Caffeine is a common ingredient used in sports nutrition, with companies using it as a sports nutrition ingredient for endurance and strength training.
Writing in Nutrients, researchers from Beijing Sport University and Rey Juan Carlos University studied 21 randomised placebo-controlled trials involving 254 participants for this particular meta-analysis.
These studies were sieved out from Medline, Scopus, Web of Science, and SportDiscus and were published as of October last year.
Out of the 21 studies, 18 provided caffeine in liquid or capsule form, with doses normalised by participants’ body mass, which were between three and nine mg/kg.
The other three studies provided absolute doses of caffeine in the form of caffeine powder, gum, and oral strips, with doses ranging from 200 to 300 mg.
Two-thirds of the trials also studied the effects of caffeine in time trial runs, while the remaining ones looked at caffeine’s effects on time-to-exhaustion runs.
Between the two, time trials offer a better scenario to study the effect of caffeine on endurance running performance, as runners’ performance is more reproducible in time-trial runs than in running to exhaustion events, the researchers said.
Effects on runners
Caffeine intake has shown to prolong the time to exhaustion as compared with the placebo in both recreational runners and trained runners.
Among the intervention groups, caffeine intake has overall increased time to exhaustion by 16.97 ± 14.65 per cent.
“Caffeine promotes the production of ß-endorphins and dopamine, which can lessen perceived effort and discomfort.
“These factors can explain the ergogenic effect of caffeine on time to exhaustion runs, as the ‘exhaustion time’ when runners decide to stop running is associated with feelings of fatigue and muscle pain,” the researchers explained.
In addition, the time to complete endurance running was shorter in participants who took caffeine as compared to the placebo.
The average reduction in time during endurance running events was 0.71 ± 0.83 per cent.
Comparing time-to-exhaustion and time-trials, the researchers said a bigger percentage change was seen between the intervention and placebo group in the time-to-exhaustion, possibly due to the characteristics of each running protocol.
“In time-to-exhaustion runs, the intensity is fixed, and runners cannot adjust the pace during the running trial.
“However, in time trials, like in real running races, runners may adjust their pace depending on their feelings of fatigue and the distance left to complete the trial. As an adenosine antagonist, caffeine can decrease fatigue by crossing the blood–brain barrier and inhibiting A1 and A2 monoadenosine receptors.
“So, caffeine may have an inhibitory effect on perceptual response during exercise, which may give participants an increased ability to tolerate the discomfort associated with fatigue during exercise, effectively masking the sensation of fatigue,” the researchers explained.
The researchers have pointed out a number of limitations with this analysis, such as the wide range of products that the placebo group in each study was exposed to.
For instance, some placebo groups were given sugar-free, or sugary, or decaffeinated products, while the intervention groups might be given caffeine with other compounds, such as carbohydrates and artificial sweeteners.
Thus, it is possible that the actual effects of caffeine may be masked or exaggerated by some of these substances.
In addition, different dosage formats of caffeine – in the form of capsules, tablets, gums, or beverages – may affect the pharmacokinetics of caffeine, thus influencing the results of different studies, the researchers said.
Nonetheless, they concluded that evidence from the meta-analysis supported the recommendation of caffeine intake for endurance runners.
“Caffeine seems more ergogenic for time-to-exhaustion runs than for time trials, suggesting that this substance may be more useful for prolonged running events where time to exhaustion is a performance factor.”
For future studies, the researchers said that the focus could be on examining the physiological and perceptual effects of caffeine intake during endurance running. This includes parameters such as heart rate, blood lactate concentration and ratings of perceived fatigue.
As the studies included in the meta-analysis mostly focused on men, more evidence is also needed to establish the effect of caffeine on endurance running in women or the best dose to maximise the benefits of caffeine supplementation.
Effects of Caffeine Intake on Endurance Running Performance and Time to Exhaustion: A Systematic Review and Meta-Analysis
Authors: Wang, Z.; Qiu, B.; Gao, J.; Del Coso, J.