Caffeine and Alertness: In Defense of Withdrawal Reversal

Genetics of caffeine and brain-related outcomes - a systematic review of observational studies and randomized trials

CONTEXT

Although the stimulant and anxiogenic properties of caffeine are widely accepted, research on its specific effects on the brain remains controversial. Growing evidence shows that interindividual differences in caffeine response may be partly due to variations in genes such as CYP1A2 and ADORA2A, which have been used to identify individuals as "fast" or "slow" caffeine metabolizers and as having a "high" or "low" caffeine sensitivity, respectively.

OBJECTIVE

The objective of this review was to identify, evaluate, and discuss current evidence on the associations between common genetic variants, caffeine consumption, and brain-related outcomes in humans.

DATA SOURCES

PubMed and Embase databases were searched for relevant reports based on a predetermined search strategy.

DATA EXTRACTION

Reports of observational and experimental studies on healthy adults who underwent (a) genetic analysis for polymorphisms in genes associated with caffeine metabolism and effects and (b) measurements of brain-related effects such as anxiety, insomnia, and cognitive performance associated with the consumption of caffeine (habitual intake or supplementation) were included.

DATA ANALYSIS

Of the 22 records included, 15 were randomized controlled trials, 6 were cross-sectional studies, and 1 was a genome-wide association study. The main outcomes identified were cognitive performance (n = 9), anxiety (n = 7), and sleep disturbance/insomnia (n = 6). Polymorphisms in the CYP1A2 gene were associated with cognitive function, while variations in the ADORA2A gene were associated with anxiety and sleep disturbance.

CONCLUSION

The present review has provided evidence that variability in the CYP1A2 and the ADORA2A genes may modulate the association between caffeine and brain-related outcomes. Future studies are warranted to investigate the specific polymorphisms implicated in each brain outcome, which cognitive functions are particularly related to caffeine (simple vs complex), whether there are gender differences in anxiety effects, and how habitual caffeine intake may influence the acute effects of caffeine.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO registration no. CRD42021257556.

Recent Caffeine Drinking Associates with Cognitive Function in the UK Biobank

Clinical evidence points to the premise that caffeine may benefit cognition, but whether these findings extend to real life settings and amidst factors that impact caffeine metabolism is unclear. The aim of this study was to investigate the impact of recent caffeine drinking on cognitive ability while additionally accounting for lifestyle and genetic factors that impact caffeine metabolism. We included up to 434,900 UK Biobank participants aged 37–73 years, recruited in 2006–2010, who provided biological samples and completed touchscreen questionnaires regarding sociodemographic factors, medical history, lifestyle, and diet. Recent caffeine drinking (yes/no in the last hour) was recorded during a physical assessment. Participants completed at least one of four self-administered cognitive function tests using the touchscreen system: prospective memory (PM), pairs matching (Pairs), fluid intelligence (FI), and reaction time (RT). Multivariable regressions were used to examine the association between recent caffeine drinking and cognition test scores. We also tested interactions between recent caffeine drinking and a genetic caffeine-metabolism score (CMS) on cognitive function. Among white participants, recent caffeine drinking was associated with higher performance on RT but lower performance on FI, Pairs, and PM (p ≤ 0.004). Similar directions of associations for FI (p = 0.09), Pairs (p = 0.03), and PM (p = 0.34) were observed among non-white participants. No significant and consistent effect modification by age, sex, smoking, test time, habitual caffeine intake, or CMS was observed. Caffeine consumed shortly before tasks requiring shorter reaction times may improve task performance. Potential impairments in memory and reasoning tasks with recent caffeine drinking warrant further study.

The Consumption of Breakfast, Fish and/or Caffeine does not Predict Study Progress in Adult Distance Education

Consumption of breakfast, fish, or caffeine are each separately often investigated in relation to learning performance in traditional education, but not in distance education (DE). The objective of this study was to investigate whether the relations between the consumption of breakfast, fish, and/or caffeine on the one hand and learning performance on the other are also found in DE students. This population is different from traditional students and characterized by a different profile in terms of age, personal and work responsibilities as well as other demographics. In an observational longitudinal study, the consumption of breakfast, fish, and caffeine of 1157 DE students (18-76 years old, mean 35.8 ± 11.1 years) was used to predict learning performance using multiple regression analysis. In an online digital survey, university students provided information about their consumption of these nutritional measures and on important covariates. Learning performance, measured as study progress (i.e., the number of successfully completed modules), was evaluated objectively after 14 months. Results showed that adding the consumption of breakfast, fish, and caffeine to the covariate model did not fit the data better, χ² (3, N = 1155) = 3.287, p = 0.35. This means that neither the consumption of breakfast nor fish nor caffeine predicted study progress in adults participating in DE. This study is important as it is the first to report on these relations in this specific age group and educational setting, which is increasingly important due to the increased preference for this type of education.

