We’re pretty big fans of caffeine, both for its ability to put some pep in our step in the morning, and for its ability to acutely enhance force production and strength endurance (likely due, primarily, to its function as a central adenosine antagonist). However, we’ve previously noted in MASS that there’s a relative dearth of caffeine studies in female subjects. Most exercise science research still uses male subjects, so questions arise regarding which research findings, resulting from research on male subjects, will generalize to female lifters. Sometimes it’s pretty safe to assume that findings in males will generalize to female – for example, protein metabolism isn’t fundamentally different between the sexes, so there’s little reason to suspect that protein synthesis research in male subjects won’t generalize to females – but with caffeine, it’s not quite as straightforward. The primary enzyme involved in caffeine metabolism (CYP1A2) is also heavily involved in estrogen metabolism and regulation, and variations in the gene that codes for that enzyme may impact the effects of caffeine, so it’s not unreasonable to wonder whether caffeine may have different effects in male and female lifters.
With that in mind, it’s unfortunate that, until very recently, almost all of the studies investigating the effects of caffeine on strength and strength endurance used exclusively male subjects. To illustrate, a 2020 meta-analysis analyzing the effects of caffeine on strength and strength endurance included 19 studies; of those 19 studies, 15 used exclusively male subjects, two used mixed-sex cohorts (one with 9 males and 2 females, and one with 13 males and 4 females), and two used exclusively female cohorts. In total, 87% of the subjects in the included studies were male.
However, we’ve seen a renaissance in the area within the past year or so. In the present meta-analysis examining the impact of caffeine on maximal strength and strength endurance, eight studies were included, including five that were published in either 2020 or 2021. All of the studies utilized a placebo-controlled crossover design, and they all included at least one test of 1RM strength or isotonic muscular endurance (typically a reps-to-failure test). Of the 8 studies, six used “resistance-trained” subjects, one used competitive karate athletes, and one used untrained middle-aged women. The caffeine dosages used were pretty standard for the literature – mostly 2-6mg/kg, ingested 60 minutes before exercise.
As it turns out, caffeine works for female lifters too. When pooling all results, caffeine was found to improve muscular endurance (ES = 0.20; p = 0.027) and 1RM strength (ES = 0.18; p < 0.001). Sub-analyses separating upper body and lower body results found that results were significant for upper body measures of strength and strength endurance, but not lower body strength and strength endurance. Honestly, I wouldn’t put much stock in the sub-analyses – they inherently have less statistical power, and the point estimates for upper and lower body performance were basically the same.
These effect estimates are virtually identical to those observed in previous meta-analyses which primarily used studies with male subjects. In previous issues of MASS, we’ve noted that the effects of caffeine seem to be similar between the sexes, based on results of individual studies. The present meta-analysis dramatically strengthens those observations, suggesting that caffeine provides an equivalent small but reliable increase in strength and strength endurance in both sexes.