Caffeine and mental alertness – part 2

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Caffeine and memory
It has been reported that caffeine intake has beneficial effects on improving working memory. Low doses of caffeine enhance working memory performance, while higher doses are found to decrease it, possibly due to over-stimulation.

  • Comparable results have been seen in low-load memory tasks versus high-load memory tasks. Caffeine has been shown to have beneficial effects on performance in both low-difficulty and low-load memory tasks. High-load and complicated tasks induced increased arousal by themselves: so in these tasks, caffeine could lead to over-arousal. Thus caffeine appears to improve working memory performance under conditions that otherwise produce low arousal states26.
  • A 2010 study suggested such an effect may be linked to personality27. Caffeine improved working memory performance in extroverts but not in introverts. Further research in this area would be of interest.
  • Another study tested college students to see if they could recall words from six different lists comprising 15 words each, after 200mg caffeine administration. The words on each list were semantically related to a single word (a “critical lure”) that was not present in the list. The students recalled more listed words and more “critical lures” with caffeine intake than with the placebo. Caffeine appeared to intensify the connections among listed words and critical lures, hence enhancing both true memory (participants memorized and recalled only words from the list) and false memory (participants quoted words that were not on the original list but were related to the list words, i.e. induced by the lure)28.

Synergistic effects of caffeine and glucose
Caffeine and glucose absorbed together have beneficial synergistic effects on sustained attention and verbal memory29.

Combined administration of glucose and caffeine modulates neural activity in a network involving the parietal and prefrontal cortex related to sustained attention. Synergistic consumption of both substances is thought to increase the efficiency of the attentional system, as subjects who received the combined beverage had similar performance to the other subjects, but required less activation of attentional brain areas30.

In a study of 150 healthy adults consuming either a placebo, 25g or 60g glucose, or 60g glucose plus caffeine, the results suggested that the caffeine-glucose group had significantly better total multi-tasking scores and were significantly faster at mental arithmetic tasks. There were no significant treatment effects on mood. The authors conclude that co-administration of glucose and caffeine allows greater allocation of attentional resources than placebo or glucose alone, although they suggest that they cannot rule out the possibility that the effects are due to caffeine alone31.

Further studies using larger samples and different levels of caffeine, glucose and cognitive effort will be necessary to better understand the combined effects of both substances.

Caffeine and mood
It is well-known that low to moderate doses of caffeine (around 2-5 cups of coffee per day) improve hedonic tone (i.e. the degree of pleasantness) and reduce anxiety. In comparison, high doses can increase tense arousal, including anxiety, nervousness, and jitteriness. A dose-related improvement in subjective measures of calmness and interest are found after caffeine, suggesting that mood improvement may depend on baseline arousal26.

Older adults are more sensitive than younger individuals to the mood-enhancing effects of caffeine, and mood effects are also influenced by the time of the day, with the most prominent effects showing in the late morning32. Moreover, mood is not only modulated by caffeine itself but also by the expectation of having consumed caffeine, which improves mood together with attention10. Caffeine tends to benefit habitual consumers’ mood more (compared to non-consumers), but there are greater improvements in performance when taken by non-consumers33.

  • Using the accepted Profile of Mood States (POMS) self-rating scale and the Bakan test for cognitive performance, a 2009 double-blind study concluded that a moderate dose of caffeine (200mg) together with a low carbohydrate intake (50g white bread) positively influenced mood and cognitive performance, while carbohydrate intake alone did not34. This indicates that, in the caffeine/carbohydrate combination group, the key element leading to improved mood and mental performance was the presence of caffeine.
  • A study investigating the role of caffeine on social support asked participants who received either caffeinated coffee (150mg caffeine), or decaffeinated coffee (9mg caffeine), to imagine a fictitious person and to play the Mixed Motive Game with that person 45 minutes after coffee consumption. Caffeinated coffee increased co-operative game behavior and sadness communication, suggesting that caffeinated coffee may improve social support and relieve depressive symptoms35.
  • A study of 50,739 women (average age 63 years), part of the Nurses’ Health Study, looked at caffeine and depression. It appeared that women who consumed 2-3 or at least 4 cups of caffeinated coffee per day were, respectively, 15% or 20% less likely to develop depression, compared to those who drank at most one cup of caffeinated coffee per week. The consumption of decaffeinated coffee had no impact on depression risk. This observational study suggests the possibility of a protective effect of caffeine on depression risk36.
  • Another cohort study, of Finnish men, reported a 77% risk reduction for depression in heavy coffee drinkers (those who consumed over 813mg caffeine daily). This effect was limited to coffee and was not found with either tea or caffeine alone37.
  • A 2013 Japanese cross-sectional study reviewed the impact of consumption of both green tea and coffee on depressive symptoms, suggesting that both green tea (more than 4 cups per day) and coffee (more than 2 cups per day) may offer protection against depression38.
  • A cross-sectional study in 10,177 Korean individuals aged 20-97 years, who participated in the fifth Korean National Health and Nutrition Examination Survey, suggested that coffee consumption may have a small protective effect on the risk of depression39.
  • A 2016 meta-analysis accounting for a total of 346,913 individuals and 8,146 cases of depression suggested that coffee consumption has a protective effect on depression, with a dose-response analysis suggesting a J-shaped curve, with the beneficial effect reported for up to approximately 300mg caffeine per day40.
  • A small pilot study reported that caffeinated coffee had a more robust positive effect on high-level mood and attention processes than decaffeinated coffee. Interestingly, the authors found that decaffeinated coffee could also improve mood and performance. This suggests that substances other than caffeine, such as chlorogenic acids, may also affect mood and performance41. However this effect needs to be confirmed in a larger group of individuals.
  • Finally, coffee and caffeine consumption might be favored by some specific patient groups, including patients with bipolar disorders, who were reported to consume more social drugs such as tobacco and coffee than the general population42, and schizophrenic patients43. It has been hypothesized that patients smoke and drink coffee to reduce medication side effects such as anhedonia (inability to feel pleasure), or to improve cognitive symptoms linked to the treatment.

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