Effect of ambient temperature on caffeine ergogenicity during endurance exercise

Common questions and misconceptions about caffeine supplementation: what does the scientific evidence really show?

Caffeine is a popular ergogenic aid that has a plethora of evidence highlighting its positive effects. A Google Scholar search using the keywords "caffeine" and "exercise" yields over 200,000 results, emphasizing the extensive research on this topic. However, despite the vast amount of available data, it is intriguing that uncertainties persist regarding the effectiveness and safety of caffeine. These include but are not limited to: 1. Does caffeine dehydrate you at rest? 2. Does caffeine dehydrate you during exercise? 3. Does caffeine promote the loss of body fat? 4. Does habitual caffeine consumption influence the performance response to acute caffeine supplementation? 5. Does caffeine affect upper vs. lower body performance/strength differently? 6. Is there a relationship between caffeine and depression? 7. Can too much caffeine kill you? 8. Are there sex differences regarding caffeine's effects? 9. Does caffeine work for everyone? 10. Does caffeine cause heart problems? 11. Does caffeine promote the loss of bone mineral? 12. Should pregnant women avoid caffeine? 13. Is caffeine addictive? 14. Does waiting 1.5-2.0 hours after waking to consume caffeine help you avoid the afternoon "crash?" To answer these questions, we performed an evidence-based scientific evaluation of the literature regarding caffeine supplementation.

Caffeine ingestion compromises thermoregulation and does not improve cycling time to exhaustion in the heat amongst males

PURPOSE

Caffeine is a commonly used ergogenic aid for endurance events; however, its efficacy and safety have been questioned in hot environmental conditions. The aim of this study was to investigate the effects of acute caffeine supplementation on cycling time to exhaustion and thermoregulation in the heat.

METHODS

In a double-blind, randomised, cross-over trial, 12 healthy caffeine-habituated and unacclimatised males cycled to exhaustion in the heat (35 °C, 40% RH) at an intensity associated with the thermoneutral gas exchange threshold, on two separate occasions, 60 min after ingesting caffeine (5 mg/kg) or placebo (5 mg/kg).

RESULTS

There was no effect of caffeine supplementation on cycling time to exhaustion (TTE) (caffeine; 28.5 ± 8.3 min vs. placebo; 29.9 ± 8.8 min, P = 0.251). Caffeine increased pulmonary oxygen uptake by 7.4% (P = 0.003), heat production by 7.9% (P = 0.004), whole-body sweat rate (WBSR) by 21% (P = 0.008), evaporative heat transfer by 16.5% (P = 0.006) and decreased estimated skin blood flow by 14.1% (P < 0.001) compared to placebo. Core temperature was higher by 0.6% (P = 0.013) but thermal comfort decreased by - 18.3% (P = 0.040), in the caffeine condition, with no changes in rate of perceived exertion (P > 0.05).

CONCLUSION

The greater heat production and storage, as indicated by a sustained increase in core temperature, corroborate previous research showing a thermogenic effect of caffeine ingestion. When exercising at the pre-determined gas exchange threshold in the heat, 5 mg/kg of caffeine did not provide a performance benefit and increased the thermal strain of participants.

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.

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 Exacerbates Hyperventilation and Reductions in Cerebral Blood Flow in Physically Fit Men Exercising in the Heat

INTRODUCTION

Caffeine is an exercise performance enhancer widely used by individuals engaged in training or competition under heat-stressed conditions. Caffeine ingestion during exercise in the heat is believed to be safe because it does not greatly affect body temperature responses, heart rate, or body fluid status. However, it remains unknown whether caffeine affects hyperthermia-induced hyperventilation or reductions in the cerebral blood flow index. We tested the hypothesis that under conditions inducing severe hyperthermia, caffeine exacerbates hyperthermia-induced hyperventilation and reduces the cerebral blood flow index during exercise.

METHODS

Using a randomized, single-blind, crossover design, 12 physically active healthy young men (23 ± 2 yr) consumed a moderate dose of caffeine (5 mg·kg-1) or placebo in the heat (37°C). Approximately 60 min after the ingestion, they cycled for ~45 min at a workload equal to ~55% of their predetermined peak oxygen uptake (moderate intensity) until their core temperature increased to 2.0°C above its preexercise baseline level.

