibitory mechanisms of the CNS, recovery of M-wave amplitude and perception of effort. This study lays the groundwork for future examinations of differences in caffeine-induced neuromuscular changes between those who are deemed to benefit from caffeine ingestion and those who are not.

Increasing Warmth in Adolescents with Anorexia Nervosa: A Randomized Controlled Crossover Trial Examining the Efficacy of Mustard and Ginger Footbaths

Objective

To analyze the thermogenic effects of footbaths with medicinal powders in adolescents with anorexia nervosa (AN) in comparison to healthy controls (HCs). Intervention and Outcomes. Forty-one female participants (21 AN, 20 HCs; 14.22 ± 1.54 years) received three footbaths-warm water and mustard (MU, Sinapis nigra), warm water and ginger (GI, Zingiber officinale), or warm water only (WA), in random order within a crossover design. Data were collected before (t1), immediately after foot immersion (maximum 20 minutes) (t2), and after 10 minutes subsequently (t3). Actual skin temperature (high resolution thermography) and perceived warmth (HeWEF questionnaire) were assessed at each time point for various body parts. The primary outcome measure was self-perceived warmth at the feet at t3. Secondary outcome measures were objective skin temperature and subjective warmth at the face, hands, and feet.

Results

Perceived warmth at the feet at t3 was significantly higher after GI compared to WA (mean difference -1.02) and MU (-1.07), with no differences between those with AN and HC (-0.29). For the secondary outcome measures, a craniocaudal temperature gradient for the skin temperature (thermography) was noted at t1 for patients with AN and HC (AN with colder feet). The craniocaudal gradient for subjective warmth was only seen for patients with AN.

Conclusion

Footbaths with ginger increased warmth perception at the feet longer than with mustard or warm water only for adolescents with AN as well as for HC. The impact of ginger footbaths on recovery of thermoregulatory disturbances in patients with AN repeated over extended periods merits further investigation.

Blood dopamine level enhanced by caffeine in men after treadmill running

The aim of this study was to investigate the plasma dopamine and serum serotonin levels in humans with and without caffeine (CAFF) ingestion during treadmill running exercise. Thirty male volunteers participated in the randomized experiment involving two groups: CON (n = 15, 200 mL of tap water) versus CAFF (n = 15, 3 mg/kg CAFF and 200 mL tap water). After treadmill running, the dopamine level was significantly increased in the CAFF group (P < 0.01) and was significantly higher than in the CON group (P < 0.01). Serotonin was significantly increased in both groups after treadmill running (P < 0.05). However, serotonin levels showed no significant statistical difference between the groups. Prolactin and cortisol were significantly increased in both groups after treadmill running (P < 0.01). However, there was no significant statistical difference between groups. β-endorphin level was significantly increased in the CAFF group at after treadmill running (P < 0.01) and was significantly higher than in CON after treadmill running (P < 0.01). In conclusion, 3 mg/kg CAFF ingestion before treadmill running stimulated dopamine release without inhibiting serotonin, which may reduce central fatigue.

Sport Nutrigenomics: Personalized Nutrition for Athletic Performance

An individual's dietary and supplement strategies can influence markedly their physical performance. Personalized nutrition in athletic populations aims to optimize health, body composition, and exercise performance by targeting dietary recommendations to an individual's genetic profile. Sport dietitians and nutritionists have long been adept at placing additional scrutiny on the one-size-fits-all general population dietary guidelines to accommodate various sporting populations. However, generic "one-size-fits-all" recommendations still remain. Genetic differences are known to impact absorption, metabolism, uptake, utilization and excretion of nutrients and food bioactives, which ultimately affects a number of metabolic pathways. Nutrigenomics and nutrigenetics are experimental approaches that use genomic information and genetic testing technologies to examine the role of individual genetic differences in modifying an athlete's response to nutrients and other food components. Although there have been few randomized, controlled trials examining the effects of genetic variation on performance in response to an ergogenic aid, there is a growing foundation of research linking gene-diet interactions on biomarkers of nutritional status, which impact exercise and sport performance. This foundation forms the basis from which the field of sport nutrigenomics continues to develop. We review the science of genetic modifiers of various dietary factors that impact an athlete's nutritional status, body composition and, ultimately athletic performance.

