Psychophysiological effects of habitual caffeine consumption

Controlling for caffeine in cardiovascular research: A critical review

Caffeine, the most widely consumed drug in the world, exerts numerous effects on cardiovascular activity. Thus, it is important and advisable to control for caffeine consumption in studies examining caffeine and/or cardiovascular activity and reactivity. This paper 1) reviews the literature concerning caffeine's effects on cardiovascular parameters; 2) summarizes the widely varying protocols used to control for the drug in extant cardiovascular literature, and 3) provide guidelines for caffeine control procedures to minimize potentially confounding acute and withdrawal effects of the drug. An abstention period equal to the average half-life of the drug is recommended for creation of methodological controls for caffeine. Additional methodological recommendations are described concerning factors that moderate the half-life of caffeine. When feasible, researchers should consider and aim to control for caffeine's acute and extended psychophysiological effects. This understudied issue has fundamental implications for caffeine-related investigations and research in psychophysiology and behavioral medicine.

Adolescent habitual caffeine consumption and hemodynamic reactivity during rest, psychosocial stress, and recovery

OBJECTIVE

Most adolescents regularly consume caffeine. Whereas observational studies have suggested that coffee may be cardio-protective, pharmacological experimentation with adults shows that caffeine at dietary doses increases blood pressure, thereby implicating regular caffeine consumption as a potential source of harm for cardiovascular health. The present study was in response to the dearth of caffeine research among younger consumers. It was hypothesised that compared to the consumption of little or no caffeine, adolescents who habitually consume caffeine have overall higher blood pressure and increased vascular resistance.

METHOD

Using a quasi-experimental design, continuous measurements of blood pressure, cardiac output, and total peripheral resistance were taken non-invasively from adolescents (n = 333) aged 14-15 years and 18-19 years who reported "low", "moderate", or "high" levels of caffeine intake. Measurements were conducted when participants generally had negligible or low systematic caffeine levels while at rest, during stress, and during recovery from stress.

RESULTS

Whereas habitual caffeine consumption did not predict blood pressure level, higher caffeine intake was associated with modestly increased vascular resistance during all phases of the experiment (i.e., at rest, during stress, and during recovery from stress).

CONCLUSIONS

Present findings are important because they suggest that early exposure to caffeine may lead to persistent increases in vascular resistance, which in turn is an acknowledged risk factor for the development of hypertension. These results highlight the need for further studies of adolescents to determine the robustness of any persistent caffeine-related hemodynamic effects, and the implications such effects could have for long-term cardiovascular health.

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.

Effects of dietary caffeine on topographic EEG after controlling for withdrawal and withdrawal reversal

BACKGROUND/AIMS

Despite several decades of research into the effects of caffeine on EEG, few consistent findings have emerged. Notwithstanding the likelihood that differences in methodology may explain some of the inconsistency, confidence in the published findings is undermined by the failure in previous studies to control for the effects of caffeine withdrawal and withdrawal reversal.

METHODS

Participants (n = 22) alternated weekly between ingesting placebo and caffeine (1.75 mg/kg) 3 times daily for 4 consecutive weeks. EEG activity was measured at 32 sites during eyes closed, eyes open, and performance of a vigilance task.

RESULTS

Caffeine was found to have few and modest effects on EEG in the theta and alpha bandwidths, and no effects in the delta and beta bandwidths. Evidence was found of withdrawal, withdrawal reversal, and tolerance in relation to observed increases in theta power during task performance; withdrawal and withdrawal reversal in relation to increases in alpha power during all three behavioural conditions (eyes closed, eyes open, and task performance), and withdrawal-induced adverse effects in relation to aspects of subjective mood.

CONCLUSION

The finding of similar increases in theta power following caffeine challenge and acute caffeine withdrawal casts doubt on whether caffeine may be viewed as having direct stimulant effects. Results could suggest that change in drug state, whether in the form of acute caffeine withdrawal or challenge, may be disruptive to electrophysiological activity in the brain.