RESULTS

In both trials, ventilation increased and the cerebral blood flow index assessed by middle cerebral artery mean blood velocity decreased as core temperature rose during exercise (P < 0.05), indicating that hyperthermia-induced hyperventilation and lowering of the cerebral blood flow occurred. When core temperature was elevated by 1.5°C or more (P < 0.05), ventilation was higher and the cerebral blood flow was lower throughout the caffeine trial than the placebo trial (P < 0.05).

CONCLUSIONS

A moderate dose of caffeine exacerbates hyperthermia-induced hyperventilation and reductions in the cerebral blood flow index during exercise in the heat with severe hyperthermia.

Acute Caffeine Mouth Rinse Does Not Change the Hydration Status following a 10 km Run in Recreationally Trained Runners

Background and Aims

Caffeine mouth rinsing has emerged as an alternative to oral caffeine consumption for improving performance without provoking lower gastrointestinal distress. However, it remains unclear if hydration status and sweat rate are negatively affected by caffeine mouth rinsing. This study is aimed at evaluating the effects of 10 seconds of caffeine mouth rinsing (1.2% anhydrous caffeine solution) on hydration status and sweat rate following a 10 km run trial.

Methods

Ten recreationally trained runners (30.1 ± 6.4 y) volunteered to participate in this double-blind, placebo-controlled, and crossover research study. Participants completed two 10 km run trials separated by approximately one week. Immediately prior to running, participants completed a 10-second mouth rinse protocol with either 300 mg of caffeine or microcrystalline cellulose (placebo) diluted in 25 mL of water. The effects of caffeine mouth rinsing on hydration status and sweat rate were assessed following a 10 km run trial.

Results

Sweat rate (placebo: 15.34 ± 9.71 vs. caffeine: 11.91 ± 6.98 mL · min⁻¹; p = 0.39), dehydration (placebo: 1.20 ± 0.57 vs. caffeine: 1.49 ± 0.29%; p = 0.15), and hydration (placebo: 15.32 ± 9.71 vs. caffeine: 11.89 ± 6.99 mL · min⁻¹; p = 0.37) measures were not significantly different between trials.

Conclusion

Caffeine mouth rinse does not appear to alter the hydration status or sweat rate following a 10 km run.

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).

Caffeine and heat have additive but not interactive effects on physiologic strain: A factorial experiment

This study tested the interactive effects of heat and caffeine on exercise-induced physiological strain by using a 2x2 within-subjects factorial design. Thirty-five physically fit Caucasians underwent a bout of exercise under four conditions wherein ambient conditions (heat vs no heat) and caffeine (placebo vs caffeine; double-blinded) were manipulated. Exercise consisted of a 60-min walk and 5-min step/squat test while wearing weighted backpack. Primary outcomes include measures of physiologic strain (Core temperature [T_r] and heart rate [HR]). Secondary measures included blood pressure, markers of sweat loss, and creatine kinase (CK). Repeated measures models were created to evaluate the individual and combined effects of heat and caffeine. Key results indicated that heat and caffeine significantly increased T_r and HR after walking and stair-stepping. No significant heat by caffeine interactions were detected, and caffeine's main effects were relatively low (≤0.17 °C for T_r and ≤6.6 bpm for HR). Of note, heat and caffeine exhibited opposite effects on blood pressure: caffeine increased both systolic and diastolic blood pressure (by 6-7 mmHg) and heat decreased them (by 4-6 mm Hg; ps < 0.05). In summary, heat and caffeine affected physiologic strain during exercise but exhibited no synergistic effects. In contrast, neither factor affected muscle damage. Clinical implications for heat illness risk in the military are discussed.

Ergogenic effects of caffeine on peak aerobic cycling power during the menstrual cycle

PURPOSE

Recent investigations have established that the ingestion of a moderate dose of caffeine (3-6 mg kg^-1) can increase exercise and sports performance in women. However, it is unknown whether the ergogenicity of caffeine is similar during all phases of the menstrual cycle. The aim of this investigation was to determine the ergogenic effects of caffeine in three phases of the menstrual cycle.