Increased Rate of Heat Storage, and No Performance Benefits, With Caffeine Ingestion Before a 10-km Run in Hot, Humid Conditions

PURPOSE

Although the effect of caffeine in thermoneutral or cool environmental conditions has generally shown performance benefits, its efficacy in hot, humid conditions is not as well known. The purpose of this study was to further examine the effect of caffeine ingestion on endurance running performance in the heat.

METHODS

Ten trained endurance runners (6 males; mean [SD] age = 26 [9] y, height = 176.7 [5.1] cm, and mass = 72.1 [8.7] kg) came to the lab for 4 visits. The first was a VO₂max test to determine cardiorespiratory fitness; the final 3 visits were 10-km runs in an environmental chamber at 30.6°C and 50% relative humidity under different conditions: 3 mg·kg^-1 body mass (low caffeine dosage), 6 mg·kg^-1 (moderate caffeine dosage), and a placebo. Repeated-measures analyses of variance were used to determine the effect of condition on the 10-km time, heart rate, core temperature, rating of perceived exertion, and thermal sensation.

RESULTS

There was no difference in the 10-km time between the placebo (53.2 [8.0] min), 3-mg·kg^-1 (53.4 [8.4]), and 6-mg·kg^-1 (52.7 [8.2]) conditions (P = .575, ηp2=.060 ). There was not a main effect of average heart rate (P = .406, ηp2=.107 ), rating of perceived exertion (P = .151, ηp2=.189 ), or thermal sensation (P = .286, ηp2=.130 ). There was a significant interaction for core temperature (P = .025, ηp2=.170 ); the moderate-dosage caffeine condition showed a higher rate of rise in core temperature (0.26 [0.08] °C·km^-1 vs 0.20 [0.06] and 0.19 [0.10] °C·km^-1 in the low-caffeine and placebo conditions, respectively).

CONCLUSION

The results support previous research showing a thermogenic effect of caffeine, as the moderate-dosage condition led to a greater rate of heat storage and no performance benefits.

Effects of Footbaths with Mustard, Ginger, or Warm Water Only on Objective and Subjective Warmth Distribution in Healthy Subjects: A Randomized Controlled Trial

OBJECTIVE

To analyze the short-term thermogenic effects of footbaths with warm water alone (WA) versus when combined with medicinal powders.

DESIGN

Randomized controlled trial with cross-over.

INTERVENTIONS AND OUTCOMES

Seventeen healthy volunteers (mean age 22.1 years, SD = 2.4; 11 female) received three footbaths with WA or WA combined with mustard (MU) or ginger (GI) in a randomized order. Self-perceived warmth (Herdecke warmth perception questionnaire) and actual skin temperatures (thermography) were assessed before (t0), immediately after footbaths (t1), and 10 minutes later (t2). The primary outcome was perceived warmth in the feet. Secondary outcomes were warmth perception in the face, hands and overall, as well as actual skin temperature in the feet, face, and hands.

RESULTS

Perceived warmth at the feet (primary outcome) increased significantly (all p's < .001) for MU and GI at t1 as well as for GI at t2 when compared to t0 with high effect sizes. At t2, GI differed significantly from WA (p < .001) and MU (p = .048). With regards to the secondary measures of outcome, no significant effects were seen for perceived warmth at the face or hands. Overall warmth was significantly higher at t1 compared to t0 (p = .01). Thermography assessments of skin temperature at the feet at t1 increased after all conditions (p < .001). No effects were seen in the face. At the hands, temperature decreased at t1 (p = .02) and t2 compared to t0 (p < .001).

CONCLUSION

The present study provides preliminary evidence that mustard and ginger increase warmth perception at the feet more than warm water alone, with only the effects for GI enduring at the brief follow-up.

The Effect of Acute Caffeine Ingestion on Endurance Performance: A Systematic Review and Meta-Analysis

BACKGROUND

Caffeine is a widely used ergogenic aid with most research suggesting it confers the greatest effects during endurance activities. Despite the growing body of literature around the use of caffeine as an ergogenic aid, there are few recent meta-analyses that quantitatively assess the effect of caffeine on endurance exercise.

