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Deborah K. Fletcher and Nicolette C. Bishop

This study investigated the effect of a high and low dose of caffeine on antigen-stimulated natural killer (NK) cell (CD3CD56+) activation after prolonged, strenuous cycling, as assessed by the early-activation molecule CD69. In a randomized crossover design, 12 healthy male endurance-trained cyclists cycled for 90 min at 70% VO2peak 60 min after ingesting either 0 (PLA), 2 (2CAF), or 6 (6CAF) mg/kg body mass of caffeine. Whole blood was stimulated with Pediacel (5 in 1) vaccine. A high dose of caffeine (6CAF) increased the number of CD3CD56+ cells in the circulation immediately postexercise compared with PLA (p < .05). For both 2CAF and 6CAF, the geometric mean fluorescence intensity (GMFI) of CD69+ expression on unstimulated CD3CD56+ cells was significantly higher than with PLA (both p < .05). When cells were stimulated with antigen, the GMFI of CD69 expression remained significantly higher with 2CAF than with PLA 1 hr postexercise (p < .05). Although not achieving statistical significance, 6CAF also followed a similar trend when stimulated (p = .09). There were no differences in GMFI of CD69 expression between 2CAF and 6CAF. These results suggest that a high (6 mg/kg) dose of caffeine was associated with the recruitment of NK cells into the circulation and that both a high and low (2 mg/kg) dose of caffeine increased unstimulated and antigen-stimulated NK-cell activation 1 hr after high-intensity exercise. Furthermore, there does not appear to be a dose-dependent effect of caffeine on NK-cell activation 1 hr after prolonged intensive cycling.

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Mark Glaister and Conor Gissane

The aim of this study was to carry out a systematic review and meta-analysis of the effects of caffeine supplementation on physiological responses to submaximal exercise. A total of 26 studies met the inclusion criteria of adopting double-blind, randomized crossover designs that included a sustained (5–30 min) fixed-intensity bout of submaximal exercise (constrained to 60–85% maximal rate of oxygen consumption) using a standard caffeine dose of 3–6 mg·kg−1 administered 30–90 min prior to exercise. Meta-analyses were completed using a random-effects model, and data are presented as raw mean difference (D) with associated 95% confidence limits (CLs). Relative to placebo, caffeine led to significant increases in submaximal measures of minute ventilation (D = 3.36 L·min−1; 95% CL, 1.63–5.08; P = .0001; n = 73), blood lactate (D = 0.69 mmol·L−1; 95% CL, 0.46–0.93; P < .00001; n = 208), and blood glucose (D = 0.42 mmol·L−1; 95% CL, 0.29–0.55; P < .00001; n = 129). In contrast, caffeine had a suppressive effect on ratings of perceived exertion (D = −0.8; 95% CL, −1.1 to −0.6; P < .00001; n = 147). Caffeine had no effect on measures of heart rate (P = .99; n = 207), respiratory exchange ratio (P = .18; n = 181), or oxygen consumption (P = .92; n = 203). The positive effects of caffeine supplementation on sustained high-intensity exercise performance are widely accepted, although the mechanisms to explain that response are currently unresolved. This meta-analysis has revealed clear effects of caffeine on various physiological responses during submaximal exercise, which may help explain its ergogenic action.

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Gary J. Walker, Phillipa Caudwell, Natalie Dixon and Nicolette C. Bishop

This study investigated the effect of caffeine ingestion on neutrophil oxidative burst responses to prolonged cycling. In a two part study, 19 endurance trained male cyclists (Part A – 11; Part B – 8) performed 90 min of exercise at 70% VO2max 1 h after ingesting 6 mg/kg body mass of caffeine (CAF) or placebo (PLA). CAF ingestion had no effect on the PMA-stimulated oxidative burst response (Part A), yet it attenuated the exercise-induced decline in f-MLP stimulated response that occurred with PLA (Part B). CAF ingestion significantly increased serum caffeine concentration and plasma adrenaline concentration following exercise. In addition, circulating lymphocyte count was increased following CAF ingestion whereas there was no effect on neutrophil number. Therefore, although CAF ingestion was associated with an increase in adrenaline, this was not associated with an expected decrease in neutrophil function. This suggests that in the present study, CAF ingestion influenced neutrophil function via alternative mechanisms.