Physical Activity, Sleep, and Nutrition Do Not Predict Cognitive Performance in Young and Middle-Aged Adults

Biological lifestyle factors (BLFs) such as physical activity, sleep, and nutrition play a role in cognitive functioning. Research concerning the relation between BLFs and cognitive performance is scarce however, especially in young and middle-aged adults. Research has not yet focused on a multidisciplinary approach with respect to this relation in the abovementioned population, where lifestyle habits are more stable. The aim of this study was to examine the contribution of these BLFs to cognitive performance. Path analysis was conducted in an observational study in which 1131 adults were analyzed using a cross-validation approach. Participants provided information on physical activity, sedentary behavior, chronotype, sleep duration, sleep quality, and the consumption of breakfast, fish, and caffeine via a survey. Their cognitive performance was measured using objective digital cognitive tests. Exploration yielded a predictive cohesive model that fitted the data properly, χ²/df = 0.8, CFI = 1.00, RMSEA < 0.001, SRMR = 0.016. Validation of the developed model indicated that the model fitted the data satisfactorily, χ²/df = 2.75, CFI = 0.95, RMSEA < 0.056, SRMR = 0.035. None of the variables within the BLFs were predictive for any of the cognitive performance measures, except for sedentary behavior. Although sedentary behavior was positively predictive for processing speed its contribution was small and unclear. The results indicate that the variables within the BLFs do not predict cognitive performance in young and middle-aged adults.

Caffeine improves reaction time, vigilance and logical reasoning during extended periods with restricted opportunities for sleep

RATIONALE

Various occupational groups are required to maintain optimal physical and cognitive function during overnight periods of wakefulness, often with less than optimal sleep. Strategies are required to help mitigate the impairments in cognitive function to help sustain workplace safety and productivity.

OBJECTIVES

To test the effectiveness of repeated 200 mg doses of caffeine on cognitive function and live-fire marksmanship with soldiers during three successive nights of sustained wakefulness followed by 4-h afternoon sleep periods.

METHODS

Twenty Special Forces personnel (28.6 ± 4.7 years, 177.6 ± 7.5 cm and 81.2 ± 8.0 kg) were randomly assigned to receive four 200-mg doses of caffeine (n = 10) or placebo (n = 10) during the late evening and early morning hours during three successive days. An afternoon 4-h sleep period followed. The psychomotor (PVT) and field (FVT) vigilance, logical reasoning (LRT) tests and a vigilance monitor assessed cognitive function throughout the study. Live-fire marksmanship requiring friend-foe discrimination was assessed.

RESULTS

Caffeine maintained speed on the PVT (p < 0.02), improved detection of events during FVT (p < 0.001), increased number of correct responses to stimuli as assessed by the vigilance monitor (p < 0.001) and increased response speed during the LRT (p < 0.001) throughout the three overnight testing periods. Live-fire marksmanship was not altered by caffeine.

CONCLUSIONS

A total daily dose of 800 mg caffeine during successive overnight periods of wakefulness is an effective strategy to maintain cognitive function when optimal sleep periods during the day are not available.

Caffeine and cognitive performance: persistent methodological challenges in caffeine research

Human cognitive performance is widely perceived to be enhanced by caffeine at usual dietary doses. However, the evidence for and against this belief continues to be vigorously contested. Controversy has centred on caffeine withdrawal and withdrawal reversal as potential sources of experimental confounding. In response, some researchers have enlisted "caffeine-naïve" experimental participants (persons alleged to consume little or no caffeine) assuming that they are not subject to withdrawal. This mini-review examines relevant research to illustrate general methodological challenges that have been the cause of enduring confusion in caffeine research. At issue are the processes of caffeine withdrawal and withdrawal reversal, the definition of caffeine-naïve, the population representativeness of participants deemed to be caffeine-naïve, and confounding due to caffeine tolerance. Attention to these processes is necessary if premature conclusions are to be avoided, and if caffeine's complex effects and the mechanisms responsible for those effects are to be illuminated. Strategies are described for future caffeine research aimed at minimising confounding from withdrawal and withdrawal reversal.