Coinciding exercise with peak serum caffeine does not improve cycling performance

Effect of caffeine supplementation on physical performance in a 5 km cycling time trial of healthy young adult women in different phases of the menstrual cycle: A parallel, randomized, double-blind, placebo-controlled clinical trial

BACKGROUND

Although caffeine is the most studied ergogenic aid, few studies have been conducted to evaluate the effect of caffeine supplementation among women in different phases of the menstrual cycle (MC).

AIM

To verify the effect of caffeine supplementation and the MC phase on the performance of a 5 km cycling time trial of female exercise practitioners (FEP).

METHODS

This was a parallel, double-blind, randomized, placebo-controlled clinical trial with a sample of women aged 18-35 years, all with regular MC. The caffeine (CAFG) and placebo (PLAG) groups performed the exercise test during the late follicular phase and mid-luteal phase. These time points were individually assessed according to each woman's MC. On the test day, they were instructed to intake a standardized meal. Subsequently, 30 min following the meal, volunteers consumed caffeine (6 mg·kg-1) or placebo. One hour following caffeine intake, the exercise protocol started. The participants were instructed to perform an all-out 5-km cycling time trial.

RESULTS

Twenty-one women with a mean age of 26.6 years (PLAG, n = 10; 26.7 y; CAFG, n = 11; 26.5 y) were evaluated. The mean test duration was approximately 10-min, with no effect of the MC phase (F = 0.410; p = 0.532), caffeine supplementation (F = 2.23; p = 0.156), or interaction (F = 0.298; p = 0.593). Likewise, we did not verify the effect of the MC phase (F = 0.249; p = 0.625), caffeine supplementation (F = 2.35; p = 0.146), or interaction (F = 0.585; p = 0.456) on the mean power.

CONCLUSION

Neither caffeine supplementation nor the different MC phases had an impact on the 5-km cycling TT performance of FEP.

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.

The effect of caffeine dose on caffeine and paraxanthine changes in serum and saliva and CYP1A2 enzyme activity in athletes: a randomized placebo-controlled crossover trial

BACKGROUND

Although caffeine (CAF) supplementation has been shown to improve exercise performance, its dose-dependent effect on CAF metabolism has not been sufficiently investigated. The aim of this study was to evaluate the effects of 3, 6 and 9 mg of CAF/kgBM on changes of CAF and paraxanthine (PRX) in the serum and saliva at four time-points.

METHODS

In a randomized, double-blind, placebo-controlled crossover design, acute pre-exercise supplementation in 26 moderately-trained athletes, participating in high-intensity functional training (HIFT), was examined. The study protocol involved CAF/PRX biochemical analyses of serum and saliva with respect to CYP1A2 polymorphism and CYP1A2 enzyme activity.

RESULTS

Despite significant differences between the serum and saliva levels of CAF and PRX, there was no difference in the PRX/CAF ratio. The interaction effect of dose and time-points for PRX concentration was revealed. The main effects of dose were observed for CAF and the PRX/CAF ratio. The main effect of time-points was registered only for serum CAF.

CONCLUSIONS

Dose- and time-dependent effect of CAF supplementation on CAF and PRX in the serum and saliva of athletes was confirmed, but there was no effect of the CAF dose on CYP1A2 enzyme activity, nor was there an interaction of CYP1A2 with enzyme inducibility. The CAF/PRX correlation indicated the possibility of interchangeable use of serum and/or saliva analyses in exercise studies.

CLINICAL TRIAL REGISTRATION

This trial was registered prospectively at ClinicalTrials.gov (NCT03822663, registration date: 30/01/2019).

Does ergogenic effect of caffeine supplementation depend on CYP1A2 genotypes? A systematic review with meta-analysis

BACKGROUND

The ergogenic effects of caffeine intake on exercise performance are well-established, even if differences exist among individuals in response to caffeine intake. The genetic variation of a specific gene, human cytochrome P450 enzyme 1A2 (CYP1A2) (rs762551), may be one reason for this difference. This systematic review and meta-analysis aimed to comprehensively evaluate the influence of CYP1A2 gene types on athletes' exercise performance after caffeine intake.

METHODS

A literature search through 4 databases (Web of Science, PubMed, Scopus, and China National Knowledge Infrastructure) was conducted until March 2023. The effect size was expressed as the weighted mean difference (WMD) by calculating fixed effects meta-analysis if heterogeneity was not significant (I2 ≤ 50% and p ≥ 0.1). Subgroup analyses were performed based on AA and AC/CC genotype of CYP1A2.