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Lawrence E. Armstrong

Recreational enthusiasts and athletes often are advised to abstain from consuming caffeinated beverages (CB). The dual purposes of this review are to (a) critique controlled investigations regarding the effects of caffeine on dehydration and exercise performance, and (b) ascertain whether abstaining from CB is scientifically and physiologically justifiable. The literature indicates that caffeine consumption stimulates a mild diuresis similar to water, but there is no evidence of a fluid-electrolyte imbalance that is detrimental to exercise performance or health. Investigations comparing caffeine (100-680 mg) to water or placebo seldom found a statistical difference in urine volume. In the 10 studies reviewed, consumption of a CB resulted in 0-84% retention of the initial volume ingested, whereas consumption of water resulted in 0-81% retention. Further, tolerance to caffeine reduces the likelihood that a detrimental fluid-electrolyte imbalance will occur. The scientific literature suggests that athletes and recreational enthusiasts will not incur detrimental fluid-electrolyte imbalances if they consume CB in moderation and eat a typical U.S. diet. Sedentary members of the general public should be at less risk than athletes because their fluid losses via sweating are smaller.

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Melissa J. Crowe, Anthony S. Leicht and Warwick L. Spinks

This study investigated the effects of caffeine on repeated, anaerobic exercise using a double-blind, randomized, crossover design. Seventeen subjects (five female) underwent cognitive (reaction time, number recall) and blood (glucose, potassium, catecholamines, lactate) testing before and after consuming caffeine (6 mg/kg), placebo, or nothing (control). An exercise test (two 60 s maximal cycling bouts) was conducted 90 min after caffeine/placebo consumption. Plasma caffeine concentrations significantly increased after caffeine ingestion, however, there were no positive effects on cognitive or blood parameters except a significant decrease in plasma potassium concentrations at rest. Potentially negative effects of caffeine included significantly higher blood lactate compared to control and significantly slower time to peak power in exercise bout 2 compared to control and placebo. Caffeine had no significant effect on peak power, work output, RPE, or peak heart rate. In conclusion, caffeine had no ergogenic effect on repeated, maximal cycling bouts and may be detrimental to anaerobic performance.

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Rory Warnock, Owen Jeffries, Stephen Patterson and Mark Waldron

Taurine, a sulfur-containing amino acid, is 1 of the primary ingredients in the most popular energy drinks. 1 In addition, most energy drinks contain caffeine, a methylxanthine drug commonly consumed by athletes as an ergogenic aid. 2 Caffeine ingestion (3–6 mg/kg body mass) has been shown to

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Craig Pickering and Jozo Grgic

coffee, decaffeinated coffee, and decaffeinated coffee with caffeine capsules, consumed ~60 min prior to the exercise trial. The caffeine dose was standardized across trials to deliver 4.45 mg/kg of caffeine, with a total volume of liquid of 7.15 ml/kg. Despite similar changes in plasma methylxanthines

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Alex M. Ehlert, Hannah M. Twiddy and Patrick B. Wilson

of methylxanthines following capsules, cola and chocolate . European Journal of Clinical Pharmacology, 51 ( 3–4 ), 319 – 325 . PubMed ID: 9010706 doi: 10.1007/s002280050205 Pallarés , J.G. , Fernandez-Elias , V.E. , Ortega , J.F. , Munoz , G. , Munoz-Guerra , J. , & Mora

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Romain Meeusen and Lieselot Decroix

plants found in the Amazon, and it contains theophylline, theobromine, and caffeine. Theobromine is a methylxanthine that is an adenosine receptor antagonist (as caffeine) and might improve cognitive function. Two studies ( Haskell et al., 2007 ; Kennedy et al., 2004 ) have investigated the effects of