We probably know that caffeine can improve various aspects of cognitive performance, but what’s less often explored or considered is if/how the amount of caffeine one regularly consumes impacts how much caffeine is necessary to provide these pro-cognitive benefits.
For example, in undergraduate students who only consumed a median of 42.5mg/day caffeine, 200mg and 400mg caffeine improved vigilance and executive control of visual attention with 200mg seeming to be more “optimal” than 400mg (1).
But in a study on undergraduate students who consumed a median of 592.3mg/day caffeine, caffeine only enhanced vigilance and executive control of visual attention at 400mg, not at 200mg (2).
So, if you consume a decent amount of caffeine daily, you may well need higher doses to get acute cognitive/performance benefits, even non-physical/athletic benefits. But there is some mixed research on the topic, and different methodologies and measures of cognition may partially explain some differences or seeming conflicts in the research I would think.
Some more research on various doses of caffeine for people with various caffeine intake (3):
Warburton (1995) examined the effects of 0, 75, and 150 mg of caffeine on attentional, verbal memory, nonverbal working memory, and problem-solving speed and accuracy in 18 men who were regular coffee drinkers (no more than 3 cups/day). Caffeine improved speed and accuracy on attentional tests (visual information processing) in a dose-dependent manner. Similar to the data of Foreman et al. (1989), there was no effect of caffeine on immediate verbal recall; however there was a dose-related effect of caffeine on delayed verbal recall. Caffeine also significantly improved the accuracy, but not the speed, of problem solving. Rogers et al. (1995) found significant improvement in reaction time with 70 mg of caffeine compared to placebo. Similarly, Lorist and Snel (1997) found that caffeine at 3 mg/kg (210 mg for a 70 kg person) given to habitual users improved reaction time and decreased false alarm rates in selective attention tasks. Streufert et al. (1997) evaluated the effects of 400 mg of caffeine added to regular caffeine consumption in moderate to heavy caffeine users (400–1,000 mg/day) and found faster responses to incoming information.
How about caffeine with theanine? Did any of these studies specify the daily/regular caffeine intake of the subjects? Because many of these studies noted synergistic cognitive benefits with pretty low doses of caffeine.
Subjects in one study consumed an average of 9.6 servings of caffeine per week, and noted benefits with only 50mg caffeine and 100mg theanine (4). It is worth noting that the group that received only 50mg caffeine (with no theanine) noted benefits, but the combination was superior, indicating synergistic effects on aspects of memory and attention. I’m not entirely certain how much caffeine is in a “serving” of caffeine though. Is it black tea (28-46mg per cup), tea or instant coffee (55-65mg per cup), or brewed coffee (107-151mg per cup) (6)? So this 9.6 servings per week could, in theory, be anywhere from 38mg/day (9.6 servings per week at 26mg per serving) to 207mg/day (9.6 servings per weeks at 151mg per serving). I suppose it could even be higher in theory if a “serving” of caffeine could have included something like a large coffee from a coffee shop, but who knows really.
Another study had subjects who consumed an average of 173mg/day caffeine (SD 86.8mg), and found that 40mg caffeine and 97mg theanine “helps to focus attention during a demanding cognitive task” (5). This study did not have a group use caffeine without theanine, only the combination and placebo.
So, in summary, if you consume a lot of caffeine regularly, you may need more caffeine to get cognitive benefits than if you consumer very little caffeine regularly, and if you consume very little caffeine regularly, the “optimal” dose of caffeine may be lower than it is if you consume a lot of caffeine regularly. That’s a pretty logical thing to expect, no? But that’s not to say that lower doses of caffeine won’t work for regular caffeine consumers, as there are studies showing that they can. And, as most of us know, theanine goes very well with caffeine.