METHODS

Thirteen well-trained eumenorrheic triathletes (age = 31 ± 6 years; body mass = 58.6 ± 7.8 kg) participated in a double-blind, cross-over, randomised experimental trial. In the (1) early follicular (EF); (2) preovulation (PO); (3) and mid luteal (ML) phases, participants either ingested a placebo (cellulose) or 3 mg kg^-1 of caffeine in an opaque and unidentifiable capsule. After a 60-min wait for substance absorption, participants performed an incremental maximal cycle ergometer test until volitional fatigue (25 W/min) to assess peak aerobic cycling power (Wmax).

RESULTS

In comparison to the placebo, caffeine increased Wmax in the EF (4.13 ± 0.69 vs. 4.24 ± 0.71 W kg^-1, Δ = 2.7 ± 3.3%, P = 0.01), in the PO (4.14 ± 0.70 vs. 4.27 ± 0.73 W kg^-1, Δ = 3.3 ± 5.0%; P = 0.03) and in the ML (4.15 ± 0.69 vs. 4.29 ± 0.67 W kg^-1, Δ = 3.6 ± 5.1%; P = 0.01) phases. The magnitude of the caffeine ergogenic effect was similar during all of the menstrual cycle phases (P = 0.85).

CONCLUSION

Caffeine increased peak aerobic cycling power in the early follicular, preovulatory, and mid luteal phases. Thus, the ingestion of 3 mg of caffeine per kg of body mass might be considered an ergogenic aid for eumenorrheic women during all three phases of the menstrual cycle.

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.

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.

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.

Moderate intensity, exercise-induced catecholamine release in the preoptic area and anterior hypothalamus in rats is enhanced in a warm environment

Thermoeffector responses and core body temperature (T_core) homeostasis during exercise are affected by both ambient temperature and exercise intensity. We have previously reported that T_core, heat loss responses, and catecholamine release in the preoptic area and anterior hypothalamus (PO/AH) increased during incremental treadmill running. However, no previous study has examined whether changes in the thermoregulatory responses at warm ambient temperature are related to catecholamine responses during moderate intensity exercise in rats. Therefore, the aim of the present study was to investigate the responsiveness of neurotransmission in the PO/AH to moderate intensity exercise at different ambient temperatures, and to relate this to changes in thermoregulation. We measured the monoamine levels in the PO/AH and the thermoregulatory responses in exercising rats simultaneously using a combination of methods, including in vivo microdialysis, biotelemetry, and animal O²/CO² metabolism measuring system. On the day of experiments, rats ran for 60min at a speed of 18mmin^-1 on a treadmill at a 5% gradient, in an ambient temperature of 23°C or 30°C. T_core, tail skin temperature (T_tail; an index of heat loss), and oxygen consumption (V̇O₂: an index of heat production) were monitored. Dopamine (DA), noradrenaline (NA), and serotonin (5-HT) levels were measured by high performance liquid chromatography (HPLC) with electrochemical detection. Exercise significantly increased the T_core, T_tail, and V̇O₂ values, as well as DA and NA release in the PO/AH at both temperatures, and the increases were more pronounced at the warm ambient temperature. The results suggest that the increase in the T_core, heat production, and heat loss responses even during moderate intensity running in a warm environment are likely associated with an increase in DA and NA release in the PO/AH region.

National Athletic Trainers' Association Position Statement: Fluid Replacement for the Physically Active

OBJECTIVE

  To present evidence-based recommendations that promote optimized fluid-maintenance practices for physically active individuals.

BACKGROUND

  Both a lack of adequate fluid replacement (hypohydration) and excessive intake (hyperhydration) can compromise athletic performance and increase health risks. Athletes need access to water to prevent hypohydration during physical activity but must be aware of the risks of overdrinking and hyponatremia. Drinking behavior can be modified by education, accessibility, experience, and palatability. This statement updates practical recommendations regarding fluid-replacement strategies for physically active individuals.