OBJECTIVES

To summarise studies that have investigated the ergogenic effects of caffeine on endurance time-trial performance and to quantitatively analyse the results of these studies to gain a better understanding of the magnitude of the ergogenic effect of caffeine on endurance time-trial performance.

METHODS

A systematic review was carried out on randomised placebo-controlled studies investigating the effects of caffeine on endurance performance and a meta-analysis was conducted to determine the ergogenic effect of caffeine on endurance time-trial performance.

RESULTS

Forty-six studies met the inclusion criteria and were included in the meta-analysis. Caffeine has a small but evident effect on endurance performance when taken in moderate doses (3-6 mg/kg) as well as an overall improvement following caffeine compared to placebo in mean power output (3.03 ± 3.07%; effect size = 0.23 ± 0.15) and time-trial completion time (2.22 ± 2.59%; effect size = 0.41 ± 0.2). However, differences in responses to caffeine ingestion have been shown, with two studies reporting slower time-trial performance, while five studies reported lower mean power output during the time-trial.

CONCLUSION

Caffeine can be used effectively as an ergogenic aid when taken in moderate doses, such as during sports when a small increase in endurance performance can lead to significant differences in placements as athletes are often separated by small margins.

The Role of Genetics in Moderating the Inter-Individual Differences in the Ergogenicity of Caffeine

Caffeine use is widespread among athletes following its removal from the World Anti-Doping Agency banned list, with approximately 75% of competitive athletes using caffeine. While literature supports that caffeine has a small positive ergogenic effect for most forms of sports and exercise, there exists a significant amount of inter-individual difference in the response to caffeine ingestion and the subsequent effect on exercise performance. In this narrative review, we discuss some of the potential mechanisms and focus on the role that genetics has in these differences. CYP1A2 and ADORA2A are two of the genes which are thought to have the largest impact on the ergogenicity of caffeine. CYP1A2 is responsible for the majority of the metabolism of caffeine, and ADORA2A has been linked to caffeine-induced anxiety. The effects of CYP1A2 and ADORA2A genes on responses to caffeine will be discussed in detail and an overview of the current literature will be presented. The role of these two genes may explain a large portion of the inter-individual variance reported by studies following caffeine ingestion. Elucidating the extent to which these genes moderate responses to caffeine during exercise will ensure caffeine supplementation programs can be tailored to individual athletes in order to maximize the potential ergogenic effect.

Establishing a relationship between the effect of caffeine and duration of endurance athletic time trial events: A systematic review and meta-analysis

OBJECTIVES

Caffeine has well-documented benefits on endurance athletic performance. Because of caffeine's ergogenic effects of reducing perceived fatigue, it is hypothesized that as duration of athletic event increases, so will the effect size of caffeine upon performance. This study aims to examine the relationship between duration of endurance athletic event and the effect size of caffeine compared to placebo for athletic performance.

DESIGN

A systematic review and meta-analysis of placebo-controlled trials assessing the effects of caffeine in adults performing endurance athletic events.

METHODS

We searched MedLine, Web of Science, and review article references published through March 2016. We performed meta-analyses on placebo-controlled trials to determine the effect of the duration of an endurance athletic event on the standardized mean difference (Cohen's d) between the caffeine and placebo groups for athletic performance.

RESULTS

Forty articles including 56 unique comparison groups were included. Pooled results showed a Cohen's d of 0.33 (95% CI=0.21, 0.45; p=1.00; I²=0%). The effect of the duration of athletic event was significantly associated with Cohen's d (Relative Risk: 0.005; 95% CI=0.001, 0.009; p=0.024). For a 30min increase in duration of the athletic event, Cohen's d will increase by 0.150.

CONCLUSIONS

This study is the first to report on the statistical finding that the effect size of caffeine increases along with the increasing duration of the time trial event. Endurance athletes may especially benefit from caffeine for performance enhancement.

Effect of a moderate caffeine dose on endurance cycle performance and thermoregulation during prolonged exercise in the heat

OBJECTIVES

This study investigated the influence of a moderate caffeine dose on endurance cycle performance and thermoregulation during prolonged exercise in high ambient temperature.

