When I was in grade school, they frequently told us a grim tale about drug use. At first, you use a little, and you feel good. Then you need more to feel good. Then, all of a sudden, you feel terrible unless you have it at all times. They could have been telling us to keep us away from hard street drugs, or warning us about overdoing it with pre-workout caffeine intake. I’m still not entirely certain. Either way, it is a great example of habituation, the process in which we become conditioned or desensitized to the effects of a substance in response to repeated usage. It also raises an interesting question: Does pre-workout caffeine still improve performance if you are a regular caffeine consumer?
Caffeine is one of the most frequently studied performance-enhancing supplements being used today. We have seen ample evidence that it enhances endurance performance , sprint performance , and resistance training performance . But, as any coffee addict will tell you (myself included), the alertness we feel in response to caffeine intake certainly seems to decrease after habitual consumption. If performance benefits are subject to the same degree of habituation, that’d be some pretty unfortunate news for me and all of my fellow daily coffee drinkers, or people who use caffeinated pre-workout supplements several times per week. Luckily, we have research available to help us uncover some answers.
Caffeine Habituation: The Evidence
While caffeine may have several mechanisms that contribute to its effects on the body, antagonism (blocking) of adenosine receptors is widely regarded as its primary mechanism of action . Research in mice has shown that chronic, high-dose caffeine intake alters the density of adenosine receptors, along with several other receptors and binding sites, in the brain . More importantly, habituation of some of caffeine’s effects has been observed in humans. For instance, caffeine has a diuretic effect (i.e., makes you urinate more), but this effect is diminished if you consume caffeine regularly . However, this habituation is not the same for all of caffeine’s physiological effects. For example, one study found that habituation was observed for caffeine’s effects on arterial blood pressure and adrenaline levels, but not cerebral blood flow . So, the question is not “Does caffeine habituation occur?” The more pertinent question is, “Do we become habituated to the performance effects of caffeine?”
We have some indirect evidence to draw from in the research literature. For example, Wiles et al. conducted a study  in which the performance effects of caffeine were measured, and habitual caffeine intake for each subject was estimated using a questionnaire. The statistical analysis revealed no direct relationship between daily caffeine consumption and the performance effects of caffeine, although the study was not designed to answer this question directly.
Other studies have addressed the question a bit more directly, by grouping subjects into “caffeine users” and “caffeine non-users.” These studies have also cast some doubt on the concept that regular caffeine users become habituated to the performance-enhancing benefits of caffeine. Dodd et al.  assessed endurance performance after caffeine consumption in non-users (less than 25 mg of caffeine per day) and heavy users (over 300 mg of caffeine per day). For each group, exercise performance was tested after the consumption of a placebo, a caffeine dose of 3 mg per kilogram of bodyweight, and a caffeine dose of 5 mg per kilogram of bodyweight. When it came to performance outcomes, users and non-users did not respond differently to caffeine supplementation.
In contrast, Bell and McLellan  found some modest differences between caffeine users (over 300 mg of caffeine per day) and non-users (less than 50 mg of caffeine per day). Several exercise tests were conducted, evaluating performance at 1, 3, and 6 hours after ingesting a dose of 5 mg of caffeine per kilogram of bodyweight, or a placebo. Both groups performed better 1 and 3 hours after caffeine ingestion, but the magnitude of this effect was a bit larger in non-users. Further, the non-users had a performance improvement 6 hours after caffeine ingestion, whereas habitual caffeine users did not.
Given these somewhat mixed findings, a recent paper  sought to settle the score and get to the bottom of this question. This study actually split subjects into three groups based on their typical caffeine use: Low users (with an average of 58 mg per day of caffeine intake), moderate users (143 mg per day), and high users (351 mg per day). Each group completed three trials to assess their endurance performance after consuming absolutely nothing, a placebo supplement, or a dose of 6 mg of caffeine per kilogram of bodyweight. Results revealed that caffeine improved time trial performance compared to the control and placebo conditions. They also determined that habitual caffeine intake did not significantly influence performance outcomes from a statistical perspective, and there was no correlation between habitual caffeine intake and caffeine-induced performance improvements.
When you measure human performance, the combined results from several different studies on a topic aren’t always cut and dried. There’s often a mixture of positive, negative, and/or null findings, and there are three pretty obvious causes for this lack of unanimity. First, human performance cannot be replicated perfectly and reliably. You have good days and bad days in the gym, and so do research subjects. Second, we cannot measure everything perfectly, so some degree of measurement error is generally present. Third, individuals respond differently to interventions. The magnitude of this individual variation differs depending on the exact intervention and population, but caffeine metabolism is influenced by several factors , and performance effects are likely to display fairly high variability by extension. So, with this in mind, what can we tell from the data that are available?
It seems safe to say that habitual caffeine users still obtain performance benefits from caffeine. This conclusion jives perfectly with the results reported by Wiles et al.  and Dodd et al. , and are strongly supported by the recent study showing no influence of habitual caffeine use across three separate levels of daily intake . If the performance effects of caffeine are in fact diminished by habitual use, this detrimental impact is minimal at best. For instance, Bell and McLellan  concluded that habitual use reduced the performance benefit of caffeine, but a significant improvement was still observed for regular users. The habitual users just didn’t see a benefit when they used the caffeine 6 hours prior to exercise (pro tip: If you’re dosing your caffeine 6 hours prior to exercise, you’re doing it wrong. Stick to 30-90 minutes before, and you’re good to go). So, while there is no question that we become habituated to some of caffeine’s effects with consistent consumption, the data indicate that habitual caffeine users can still get a performance boost fromm their pre-workout caffeine. When it comes to performance, we can have our coffee, and drink it too.
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