RESULTS

The final number of studies meeting the inclusion criteria was 12 (n = 666 participants). The overall analysis showed that the cycling time trial significantly improved after caffeine intake (WMD = -0.48, 95% confidence interval (95%CI): -0.83 to -0.13, p = 0.007). In subgroup analyses, acute caffeine intake improved cycling time trial only in individuals with the A allele (WMD = -0.90, 95%CI: -1.48 to -0.33, p = 0.002), but not the C allele (WMD = -0.08, 95%CI: -0.32 to 0.17, p = 0.53). Caffeine supplementation did not influence the Wingate (WMD = 8.07, 95%CI: -22.04 to 38.18, p = 0.60) or countermovement jump test (CMJ) performance (WMD = 1.17, 95%CI: -0.02 to 2.36, p = 0.05), and these outcomes were not influenced by CYP1A2 genotype.

CONCLUSION

Participants with the CYP1A2 genotype with A allele improved their cycling time trials after caffeine supplementation. However, compared to placebo, acute caffeine supplementation failed to increase the Wingate or CMJ performance, regardless of CYP1A2 genotype.

Effects of Various Caffeine Doses on Cognitive Abilities in Female Athletes with Low Caffeine Consumption

Caffeine (CAF), a prevalent psychoactive stimulant, is believed to potentially enhance cognitive ability. However, studies on the effects of various doses are limited and yield inconsistent results, particularly in female athletes. Therefore, we aimed to assess the association between three different dosages of CAF intake (low, moderate, and high) and cognitive skills in female athletes with low CAF consumption. This study had a randomized, crossover, double-blind design in which each athlete performed four experimental sessions after ingesting either a placebo (PLAC), 3 mg·kg-1 of CAF (3 mg of CAF), 6 mg·kg-1 of CAF (6 mg of CAF), or 9 mg·kg-1 of CAF (9 mg of CAF) with an in-between washout period of at least 72 h. Following a 60 min window post-capsule consumption, fourteen female athletes (age: 17.4 ± 0.8 years) were assessed through various cognitive tests, namely, simple reaction time (SRT), choice reaction time (CRT), and attentional task (AT) tests, along with the mental rotation test (MRT). Additionally, they were required to complete a questionnaire about the undesirable side effects of CAF. Our results indicated that, compared to those of PLAC, the SRT, CRT, and AT performance were significantly improved following the administration of both 3 mg of CAF and 6 mg of CAF. While the greatest enhancement was observed after consuming 3 mg of CAF, no significant differences were found between the effects of 3 mg and 6 mg of CAF. Interestingly, MRT performance did not improve with any of the CAF dosages. Moreover, the ingestion of 9 mg of CAF did not enhance cognitive skills and was linked to the highest occurrence of CAF-related side effects. In conclusion, our results highlight the recommendation for a low CAF dosage of 3 mg·kg-1, in contrast to a higher dose of 6 mg·kg-1 or 9 mg·kg-1 of CAF, to enhance various aspects of cognitive performance in female athletes with low CAF consumption without adverse side effects.

Acute effects of caffeine supplementation on taekwondo performance: the influence of competition level and sex

The purpose of this study was to assess the effects of acute caffeine supplementation on physical performance and perceived exertion during taekwondo-specific tasks in male and female athletes with varying expertise. In a double-blinded, randomized, placebo-controlled crossover study design, 52 young athletes from elite (n = 32; 16 males and 16 females) and sub-elite competitive level (n = 20; 10 males and 10 females) participated. Athletes performed taekwondo-specific tasks including the taekwondo-specific agility test (TSAT), 10 s frequency speed of kick test (FSKT-10 s) and multi-bout FSKT (FSKT-multi) under the following conditions: (1) Caffeine (CAF; 3 mg kg-1), placebo (PLA), and no supplement control (CON). Session rating of perceived exertion (s-RPE) was determined after the tests. Findings show that regardless of condition, males performed better than females (p < 0.05) and elite athletes had superior performance compared to their sub-elite counterparts (p < 0.05). For the TSAT (p < 0.001), FSKT-10s (p < 0.001), and FSKT-multi (p < 0.001), CAF enhanced performance in elite female athletes compared to sub-elite females. Likewise, CAF ingestion resulted in superior performance in elite males compared to sub-elite males for FSKT-10s (p = 0.003) and FSKT-multi (p < 0.01). The ergogenic potential of CAF during taekwondo-specific tasks appears to be related to a competitive level, with greater benefits in elite than sub-elite athletes.