DESIGN

Double-blind cross-over study.

METHODS

Eight healthy, recreationally active males (mean±SD; age: 22±1 years; body mass: 71.1±8.5kg; VO_2peak: 55.9±5.8mLkg^-1min^-1; W_max: 318±37W) completed one VO_2peak test, one familiarisation trial and two experimental trials. After an overnight fast, participants ingested a placebo or a 6mgkg^-1 caffeine dose 60min before exercise. The exercise protocol consisted of 60min of cycle exercise at 55% W_max, followed by a 30min performance task (total kJ produced) in 30°C and 50% RH.

RESULTS

Performance was enhanced (Cohen's d effect size=0.22) in the caffeine trial (363.8±47.6kJ) compared with placebo (353.0±49.0kJ; p=0.004). Caffeine did not influence core (p=0.188) or skin temperature (p=0.577) during exercise. Circulating prolactin (p=0.572), cortisol (p=0.842) and the estimated rates of fat (p=0.722) and carbohydrate oxidation (p=0.454) were also similar between trial conditions. Caffeine attenuated perceived exertion during the initial 60min of exercise (p=0.033), with no difference in thermal stress across trials (p=0.911).

CONCLUSIONS

Supplementation with 6mgkg^-1 caffeine improved endurance cycle performance in a warm environment, without differentially influencing thermoregulation during prolonged exercise at a fixed work-rate versus placebo. Therefore, moderate caffeine doses which typically enhance performance in temperate environmental conditions also appear to benefit endurance performance in the heat.

Effects of caffeine on endurance capacity and psychological state in young females and males exercising in the heat

Acute caffeine ingestion is considered effective in improving endurance capacity and psychological state. However, current knowledge is based on the findings of studies that have been conducted on male subjects mainly in temperate environmental conditions, but some physiological and psychological effects of caffeine differ between the sexes. The purpose of this study was to compare the physical performance and psychological effects of caffeine in young women and men exercising in the heat. Thirteen male and 10 female students completed 2 constant-load walks (60% of thermoneutral peak oxygen consumption on a treadmill until volitional exhaustion) in a hot-dry environment (air temperature, 42 °C; relative humidity, 20%) after caffeine (6 mg·kg–1) and placebo (wheat flour) ingestion in a double-blind, randomly assigned, crossover manner. Caffeine, compared with placebo, induced greater increases (p < 0.05) in heart rate (HR) and blood lactate concentrations in both males and females but had no impact on rectal or skin temperatures or on walking time to exhaustion in subjects of either gender. Caffeine decreased (p < 0.05) ratings of perceived exertion and fatigue in males, but not in females. In females, but not in males, a stronger belief that they had been administered caffeine was associated with a shorter time to exhaustion. In conclusion, acute caffeine ingestion increases HR and blood lactate levels during exercise in the heat, but it has no impact on thermoregulation or endurance capacity in either gender. Under exercise-heat stress, caffeine reduces ratings of perceived exertion and fatigue in males but not in females.

The Effects of Caffeine Supplementation on Physiological Responses to Submaximal Exercise in Endurance-Trained Men

OBJECTIVES

The aim of this study was to evaluate the effects of caffeine on physiological responses to submaximal exercise, with a focus on blood lactate concentration ([BLa]).

METHODS

Using a randomised, single-blind, crossover design; 16 endurance-trained, male cyclists (age: 38 ± 8 years; height: 1.80 ± 0.05 m; body mass: 76.6 ± 7.8 kg; [Formula: see text]: 4.3 ± 0.6 L∙min-1) completed four trials on an electromagnetically-braked cycle ergometer. Each trial consisted of a six-stage incremental test (3 minute stages) followed by 30 minutes of passive recovery. One hour before trials 2-4, participants ingested a capsule containing 5 mg∙kg-1 of either caffeine or placebo (maltodextrin). Trials 2 and 3 were designed to evaluate the effects of caffeine on various physiological responses during exercise and recovery. In contrast, Trial 4 was designed to evaluate the effects of caffeine on [BLa] during passive recovery from an end-exercise concentration of 4 mmol∙L-1.

