***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Caffeine , an adenosine receptor antagonist , has shown to improve performance in normal ambient temperature , presumably via an effect on dopaminergic neurotransmission through the antagonism of adenosine receptors . However , there is very limited evidence from studies that administered caffeine and examined its effects on exercise in the heat . Therefore , we wanted to study the effects of caffeine on performance and thermoregulation in high ambient temperature . Eight healthy trained male cyclists completed two experimental trials ( in 30 ° C ) in a double-blind-r and omized crossover design . Subjects ingested either placebo ( 6 mg/kg ) or caffeine ( 6 mg/kg ) 1 h prior to exercise . Subjects cycled for 60 min at 55 % Wmax , immediately followed by a time trial to measure performance . The significance level was set at p < 0.05 . Caffeine did not change performance ( p = 0.462 ) . Rectal temperature was significantly elevated after caffeine administration ( p < 0.036 ) . Caffeine  significantly increased B-endorphin plasma concentrations at the end of the time trial ( p = 0.032 ) . The present study showed no ergogenic effect of caffeine when administered 1 h before exercise in 30 ° C . This confirms results from a previous study that examined the effects of caffeine administration on a short ( 15 min ) time trial in 40 ° C . However , caffeine  increased core temperature during exercise . Presumably , the rate of increase in core temperature may have counteracted the ergogenic effects of caffeine . However , other factors such as interindividual differences in response to caffeine and changes in neurotransmitter concentrations might also be responsible for the lack of performance improvement of caffeine in high ambient temperature This study compared the effects of a single and divided dose of caffeine on endurance performance and on  postexercise urinary caffeine and plasma paraxanthine concentrations .  Nine male cyclists and triathletes cycled for 90 min at 68 % of maximal oxygen uptake , followed by a self-paced time trial ( work equivalent to 80 % of maximal oxygen uptake workload over 30 min ) with three r and omized , balanced , and double-blind interventions : 1 ) placebo 60 min before and 45 min into exercise ( PP ) ; 2 ) single caffeine dose ( 6 mg/kg ) 60 min before exercise and placebo 45 min into exercise ( CP ) ; and 3 ) divided caffeine dose ( 3 mg/kg ) 60 min before and 45 min into exercise ( CC ) . Time trial performance was unchanged with caffeine ingestion ( P = 0.08 ) , but it tended to be faster in the caffeine trials ( CP : 24.2 min and CC : 23.4 min ) compared with placebo ( PP : 28.3 min ) . Postexercise urinary caffeine concentration was significantly lower in CC ( 3.8 micro g/ml ) compared with CP ( 6.8 micro g/ml ) . Plasma paraxanthine increased in a dose-dependent fashion and did not peak during exercise . In conclusion , dividing a caffeine dose provides no ergogenic effect over a bolus dose but reduces postexercise urinary concentration AIM The purpose of this work was to determine the effects of caffeine ingestion on cycling time trial ( TT ) performance in well trained male subjects . METHODS Eight males , with the following physical characteristics ( Mean + /- SD ) age 30.2+/-10.1 years , height 180.3+/-7.1 cm , mass 70.4+/-5.1 kg , VO2max 63.6+/-4.4 mL.kg(-1).min(-1 ) undertook three 1 h TT performances on a VelotronPro cycle ergometer , in a double blind , r and om fashion . The trials were Control ( C ) , Placebo ( Pl ) and Caffeine ( CAF ) . The CAF and Pl were given 60 min prior to exercise in a dose of 6 mg.kg(-1 ) body mass . Prior to ingestion , 60 min post ingestion , and at the end of the TT , subjects gave 10 mL of venous blood which was analysed for lactate , glucose , and free fatty acids . Expired air was collected throughout each test by indirect calorimetry . RESULTS The cyclists rode significantly further in CAF trial ( 28.11+/-1.32 km ) than they did in the C ( 26.69+/-1.5 km , P < 0.03 ) or Pl ( 27.0+/-1.5 km , P < 0.03 ) trials . No significant differences were seen between C and Pl trials ( P > 0.88 ) . No differences between C and Pl were seen in heart rate data throughout the TT ( p > 0.05 ) . The free fatty acid ( FFA ) concentrations were significantly higher in the CAF trials both post ingestion ( P < 0.005 ) and post exercise ( P < 0.0001 ) than either C or Pl trials . CONCLUSION We concluded that performance was improved possibly based upon a greater reliance on fat metabolism , as indicated by increased FFA and a lower respiratory exchange ratio ( RER ) This study analyzed the effect of caffeine ingestion on performance during a repeated- measures , 100-km , laboratory cycling time trial that included bouts of 1- and 4-km high intensity epochs ( HIE ) . Eight highly trained cyclists participated in 3 separate trials ' placebo ingestion before exercise with a placebo carbohydrate solution and placebo tablets during exercise ( Pl ) , or placebo ingestion before exercise with a 7 % carbohydrate drink and placebo tablets during exercise ( Cho ) , or caffeine tablet ingestion before and during exercise with 7 % carbohydrate ( Caf ) . Placebo ( twice ) or 6 mg.kg(-1 ) caffeine was ingested 60 min prior to starting 1 of the 3 cycling trials , during which subjects ingested either additional placebos or a caffeine maintenance dose of 0.33 mg.kg(-1 ) every 15 min to trial completion . The 100-km time trial consisted of five 1-km HIE after 10 , 32 , 52 , 72 , and 99 km , as well as four 4-km HIE after 20 , 40 , 60 , and 80 km . Subjects were instructed to complete the time trial