Behavioral Effects of Caffeine - American Chemical Society

Chapter 4

Behavioral Effects of Caffeine Andrew Smith and Carolyn Brice

Downloaded by EAST CAROLINA UNIV on November 13, 2016 | http://pubs.acs.org Publication Date: January 15, 2000 | doi: 10.1021/bk-2000-0754.ch004

Health Psychology Research Unit, Department of Experimental Psychology, University of Bristol, Bristol BS8 1TN, United Kingdom The present paper will give a review of 10 years of our research on the effects of caffeine on cognition and mood. Research has demonstrated that caffeine has beneficial effects on performance of tasks requiring sustained attention and that these effects are readily observed in low alertness situations (after lunch; at night; when a person has a cold). Although many of the studies of caffeine have used very large single doses, recent studies have demonstrated that beneficial effects can be observed with more realistic doses and drinking regimes that equate to real-life intake. Furthermore, the improved performance can be demonstrated using simulations of real-life tasks and in the context of a normal working day.

A n Overview of the Published Literature Smith (/) has reviewed the published literature on caffeine and behavior. This review has been sub-divided into a number of different sections and the conclusions about each section are summarized below. The aim of the article was to review the effects of caffeine on the behavior of adult humans. The main areas covered were effects on mood, the efficiency of mental performance and sleep. In all areas it is important to make a distinction between effects of amounts of caffeine that are normally consumed from food and drinks and the very different effects observed with excessive amounts or in very sensitive individuals. Unlike other areas of research (e.g. studies of health effects) most studies of the behavioral effects of caffeine have examined acute changes following a single dose. Less is known about effects of regular consumption although there is now enough data on this topic to draw tentative conclusions. In addition to studying effects of caffeine consumption the research has also considered possible changes in behavior as a function of caffeine withdrawal. This issue is usually considered in the context of whether caffeine leads to dependence and this

30

© 2000 American Chemical Society

Parliment et al.; Caffeinated Beverages ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

31 topic is not reviewed here. Rather a critical appraisal of claims that caffeine withdrawal influences performance and mood is provided. Overall, the literature suggests that the following effects may occur when individuals consume moderate amounts of caffeine: 1. 2.


3.

4.

5. 6.

Caffeine increases alertness and reduces fatigue. This may be especially important in low arousal situations (e.g. working at night). Caffeine improves performance on vigilance tasks and simple tasks which require sustained response. Again, these effects are often clearest when alertness is reduced although there is evidence that benefits may still occur when the person is unimpaired. Effects on more complex tasks (e.g. memory tasks) are difficult to assess and probably involve interactions between the caffeine and other variables which increase alertness (e.g. personality and time of day). Tn contrast to the effects of caffeine consumption, withdrawal of caffeine has few effects on performance. There is often an increase in negative mood following withdrawal of caffeine but such effects may largely reflect the expectancies of the volunteers and the failure to conduct "blind" studies. Regular caffeine usage appears to be beneficial, with higher users having better menial functioning. Most people are very good at controlling their caffeine consumption to maximise the above positive effects. For example, the pattern of consumption over the day shows that caffeine is often consumed to increase alertness. Indeed, many people do not consume much caffeine later in the day as it is important not to be alert when you want to go to sleep.

In contrast to effects found with normal caffeine intake, there are studies which have demonstrated negative effects when very large amounts are given or sensitive groups (e.g. patients with anxiety disorders) studied. In this context caffeine has been shown to increase anxiety and impair sleep. There is also some evidence that fine motor control may be impaired as a function of the increase in anxiety. Overall, the global picture that emerges depends on whether one focuses on effects that are likely to be present when caffeine is consumed in moderation by the majority of the population or on the effects found in extreme conditions. The evidence clearly shows that levels of caffeine consumed by most people have largely positive effects on behavior. Like most things excessive consumption will lead to problems and there are also some individuals who are more sensitive than others.

Effects of Low Doses of Caffeine on Mood and Performance Many of the generalizations about the behavioral effects of caffeine are based on studies which lune used very large doses of caffeine, used students as subjects and tested them in the early morning only. It is clearly desirable to have further


32


information on the effects of the lower doses of caffeine which are more typically consumed, to examine effects in a representative sample of the population at several times of day and to consider the possible modifying effects of habitual caffeine consumption and individual differences. Smith et al. (2) carried out a study to examine the above issues and comparing 60 mg caffeine with placebo. The results showed that even this dose of caffeine led to improved mood and enhanced encoding of new information. Interactions between caffeine and other factors were observed although these were usually specific to particular tasks. In other words, the general effects of caffeine were not influenced by demographic factors, lime of testing, personality or habitual caffeine usage.

Importance of the Mode of Administering the Caffeine It is important to consider whether effects of caffeine are modified by the vehicle in which it is administered. Smith, Sturgess and Gallagher (3) examined this issue in a study with the following features. A n experiment was carried out to examine the effects of 40 mg of caffeine given in different drinks (coffee, water, tea, cola) on mood and performance. One hundred and forty four volunteers were randomly assigned to one of the groups formed by combining the caffeine/placebo and drinks conditions. Following a baseline session measuring mood and different aspects of performance the volunteers were given their drink and then carried out another test session one hour later. Administration of the caffeine/placebo was double blind. The results showed that those given caffeine reported greater alertness and anxiety at the end of the test session as well as improved performance on choice reaction time tasks involving focused attention and categoric search, a semantic memory task and a delayed recognition memory task. The effect of the caffeine was not modified by the nature of the drink in which it was given. Overall, these results show that a dose of caffeine typical of the level found in commercial products can improve alertness and performance efficiency.