RESULTS

Relative to placebo, caffeine increased [BLa] during exercise, independent of exercise intensity (mean difference: 0.33 ± 0.41 mmol∙L-1; 95% likely range: 0.11 to 0.55 mmol∙L-1), but did not affect the time-course of [BLa] during recovery (p = 0.604). Caffeine reduced ratings of perceived exertion (mean difference: 0.5 ± 0.7; 95% likely range: 0.1 to 0.9) and heart rate (mean difference: 3.6 ± 4.2 b∙min-1; 95% likely range: 1.3 to 5.8 b∙min-1) during exercise, with the effect on the latter dissipating as exercise intensity increased. Supplement × exercise intensity interactions were observed for respiratory exchange ratio (p = 0.004) and minute ventilation (p = 0.034).

CONCLUSIONS

The results of the present study illustrate the clear, though often subtle, effects of caffeine on physiological responses to submaximal exercise. Researchers should be aware of these responses, particularly when evaluating the physiological effects of various experimental interventions.

Neurophysiological effects of exercise in the heat

Fatigue during prolonged exercise is a multifactorial phenomenon. The complex interplay between factors originating from both the periphery and the brain will determine the onset of fatigue. In recent years, electrophysiological and imaging tools have been fine-tuned, allowing for an improved understanding of what happens in the brain. In the first part of the review, we present literature that studied the changes in electrocortical activity during and after exercise in normal and high ambient temperature. In general, exercise in a thermo-neutral environment or at light to moderate intensity increases the activity in the β frequency range, while exercising at high intensity or in the heat reduces β activity. In the second part, we review literature that manipulated brain neurotransmission, through either pharmacological or nutritional means, during exercise in the heat. The dominant outcomes were that manipulations changing brain dopamine concentration have the potential to delay fatigue, while the manipulation of serotonin had no effect and noradrenaline reuptake inhibition was detrimental for performance in the heat. Research on the effects of neurotransmitter manipulations on brain activity during or after exercise is scarce. The combination of brain imaging techniques with electrophysiological measures presents one of the major future challenges in exercise physiology/neurophysiology.

Thermoregulatory responses in exercising rats: methodological aspects and relevance to human physiology

Rats are used worldwide in experiments that aim to investigate the physiological responses induced by a physical exercise session. Changes in body temperature regulation, which may affect both the performance and the health of exercising rats, are evident among these physiological responses. Despite the universal use of rats in biomedical research involving exercise, investigators often overlook important methodological issues that hamper the accurate measurement of clear thermoregulatory responses. Moreover, much debate exists regarding whether the outcome of rat experiments can be extrapolated to human physiology, including thermal physiology. Herein, we described the impact of different exercise intensities, durations and protocols and environmental conditions on running-induced thermoregulatory changes. We focused on treadmill running because this type of exercise allows for precise control of the exercise intensity and the measurement of autonomic thermoeffectors associated with heat production and loss. Some methodological issues regarding rat experiments, such as the sites for body temperature measurements and the time of day at which experiments are performed, were also discussed. In addition, we analyzed the influence of a high body surface area-to-mass ratio and limited evaporative cooling on the exercise-induced thermoregulatory responses of running rats and then compared these responses in rats to those observed in humans. Collectively, the data presented in this review represent a reference source for investigators interested in studying exercise thermoregulation in rats. In addition, the present data indicate that the thermoregulatory responses of exercising rats can be extrapolated, with some important limitations, to human thermal physiology.

Central dopaminergic neurotransmission plays an important role in thermoregulation and performance during endurance exercise

Dopamine (DA) has been widely investigated for its potential role in determining exercise performance. It was originally thought that DA's ergogenic effect was by mediating psychological responses. Recently, some studies have also suggested that DA may regulate physiological responses, such as thermoregulation. Hyperthermia has been demonstrated as an important limiting factor during endurance exercise. DA is prominent in the thermoregulatory centre, and changes in DA concentration have been shown to affect core temperature regulation during exercise. Some studies have proposed that DA or DA/noradrenaline (NA) reuptake inhibitors can improve exercise performance, despite hyperthermia during exercise in the heat. DA/NA reuptake inhibitors also increase catecholamine release in the thermoregulatory centre. Intracerebroventricularly injected DA has been shown to improve exercise performance through inhibiting hyperthermia-induced fatigue, even at normal ambient temperatures. Further, caffeine has been reported to increase DA release in the thermoregulatory centre and improves endurance exercise performance despite increased core body temperature. Taken together, DA has been shown to have ergogenic effects and increase heat storage and hyperthermia tolerance. The mechanisms underlying these effects seem to involve limiting/overriding the inhibitory signals from the central nervous system that result in cessation of exercise due to hyperthermia.