and all HIE as fast as possible . Plasma ( caffeine ) was significantly higher during Caf ( 0.43 + /- 0.56 and 1.11 + /- 1.78 mM pre vs. post Pl ; and 47.32 + /- 12.01 and 72.43 + /- 29.08 mM pre vs. post Caf ) . Average power , HIE time to completion , and 100-km time to completion were not different between trials . Mean heart rates during both the 1-km HIE ( 184.0 + /- 9.8 Caf ; 177.0 + /- 5.8 Pl ; 177.4 + /- 8.9 Cho ) and 4-km HIE ( 181.7 + /- 5.7 Caf ; 174.3 + /- 7.2 Pl ; 175.6 + /- 7.6 Cho ; p < .05 ) was higher in Caf than in the other groups . No significant differences were found between groups for either EMG amplitude ( IEMG ) or mean power frequency spectrum ( MPFS ) . IEMG activity and performance were not different between groups but were both higher in the 1-km HIE , indicating the absence of peripheral fatigue and the presence of a central ly-regulated pacing strategy that is not altered by caffeine ingestion . Caffeine may be without ergogenic benefit during endurance exercise in which the athlete begins exercise with a defined , predetermined goal measured as speed or distance Purpose Many studies have examined the effect of caffeine on exercise performance , but findings have not always been consistent . The objective of this study was to determine whether variation in the CYP1A2 gene , which affects caffeine metabolism , modifies the ergogenic effects of caffeine in a 10-km cycling time trial . Methods Competitive male athletes ( n = 101 ; age = 25 ± 4 yr ) completed the time trial under three conditions : 0 , 2 , or 4 mg of caffeine per kilogram body mass , using a split-plot r and omized , double-blinded , placebo-controlled design . DNA was isolated from saliva and genotyped for the −163A > C polymorphism in the CYP1A2 gene ( rs762551 ) . Results Overall , 4 mg·kg−1 caffeine  decreased cycling time by 3 % ( mean ± SEM ) versus placebo ( 17.6 ± 0.1 vs 18.1 ± 0.1 min , P = 0.01 ) . However , a significant ( P < 0.0001 ) caffeine – gene interaction was observed . Among those with the AA genotype , cycling time decreased by 4.8 % at 2 mg·kg−1 ( 17.0 ± 0.3 vs 17.8 ± 0.4 min , P = 0.0005 ) and by 6.8 % at 4 mg·kg−1 ( 16.6 ± 0.3 vs 17.8 ± 0.4 min , P < 0.0001 ) . In those with the CC genotype , 4 mg·kg−1 increased cycling time by 13.7 % versus placebo ( 20.8 ± 0.8 vs 18.3 ± 0.5 min , P = 0.04 ) . No effects were observed among those with the AC genotype . Conclusion Our findings show that both 2 and 4 mg·kg−1 caffeine improve 10-km cycling time , but only in those with the AA genotype . Caffeine had no effect in those with the AC genotype and diminished performance at 4 mg·kg−1 in those with the CC genotype . CYP1A2 genotype should be considered when deciding whether an athlete should use caffeine for enhancing endurance performance PURPOSE To determine the effects of ingesting caffeine ( CAFF ) and sodium bicarbonate ( SB ) , taken individually and simultaneously , on 3-km cycling time-trial ( TT ) performance . METHOD Ten well-trained cyclists , age 24.2 ± 5.4 yr , participated in this acute-treatment , double-blind , crossover study that involved four 3-km cycling TTs performed on separate days . Before each TT , participants ingested either 3 mg/kg body mass ( BM ) of CAFF , 0.3 g · kg⁻¹ · BM⁻¹ of SB , a combination of the two ( CAFF+SB ) , or a placebo ( PLAC ) . They completed each 3-km TT on a laboratory-based cycle ergometer , during which physiological , perceptual , and performance measurements were determined . For statistical analysis , the minimal worthwhile difference was considered ~1 % based on previous research . RESULTS Pretrial pH and HCO₃ were higher in SB and CAFF+SB than in the CAFF and PLAC trials . Differences across treatments for perceived exertion and gastric discomfort were mostly unclear . Compared with PLAC , mean power output during the 3-km TT was higher in CAFF , SB , and CAFF+SB trials ( 2.4 % , 2.6 % , 2.7 % respectively ) , result ing in faster performance times ( -0.9 , -1.2 , -1.2 % respectively ) . Effect sizes for all trials were small ( 0.21 - 0.24 ) . CONCLUSIONS When ingested individually , both CAFF and SB  enhance high-intensity cycling TT performance in trained cyclists . However , the ergogenic effect of these 2 popular supplements was not additive , bringing into question the efficacy of coingesting the 2 supplements before short- duration high-intensity exercise . In this study there were no negative effects of combining CAFF and SB , 2 relatively inexpensive and safe supplements BACKGROUND The ingestion of either caffeine ( C ) or ephedrine ( E ) has been shown to improve performance during high-intensity aerobic activity lasting 10 - 20 min , with an additive effect being found when the combination ( C + E ) was ingested . It was the purpose of this study to determine if the addition of E to C would improve performance in activity lasting longer than 20 min . METHODS One and one half hours after ingesting a placebo ( P ) , C ( 4 mg/kg ) , E ( 0.8 mg/kg ) , or C + E , 12 subjects performed a 10-km run while wearing a helmet and backpack weighing 11 kg . The trials were performed in a climatic suite at 12 - 13 degrees C , on a treadmill where the speed was regulated by the subject . VO(2 ) , VCO(2 ) , V(E ) , heart rate ( HR ) , and rating of perceived exertion ( RPE ) were measured during the run at 15 and 30 min , and again when the individual reached 9 km . Blood was sample d at 15 and 30 min and again at the end of the run and assayed for lactate , glucose , and catecholamines . RESULTS Run times ( mean + /- SD ) , in minutes , were for C ( 46.0 + /- 2 
    Output:
    
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