Caffeine, sleep and wakefulness: implications of new understanding about withdrawal reversal

The broad aim of this review is to critically examine the implications of new understanding concerning caffeine withdrawal and withdrawal reversal in the context of research concerned with the effects of caffeine on sleep and wakefulness. A comprehensive search was conducted for relevant experimental studies in the PubMED and PsycINFO databases. Studies were assessed with particular reference to methodological adequacy for controlling against confounding due to caffeine withdrawal and withdrawal reversal. This assessment was used to clarify evidence of effects, highlight areas of ambiguity and derive recommendations for future research. It was found that researchers have generally failed to take account of the fact that habitual use of caffeine, even at moderate levels, leads to physical dependence evidenced by physiological, behavioural and subjective withdrawal effects during periods of abstinence. Consequently, there has been near-complete absence of adequate methodological controls against confounding due to reversal of withdrawal effects when caffeine is experimentally administered. The findings of what has been a substantial research effort to elucidate the effects of caffeine on sleep and wakefulness, undertaken over a period spanning decades, are ambiguous. Current shortcomings can be redressed by incorporating suitable controls in new experimental designs.

Effects of caffeine on performance and mood: withdrawal reversal is the most plausible explanation

RATIONALE

Although it is widely believed that caffeine can enhance human performance and mood, the validity of this belief has been questioned, giving rise to debate. The central question is whether superior performance and mood after caffeine represent net benefits, or whether differences between caffeine and control conditions are due to reversal of adverse withdrawal effects.

OBJECTIVES

To provide a focussed review of relevant experimental studies with the aim of clarifying current understanding regarding the effects of caffeine on human performance and mood.

METHODS

To avoid the shortcomings of standard placebo-controlled studies, which are ambiguous due to failure to control for the confounding influence of withdrawal reversal, three main experimental approaches have been employed: studies that compare consumers and low/non-consumers, pre-treatment and ad lib consumption studies, and long-term withdrawal studies.

RESULTS

Of the three approaches, only long-term withdrawal studies are capable of unambiguously revealing the net effects of caffeine. Overall, there is little evidence of caffeine having beneficial effects on performance or mood under conditions of long-term caffeine use vs abstinence. Although modest acute effects may occur following initial use, tolerance to these effects appears to develop in the context of habitual use of the drug.

CONCLUSIONS

Appropriately controlled studies show that the effects of caffeine on performance and mood, widely perceived to be net beneficial psychostimulant effects, are almost wholly attributable to reversal of adverse withdrawal effects associated with short periods of abstinence from the drug.

Effects of dietary caffeine on mood when rested and sleep restricted

Prolonged use of caffeine can lead to physical dependence evidenced by characteristic withdrawal symptoms during abstinence. Debate exists as to whether mood enhancement by caffeine represents a net effect or merely the restoration of abstinence-induced mood decrements. One aim of this study was to determine the net effects on mood of dietary caffeine compared with prolonged abstinence. In addition, the study aimed to determine whether caffeine restores mood degraded by a non-caffeine source, namely, sleep restriction. A double-blind placebo-controlled cross-over design was employed in which 48 male and female volunteers alternated weekly between ingesting placebo and caffeine (1.75 mg/kg) three times daily for 4 consecutive weeks, while being either rested or sleep restricted. Mood was assessed using a computerized version of the profile of mood states (POMS), giving scores for overall mood and six mood dimensions. Gender had small effects on mood, whereas all mood dimensions were markedly adversely affected by sleep restriction. Caffeine had no significant net enhancing effects on mood when participants were rested, and produced no net restorative effects when mood was degraded by sleep restriction. On the contrary, caffeine-induced decrements in mood were observed during both conditions of rest and sleep restriction.

Physiological and emotional response to psychological stressors in psychiatric and psychosomatic disorders

Deviations from emotional and physiological homeostasis in humans occur under episodic, chronic, or traumatic stress experiences. These dysregulations can provoke emotional and physical disturbances that may result in stress-related psychiatric or psychosomatic disorders. This brief review discusses a variety of both naturally occurring and experimental stressors and the complexity of intervening variables on cardiovascular and hormonal dysregulations of the hypothalamic-pituitary-adrenal (HPA) axis. In addition to the type, intensity, and duration of the stressor, subject variables such as dispositional factors (e.g., age and sex), personality traits, and coping styles are considered. Finally, emotional and physiological stress responses to naturally occurring stressors and psychological challenge tests in depressed patients and anxiety disorder patients are presented and discussed. Stressors in humans induce emotional arousal, which leads to physiological activation of the central nervous system, an increase of hormones of the HPA axis, and coping reactions on the psychological and behavioral level. Depending on the type, intensity, and duration of the stressor, on personal and psychosocial resources of the individual, stressful situations may provoke emotional and physical disturbances. These dysfunctions may result in psychosomatic or psychiatric disorders. The following brief review intends to discuss a variety of stressors that have been examined in humans in order to gain insight into the complex relationship between physiological and behavioral stress reactions and the occurrence or maintenance of stress related psychosomatic and psychiatric disorders.