Are Caffeine's Effects on Resistance Exercise and Jumping Performance Moderated by Training Status?

This study aimed to explore if the effects of caffeine intake on resistance exercise and jumping performance are moderated by training status. We included ten resistance-trained and ten recreationally active males in a randomized, double-blind, crossover study. Participants were categorized into groups according to their resistance to training experience and muscular strength levels. Exercise performance outcomes included weight lifted and mean velocity during a one-repetition maximum (1RM) bench press and squat; repetitions were performed to muscular failure in the same exercises with 70% of 1RM and countermovement jump (CMJ) height. Exercise performance was evaluated on three occasions, following no substance ingestion (control), caffeine (6 mg/kg), and placebo. There was a main effect on the condition for all the performance outcomes (all p ≤ 0.02), except for the 1RM squat mean velocity (p = 0.157) and 1RM bench press mean velocity (p = 0.719). For weight lifted in the 1RM bench press, there was a significant difference when comparing the caffeine vs. control, caffeine vs. placebo, and placebo vs. control. For weight lifted in the 1RM squat, a significant difference was found when comparing the caffeine vs. control. For muscular endurance outcomes and jump height, a significant difference was found when caffeine was compared to the control or placebo. Effect sizes were trivial for muscular strength (Hedges' g: 0.04-0.12), small for the jump height (Hedges' g: 0.43-0.46), and large for muscular endurance (Hedges' g: 0.89-1.41). Despite these ergogenic effects, there was no significant training status × caffeine interaction in any of the analyzed outcomes. In summary, caffeine ingestion is ergogenic for muscular strength, endurance, and jump height. These effects are likely to be of a similar magnitude in resistance-trained and recreationally active men.

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 Timing Improves Lower-Body Muscular Performance: A Randomized Trial

Little is known about the optimal time to consume caffeine prior to exercise to maximize the ergogenic benefits of the substance.

Purpose: To determine the optimal pre-exercise time interval to consume caffeine to improve lower-body muscular performance. A secondary aim was to identify the presence of any sex differences in responses to timed caffeine administration.

Methods: Healthy, resistance-trained males (n = 18; Mean±SD; Age: 25.1 ± 5.7 years; Height: 178.4 ± 7.1 cm; Body mass: 91.3 ± 13.5 kg; Percent body fat: 20.7 ± 5.2; Average caffeine consumption: 146.6 ± 100.3 mg/day) and females (n = 11; Mean ± SD; Age: 20.1 ± 1.6 years; Height: 165.0 ± 8.8 cm; Body mass: 65.8 ± 10.0 kg; Percent bodyfat: 25.8 ± 4.2; Average caffeine consumption: 111.8 ± 91.7 mg/day) participated in this investigation. In a randomized, double-blind, placebo-controlled, crossover fashion, participants consumed 6 mg·kg⁻¹ caffeine or placebo solution at three time points: 2 h prior (2H), 1 h prior (1H), or 30 min prior (30M) to exercise testing. During three visits, caffeine was randomly administered at one time point, and placebo was administered at the other two time points. During one visit, placebo was administered at all three time points. Next, participants performed isometric mid-thigh pulls (IMTP), countermovement vertical jumps (CMVJ), and isometric/isokinetic knee extensor testing (ISO/ISOK).

Results: Caffeine administered at 1H significantly improved absolute CMVJ and ISO performance relative to placebo. Mean CMVJ jump height was significantly higher during 1H compared to 30M. However, only caffeine administered at 30M significantly improved absolute measures of isokinetic performance. Analysis of the pooled caffeine conditions revealed that muscular performance was more consistently augmented by caffeine in males compared to females.

Conclusions: Pre-exercise caffeine timing significantly modulated participant responses to the substance, with 1H exerting the most consistent ergogenic benefits relative to other time points, particularly compared to 2H. Male participants were found to respond more consistently to caffeine compared to female participants. These results suggest that active individuals can maximize the ergogenic effects of caffeine by consuming the substance ~1 h prior to the point when peak muscular performance is desired.

