Abstract
Pre-exercise caffeine ingestion has been shown to increase the workload at ventilatory threshold, suggesting an ergogenic effect of this stimulant on submaximal aerobic exercise. However, the time course of tolerance to the effect of caffeine on ventilatory threshold is unknown. This study aimed to determine the evolution of tolerance to the ergogenic effect of caffeine on the ventilatory threshold. Methods: Eleven participants (age 32.3 ± 4.9 yrs, height 171 ± 8 cm, body mass 66.6 ± 13.6 kg, VO2max = 48.0 ± 3.8 mL/kg/min) took part in a longitudinal, double-blind, placebo-controlled, randomized, crossover experimental design. Each participant took part in two identical treatments: in one treatment, participants ingested a capsule containing 3 mg of caffeine per kg of body mass per day (mg/kg/day) for twenty consecutive days; in the other treatment, participants ingested a capsule filled with a placebo for the same duration and frequency. During these treatments, participants performed a maximal ramp test on a cycle ergometer three times per week and the second ventilatory threshold (VT2) was assessed by using the ventilatory equivalents for oxygen and carbon dioxide. Results: A two-way ANOVA with repeated measures (substance × time) revealed statistically significant main effects of caffeine (p < 0.01) and time (p = 0.04) on the wattage obtained at VT2, although there was no interaction (p = 0.09). In comparison to the placebo, caffeine increased the workload at VT2 on days 1, 4, 6 and 15 of ingestion (p < 0.05). The size of the ergogenic effect of caffeine over the placebo on the workload at VT2 was progressively reduced with the duration of the treatment. In addition, there were main effects of caffeine (p = 0.03) and time (p = 0.16) on VO2 obtained at VT2, with no interaction (p = 0.49). Specifically, caffeine increased oxygen uptake at VT2 on days 1 and 4 (p < 0.05), with no other caffeine–placebo differences afterwards. For heart rate obtained at VT2, there was a main effect of substance (p < 0.01), while the overall effect of time (p = 0.13) and the interaction (p = 0.22) did not reach statistical significance. Heart rate at VT2 was higher with caffeine than with the placebo on days 1 and 4 (p < 0.05). The size of the effect of caffeine on VO2 and heart at VT2 tended to decline over time. Conclusion: Pre-exercise intake of 3 mg/kg/day of caffeine for twenty days enhanced the wattage obtained at VT2 during cycling ramp tests for ~15 days of ingestion, while there was a progressive attenuation of the size of the ergogenic effect of caffeine on this performance variable. Therefore, habituation to caffeine through daily ingestion may reduce the ergogenic effect of this stimulant on aerobic exercise of submaximal intensity.
Highlights
Caffeine (1,3,7-trimethylxanthine) is a substance naturally found in coffee, tea, yerba mate and cocoa and is added to foods and beverages due to its potent effect to increase alertness and to reduce fatigue
The two-way analysis of variance (ANOVA) revealed statistically significant main effects of substance (F = 26.11; p < 0.01) and time (F = 1.99; p = 0.04) on the wattage obtained at VT2, there was no interaction between these two factors (F = 1.73; p = 0.09)
The acute intake of caffeine at a dose of 3 mg per kg of body mass, prior to exercise, increased the workload attained at VT2 with additional and concomitant benefits on VO2 and heart rate at VT2
Summary
Caffeine (1,3,7-trimethylxanthine) is a substance naturally found in coffee, tea, yerba mate and cocoa and is added to foods and beverages due to its potent effect to increase alertness and to reduce fatigue. There has been a strong body of evidence, based on systematic reviews and meta-analyses, pointing towards an ergogenic effect of caffeine during endurance-like exercise activities [4,5,6,7] when caffeine is taken acutely in moderate doses (from 3 to 9 mg of caffeine per kg of body mass) These studies confirm the utility of acute caffeine intake to enhance the time to fatigue [8] and the mean power output during prolonged exercise trials [6], and the reduction in the time employed to complete a fixed amount of work during time trials [9]. To date, it is unknown if those endurance athletes consuming caffeine daily are benefiting from this substance during training and competition
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