RECOMMENDATIONS

  Educate physically active people regarding the benefits of fluid replacement to promote performance and safety and the potential risks of both hypohydration and hyperhydration on health and physical performance. Quantify sweat rates for physically active individuals during exercise in various environments. Work with individuals to develop fluid-replacement practices that promote sufficient but not excessive hydration before, during, and after physical activity.

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.

Supplementation with a low-dose of octopamine does not influence endurance cycling performance in recreationally active men

OBJECTIVES

The aim of this study was to examine the influence of octopamine supplementation on endurance performance and exercise metabolism.

DESIGN

Double-blind cross-over study.

METHODS

Ten healthy, recreationally active men (Mean±SD; age: 24±2 years; body mass: 78.4±8.7kg; VO_2peak: 50.5±6.8 mLkg^-1min^-1) completed one VO_2peak test, one familiarisation trial and two experimental trials. After an overnight fast, participants ingested either a placebo or 150mg of octopamine 60min prior to exercise. Trials consisted of 30min of cycle exercise at 55% peak power output, followed by a 30min performance task whereby participants completed as much work (kJ) as possible.

RESULTS

Performance was similar between the experimental trials (placebo: 352.8±39kJ; octopamine: 350.9±38.3kJ; Cohen's d effect size=0.05; p=0.380). Substrate oxidation and circulating concentrations of free fatty acids, prolactin and cortisol were similar between trial conditions (all p>0.05). There were also no differences across trials for heart rate or perceived exertion during exercise (both p>0.05).

CONCLUSIONS

Acute supplementation with a low dose of octopamine did not influence endurance cycle performance, substrate oxidation or circulating hormonal concentrations, which could be due to the low serum octopamine concentrations observed. Future studies should investigate the influence of larger doses of octopamine in recreationally active and well-trained individuals during prolonged exercise in temperate and high ambient conditions.

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.

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.

Effect of caffeine intake on finger cold-induced vasodilation

OBJECTIVE

The purpose of the study was to investigate the effect of caffeine intake on finger cold-induced vasodilation (CIVD).

METHODS

Ten healthy men underwent 6 experimental trials characterized by control (NCAFF) or caffeine intake (CAFF) via chewing gum (300 mg of caffeine) while resting on a chair or performing submaximal (70% maximal oxygen consumption) or maximal (100% maximal oxygen consumption) treadmill exercise (Bruce protocol) followed by immersion of the middle finger in a water bath (5°C) for 20 minutes. Finger temperature (Tf ) and time parameters of the first CIVD cycle and post-test norepinephrine were measured.

RESULTS

Exercise duration for submaximal and maximal exercise was 8.9 ± 0.9 and 12.4 ± 0.8 minutes, respectively. CAFF had no effect on Tf, but exercise increased minimal Tf in NCAFF (9.08 ± 1.27°C, 13.02 ± 2.13°C, and 13.25 ± 1.63°C in rest, submaximal, and maximal exercise, respectively) and CAFF (8.76 ± 1.39°C, 12.50 ± 1.91°C, and 12.79 ± 1.20°C). Maximal Tf was significantly higher in NCAFF (15.98 ± 1.04°C, 16.18 ± 1.56°C, and 15.14 ± 1.52°C) than in CAFF (13.56 ± 1.19°C, 15.52 ± 1.31°C, and 14.39 ± 1.43°C), resulting in a significant difference between minimal and maximal Tf in rest (NCAFF, 6.89 ± 1.56°C and CAFF, 4.79 ± 1.23°C), but not in exercise conditions. CAFF had no effect on CIVD time responses, but exercise significantly shortened CIVD onset and peak time compared with rest in both NCAFF and CAFF. Norepinephrine concentration was significantly greater in CAFF (290.6 ± 113.0 pg/mL, 278.1 ± 91.4 pg/mL, and 399.8 ± 125.5 pg/mL) than NCAFF (105.6 ± 29.5 pg/mL, 199.6 ± 89.6 pg/mL, and 361.5 ± 171.3 pg/mL).

CONCLUSIONS

Caffeine intake before finger immersion in cold water does not result in a thermogenic effect and adversely affects CIVD responses, whereas exercise modifies CIVD temperature and time responses.