No difference found in caffeine's effects on exercise power among 'fast' or 'slow' metabolizers

The study was published in the International Journal of Sports Nutrition. It was the work of researchers associated with universities in Australia, Croatia, the UK and the US.

The researchers’ aim was to further elucidate how caffeine boosts acute exercise performance and whether it does so more for some people than others and why. 

Caffeine is one of the oldest botanical stimulants known to man.  It has been ingested for more than a thousand years in the form of tea, whose common consumption in China stated during the Tang dynasty around 700 CE.

Long prized for its effects on mental clarity, and to some extent for its ability to help those who consume it to stay awake longer, caffein has more recently been researched for its performance benefits.  Early research focused more on endurance events such as cycling time trial performance​​, partly, it seems, because this particular activity is easy to conduct in a controlled manner in a lab.  These studies consistently found statistically significant performance improvements.  The authors of the present study noted that previous studies have linked genetic variation to a difference in caffeine’s effects on endurance exercise, but there is less know about how these genetic difference affect power movements.

How does genetic availability affect caffeine’s effect on high intensity exercise?

It has already been well established that the CYP1A2 gene encodes an enzyme known as P450-​​1A2 that is responsible for 95% of caffeine metabolism​.  Some individuals make a lot of this enzyme, others less, and so have come to be known as ‘fast’ metabolizers or ‘slow’ metabolizers.

“Significantly greater ergogenic effects of caffeine on aerobic endurance have been reported for individuals with the AA genotype [‘fast’ metabolizers], compared with C allele carriers [‘slow’ metabolizers]. However, for high-intensity exercise tasks of a shorter duration, the evidence is less clear,”​ they wrote.

It stands to reason that the different speed of metabolism would mean there would always be different amounts of caffeine in the subjects’ systems, so there should be some variation in how caffeine affects even fairly brief, high intensity exercise tests.

However, with the little amount of data available, the picture is not clear, the researchers noted. One study done with 19 basketball players doing jump tests and another using trained subjects doing sprint tests found no difference in caffeine’s effects for the different genotypes.  But a 2019 study published in an Irish journal did find a difference when measuring things like total number of bench presses and leg presses, with the fast metabolizers benefiting more.

The present study recruited 22 young men who were trained in resistance exercise.  Of the group, 13 carried the AA genotype, while 9 were C allele carriers.  The test measured bench press speed and power as well as how many repetitions the subjects could manage to exhaustion at 85% of their peak 1 rep weight. The study also included a Wingate sprint test and a vertical leap measurement.

Caffeine helped both groups similarly

All subjects ingested caffeine at a dose of 3 gramps per kilogram of body weight  or a placebo. 90 minutes before they were tested.  The researchers found that for these measures, caffeine boosted performance for both groups and there was no statistically significant difference between how the two groups performed versus placebo.

“This study found that caffeine is acutely ergogenic for movement velocity, power output, and muscle endurance in resistance exercise, vertical jump height, and peak, mean, and minimum power in a Wingate test.  . . . The comparisons of the effects of caffeine on exercise performance between individuals with the AA genotype and AC/CC genotypes found no significant differences,” ​they concluded.

The authors noted that about 60% of participants could tell when they had ingested the caffeine as opposed to placebo, which is a potential limitation of the study.  But they cited previous research that showed that in other caffeine studies this incomplete blinding did not significant alter the placebo effect, so they said the effect of this issue on their results was likely small.

“We would suggest that future research in this area explores both the quality and quantity of the performed repetitions, to provide a more comprehensive assessment of possible effects of caffeine,” ​they added.

Source:​ Journal of the International Society of Sports Nutrition​​​
 17​, Article number: 21 (2020)
CYP1A2 genotype and acute effects of caffeine on resistance exercise, jumping, and sprinting performance​
Authors: Grgic J, et al.