The Effects of Caffeine Mouth Rinsing on Exercise Performance: A Systematic Review

Caffeine ingestion can improve performance across a variety of exercise modalities but can also elicit negative side effects in some individuals. Thus, there is a growing interest in the use of caffeine mouth rinse solutions to improve sport and exercise performance while minimizing caffeine’s potentially adverse effects. Mouth rinse protocols involve swilling a solution within the oral cavity for a short time (e.g., 5–10 s) before expectorating it to avoid systemic absorption. This is believed to improve performance via activation of taste receptors and stimulation of the central nervous system. Although reviews of the literature indicate that carbohydrate mouth rinsing can improve exercise performance in some situations, there has been no attempt to systematically review the available literature on caffeine mouth rinsing and its effects on exercise performance. To fill this gap, a systematic literature search of three databases (PubMed, SPORTDiscus, and Web of Science) was conducted by two independent reviewers. The search resulted in 11 randomized crossover studies that were appraised and reviewed. Three studies found significant positive effects of caffeine mouth rinsing on exercise performance, whereas the remaining eight found no improvements or only suggestive benefits. The mixed results may be due to heterogeneity in the methods across studies, interindividual differences in bitter tasting, and differences in the concentrations of caffeine solutions. Future studies should evaluate how manipulating the concentration of caffeine solutions, habitual caffeine intake, and genetic modifiers of bitter taste influence the efficacy of caffeine mouth rinsing as an ergogenic strategy.

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).

Women Experience the Same Ergogenic Response to Caffeine as Men

PURPOSE

This study aimed to determine whether 1) consumption of caffeine improves endurance cycling performance in women and 2) sex differences exist in the magnitude of the ergogenic and plasma responses to caffeine supplementation.

METHODS

Twenty-seven (11 women and 16 men) endurance-trained cyclists and triathletes participated in this randomized, double-blind, placebo-controlled, crossover study. Participants completed an incremental exercise test to exhaustion, two familiarization trials, and two performance trials. Ninety minutes before the performance trials, participants ingested opaque capsules containing either 3 mg·kg body mass of anhydrous caffeine or a placebo. They then completed a set amount of work (75% of peak sustainable power output) in the fastest possible time. Plasma was sampled at baseline, preexercise, and postexercise for caffeine. Strict standardization and verification of diet, hydration, training volume and intensity, and contraceptive hormone phase (for women) were implemented.

RESULTS

Performance time was significantly improved after caffeine administration in women (placebo: 3863 ± 419 s, caffeine: 3757 ± 312 s; P = 0.03) and men (placebo: 3903 ± 341 s, caffeine: 3734 ± 287 s; P < 0.001). The magnitude of performance improvement was similar for women (mean = 4.3%, 95% CI = 0.4%-8.2%) and men (4.6%, 2.3%-6.8%). Plasma caffeine concentrations were similar between sexes before exercise, but significantly greater in women after exercise (P < 0.001).

CONCLUSIONS

Ingestion of 3 mg·kg body mass of caffeine enhanced endurance exercise performance in women. The magnitude of the performance enhancement observed in women was similar to that of men, despite significantly greater plasma caffeine concentrations after exercise in women. These results suggest that the current recommendations for caffeine intake (i.e., 3-6 mg·kg caffeine before exercise to enhance endurance performance), which are derived almost exclusively from studies on men, may also be applicable to women.

MUSCLE STRENGTH AND CAFFEINE SUPPLEMENTATION: ARE WE DOING MORE OF THE SAME?

ABSTRACT The purpose of this review was to examine in the current literature the advances made in terms of the effects of caffeine supplementation on maximum strength and its associated mechanisms since the publication of two important papers in 2010. Searches were carried out in the PubMed, Medline, Scielo and Web of Science databases for articles published after 2010. Sixteen studies were included based on inclusion and exclusion criteria. Five studies did not report changes in maximal voluntary strength (31.3%). Four of them used isometric muscle contractions, although this may not be a key factor because five other studies also used isometric contractions and reported ergogenic effects. Furthermore, these four studies evaluated small muscle groups and volunteers were not accustomed to consuming caffeine. Caffeine produced ergogenic effects in eleven of the sixteen studies analyzed (68.8%). None of the doses were clearly related to ergogenic effects; however, a dose of at least 3 mg/kg of caffeine is probably necessary. Caffeine ergogenicity was affected by various factors. There was a lack of standardized protocols and controls for intervening factors (e.g., circadian cycles and nutritional states), which could affect results. An ideal caffeine supplementation protocol that is useful for future research, athletes, and physical activity practitioners, has yet to be defined. A small advance made since 2010 involved a possible lack of gender difference; it would appear that caffeine supplementation affects men and women equally. Level of Evidence I; Systematic Review of Level I Studies.