Performance effects and metabolic consequences of caffeine and caffeinated energy drink consumption on glucose disposal

This review documents two opposing effects of caffeine and caffeine-containing energy drinks, i.e., their positive effects on athletic performance and their negative impacts on glucose tolerance in the sedentary state. Analysis of studies examining caffeine administration prior to performance-based exercise showed caffeine improved completion time by 3.6%. Similar analyses following consumption of caffeine-containing energy drinks yielded positive, but more varied, benefits, which were likely due to the diverse nature of the studies performed, the highly variable composition of the beverages consumed, and the range of caffeine doses administered. Conversely, analyses of studies administering caffeine prior to either an oral glucose tolerance test or insulin clamp showed a decline in whole-body glucose disposal of ~30%. The consequences of this resistance are unknown, but there may be implications for the development of a number of chronic diseases. Both caffeine-induced performance enhancement and insulin resistance converge with the primary actions of caffeine on skeletal muscle.

Performance in the heat-physiological factors of importance for hyperthermia-induced fatigue

This article presents a historical overview and an up-to-date review of hyperthermia-induced fatigue during exercise in the heat. Exercise in the heat is associated with a thermoregulatory burden which mediates cardiovascular challenges and influence the cerebral function, increase the pulmonary ventilation, and alter muscle metabolism; which all potentially may contribute to fatigue and impair the ability to sustain power output during aerobic exercise. For maximal intensity exercise, the performance impairment is clearly influenced by cardiovascular limitations to simultaneously support thermoregulation and oxygen delivery to the active skeletal muscle. In contrast, during submaximal intensity exercise at a fixed intensity, muscle blood flow and oxygen consumption remain unchanged and the potential influence from cardiovascular stressing and/or high skin temperature is not related to decreased oxygen delivery to the skeletal muscles. Regardless, performance is markedly deteriorated and exercise-induced hyperthermia is associated with central fatigue as indicated by impaired ability to sustain maximal muscle activation during sustained contractions. The central fatigue appears to be influenced by neurotransmitter activity of the dopaminergic system, but inhibitory signals from thermoreceptors arising secondary to the elevated core, muscle and skin temperatures and augmented afferent feedback from the increased ventilation and the cardiovascular stressing (perhaps baroreceptor sensing of blood pressure stability) and metabolic alterations within the skeletal muscles are likely all factors of importance for afferent feedback to mediate hyperthermia-induced fatigue during submaximal intensity exercise. Taking all the potential factors into account, we propose an integrative model that may help understanding the interplay among factors, but also acknowledging that the influence from a given factor depends on the exercise hyperthermia situation.

Exercise and sport performance with low doses of caffeine

Caffeine is a popular work-enhancing supplement that has been actively researched since the 1970s. The majority of research has examined the effects of moderate to high caffeine doses (5-13 mg/kg body mass) on exercise and sport. These caffeine doses have profound effects on the responses to exercise at the whole-body level and are associated with variable results and some undesirable side effects. Low doses of caffeine (<3 mg/kg body mass, ~200 mg) are also ergogenic in some exercise and sport situations, although this has been less well studied. Lower caffeine doses (1) do not alter the peripheral whole-body responses to exercise; (2) improve vigilance, alertness, and mood and cognitive processes during and after exercise; and (3) are associated with few, if any, side effects. Therefore, the ergogenic effect of low caffeine doses appears to result from alterations in the central nervous system. However, several aspects of consuming low doses of caffeine remain unresolved and suffer from a paucity of research, including the potential effects on high-intensity sprint and burst activities. The responses to low doses of caffeine are also variable and athletes need to determine whether the ingestion of ~200 mg of caffeine before and/or during training and competitions is ergogenic on an individual basis.