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.

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.

Spinal Cord Injury Level Influences Acute Plasma Caffeine Responses

PURPOSE

This study aimed to investigate the absorption curve and acute effects of caffeine at rest in individuals with no spinal cord injury (SCI), paraplegia (PARA), and tetraplegia (TETRA).

METHODS

Twenty-four healthy males (eight able-bodied [AB], eight PARA, and eight TETRA) consumed 3 mg·kg caffeine anhydrous (CAF) in a fasted state. Plasma caffeine [CAF], glucose, lactate, free fatty acid, and catecholamine concentrations were measured during a 150-min rest period.

RESULTS

Peak [CAF] was greater in TETRA (21.5 μM) compared with AB (12.2 μM) and PARA (15.1 μM), and mean peak [CAF] occurred at 70, 80, and 80 min, respectively. Moderate and large effect sizes were revealed for TETRA compared with PARA and AB (-0.55 and -1.14, respectively) for the total area under the [CAF] versus time curve. Large interindividual responses were apparent in SCI groups. The change in plasma catecholamine concentrations after CAF did not reach significance (P > 0.05); however, both adrenaline and noradrenaline concentrations were lowest in TETRA. Significant increases in free fatty acid were seen over time (P < 0.0005), but there was no significant influence of SCI level. Blood lactate concentration reduced over time (P = 0.022), whereas blood glucose concentration decreased modestly (P = 0.695), and no difference between groups was seen (P > 0.05).

CONCLUSION

The level of SCI influenced the caffeine absorption curve, and there was large interindividual variation within and between groups. Individual curves should be considered when using caffeine as an ergogenic aid in athletes with an SCI. The results indicate TETRA should trial low doses in training and PARA may consider consuming caffeine greater than 60 min before exercise performance. The study also supports caffeine's direct effect on adipose tissue, which is not secondary to catecholamine release.

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.

Pre-exercise nutrition: the role of macronutrients, modified starches and supplements on metabolism and endurance performance

Endurance athletes rarely compete in the fasted state, as this may compromise fuel stores. Thus, the timing and composition of the pre-exercise meal is a significant consideration for optimizing metabolism and subsequent endurance performance. Carbohydrate feedings prior to endurance exercise are common and have generally been shown to enhance performance, despite increasing insulin levels and reducing fat oxidation. These metabolic effects may be attenuated by consuming low glycemic index carbohydrates and/or modified starches before exercise. High fat meals seem to have beneficial metabolic effects (e.g., increasing fat oxidation and possibly sparing muscle glycogen). However, these effects do not necessarily translate into enhanced performance. Relatively little research has examined the effects of a pre-exercise high protein meal on subsequent performance, but there is some evidence to suggest enhanced pre-exercise glycogen synthesis and benefits to metabolism during exercise. Finally, various supplements (i.e., caffeine and beetroot juice) also warrant possible inclusion into pre-race nutrition for endurance athletes. Ultimately, further research is needed to optimize pre-exercise nutritional strategies for endurance performance.

The impact of acute and chronic catecholamines on respiratory responses to hypoxic stress in the rat

Chronic catecholamine production is associated with desensitisation and down-regulation of adrenergic receptors and occurs in conditions, such as heart failure and myocardial infarction. The effects of further acute adrenergic stimulation, which may occur during exercise, and their subsequent effects on chemosensitivity and ventilation are unclear. Chronic isoprenaline (ISO) increased ventilation by 50 % (P < 0.05) yet the sensitivity to graded hypoxia was preserved. Acute noradrenaline (NA) in control animals led to a doubling of ventilation in hyperoxia (P < 0.001), and this difference was preserved in graded hypoxia (P < 0.001). Yet, combination of NA + ISO did not increase ventilation beyond ISO at baseline or in hypoxia. ISO, NA, and NA + ISO all induced a metabolic acidosis (P < 0.05) with enhanced ventilation in partial compensation. Carotid sinus nerve (CSN) section led to a partial loss of catecholamine-induced augmentation in ventilation (P < 0.05), yet direct recording from CSN in vitro suggests catecholamine is inhibitory for CSN discharge. These observations suggest that chronic catecholamine exposure may result in decreased exercise performance as a direct consequence of the hyperpnea to compensate for an increased metabolic rate coupled with acidosis and leading to increased central chemosensitivity. A limited contribution from peripheral chemoreceptors was noted but was not a consequence of catecholamine stimulation of the carotid body.