The effect of caffeine mouth rinse on self-paced cycling performance

The aim of the study was to determine whether caffeine mouth rinse would improve 30 min self-paced cycling trial. Twelve healthy active males (age 20.5±0.7 years, mass 87.4±18.3 kg) volunteered for the study. They attended the laboratory on 3 separate occasions performing a 30 min self-paced cycling trial. On one occasion water was given as a mouth rinse for 5 s (PLA), on another occasion a 6.4% maltodextrin (CHO) solution was given for 5 s and finally a caffeine solution (containing 32 mg of caffeine dissolved in 125 ml water; CAF) was given for 5 s. Distance cycled, heart rate, ratings of perceived exertion, cadence, speed and power output were recorded throughout all trials. Distance cycled during the CAF mouth rinse trial (16.2±2.8 km) was significantly greater compared to PLA trial (14.9±2.6 km). There was no difference between CHO and CAF trials (P=0.89). Cadence, power and velocity were significantly greater during the CAF trial compared to both PLA and CHO (P<0.05). There were no differences between trials for heart rate and perceived exertion (P>0.05). Caffeine mouth rinse improves 30 min cycling performance by allowing the participant to increase cadence, power and velocity without a concurrent increase in perceived exertion and heart rate.

Neurophysiological determinants of theoretical concepts and mechanisms involved in pacing

Fatigue during prolonged exercise is often described as an acute impairment of exercise performance that leads to an inability to produce or maintain a desired power output. In the past few decades, interest in how athletes experience fatigue during competition has grown enormously. Research has evolved from a dominant focus on peripheral causes of fatigue towards a complex interplay between peripheral and central limitations of performance. Apparently, both feedforward and feedback mechanisms, based on the principle of teleoanticipation, regulate power output (e.g., speed) during a performance. This concept is called 'pacing' and represents the use of energetic resources during exercise, in a way such that all energy stores are used before finishing a race, but not so far from the end of a race that a meaningful slowdown can occur.It is believed that the pacing selected by athletes is largely dependent on the anticipated exercise duration and on the presence of an experientially developed performance template. Most studies investigating pacing during prolonged exercise in ambient temperatures, have observed a fast start, followed by an even pace strategy in the middle of the event with an end sprint in the final minutes of the race. A reduction in pace observed at commencement of the event is often more evident during exercise in hot environmental conditions. Further, reductions in power output and muscle activation occur before critical core temperatures are reached, indicating that subjects can anticipate the exercise intensity and heat stress they will be exposed to, resulting in a tactical adjustment of the power output. Recent research has shown that not only climatic stress but also pharmacological manipulation of the central nervous system has the ability to cause changes in endurance performance. Subjects seem to adapt their strategy specifically in the early phases of an exercise task. In high-ambient temperatures, dopaminergic manipulations clearly improve performance. The distribution of the power output reveals that after dopamine reuptake inhibition, subjects are able to maintain a higher power output compared with placebo. Manipulations of serotonin and, especially, noradrenaline, have the opposite effect and force subjects to decrease power output early in the time trial. Interestingly, after manipulation of brain serotonin, subjects are often unable to perform an end sprint, indicating an absence of a reserve capacity or motivation to increase power output. Taken together, it appears that many factors, such as ambient conditions and manipulation of brain neurotransmitters, have the potential to influence power output during exercise, and might thus be involved as regulatory mechanisms in the complex skill of pacing.

Brain temperature and exercise performance

Events arising within the central nervous system seem to be a major factor in the aetiology of hyperthermia-induced fatigue. Thus, various studies with superimposed electrical nerve stimulation or transcranial magnetic stimulation have shown that both passive and exercise-induced hyperthermia will impair voluntary motor activation during sustained maximal contractions. In humans, the brain temperature increases in parallel with that of the body core, making it very difficult to evaluate the independent effect of the cerebral temperature. Experiments with separate manipulation of the brain temperature in exercising goats indicate that excessive brain hyperthermia will directly affect motor performance. However, several homeostatic changes arise in parallel with hyperthermia, including factors that may influence both peripheral and central fatigue, and it is likely that these changes interact with the inhibitory effect of an elevated brain temperature.