Effects of Regular Consumption Levels About 25% of the participants in the above study consumed virtually no caffeine on a day to clay basis. These volunteers showed similar beneficial effects of caffeine to those who regularly consumed greater amounts. This finding has been replicated in another as yet unpublished study and the presence of beneficial effects of caffeine in very low/non-consumers argues against the beneficial effects of caffeine merely reflecting removal of the negative effects of caffeine withdrawal. These findings also support the view that regular levels of caffeine consumption have little effect on the caffeine-induced behavioral changes (4).


33

Caffeine and caffeine withdrawal


In the majority of studies of caffeine the volunteers have caffeine withdrawn (usually over night) prior to testing. Positive effects of caffeine could, therefore, reflect removal of impairments produced by withdrawal. However, several studies (4, 5, 6) have demonstrated that one obtains similar effects of caffeine when volunteers have had caffeine withdrawn for at least 8 hours and when they have been free to consume caffeine prior to the experiment. Such results cause problems for an explanation based solely on the effects of caffeine withdrawal.

Consumption Regime Most studies of the effects of caffeine have administered a single large dose, often equivalent to the person's total daily consumption level. Caffeine is usually ingested in a number of smaller doses and it is unclear whether effects observed after a single large dose are the same as those produced by an identical level produced by consuming several caffeine containing drinks over a longer time period. Brice and Smith (unpublished,) examined this issue and found that the improved mood and enhanced performance found after a single dose of 200 mg were also observed following 4 doses of 65 mg given at hourly intervals (which resulted in an identical final level to the single 200 mg dose).

Metabolism of Caffeine Most of the beneficial effects of caffeine show a linear dose-response relationship up to about 300 mg and this is then followed by cither a flattening of the curve or, sometimes, impaired performance at higher doses. If one looks at the relationship between metabolism of a fixed dose of caffeine (as indicated by saliva levels) and mood and performance changes one finds that there is no strong association between the two. This is not too surprising in that it is not caffeine levels in the periphery per se which produce the behavioral changes but secondary C N S mechanisms. The individual differences in the metabolism of the caffeine may be very different from the individual differences in the C N S mechanisms which, plausibly, accounts for the lack of a strong association between plasma (or saliva levels) and behavioral changes.

Effects of Caffeine on Real-life Performance The previous sections have shown that doses of caffeine typically consumed in real-life, and presented in commercial products, can improve mood and aspects of


34 performance. The majority of studies have been laboratory experiments using artificial tasks. It is now important to ask whether similar effects are observed i n simulations of real-life activities (e.g. driving).


Caffeine and Driving A number of studies (e.g. 8) show that caffeine can improve driving performance of fatigued drivers. In a recent as yet unpublished study caffeine was found to improve steering accuracy in drivers carrying out a one hour drive. Measures of mood and performance of artificial tasks were also taken during the last study and these also showed benefits of consuming caffeine. This suggests that changes in the laboratory may reflect a general benefit of caffeine that is also observed in real-life situations.

Changes Over the W o r k i n g Day Another method of assessing the effects of caffeine involves monitoring changes over the course of the working day. Indeed, i f performance is measured before starting work and then again at the end of the working day, then the difference between the two times reflects performance efficiency over the course of the day. Smith et al. (9) examined both the effects of controlled consumption and free choice of caffeinated drinks over the course of the day. The results showed that caffeine consumption was greatest in the morning and that similar diurnal trends were seen in free living and free choice conditions. Consumption of decaffeinated drinks led to a reduction in alertness over the day and also slowed response times. Indeed, regular monitoring of alertness showed that decaffeinated drinks were associated with reduced alertness from 10.00 to 19.00. In the free living condition high caffeine consumption was associated with faster reaction times in the evening. Furthermore, there was evidence to suggest that it was the subjects with lower levels of alertness who subsequently drank more caffeine. Overall, these results demonstrate that caffeine consumption maximizes alertness over the working day and is often consumed to produce this effect.

Mechanisms Underlying the Effects of Caffeine on Mood and Performance It is highly likely that many different C N S mechanisms underlie the effects of caffeine on behavior. Caffeine's major effect is as an antagonist of the adenosine receptors which in turn affects the release of a variety of neurotransmitters (e.g. noradrenaline, acetylcholine, dopamine and the GABA/benzodiazepine system). It is important to link specific behavioral changes with C N S mechanisms and also to



35 develop profiles of the stages of information processing involved in and the energetics underlying the effects of caffeine. Mechanisms can also be considered at the level of the cognitive processes and energetical mechanisms influenced by caffeine. Smith, Clark and Gallagher (10) conducted a study to elucidate the stages of processing underlying enhanced performance by caffeine of choice reaction time tasks. In addition, they attempted to identify the energetics of the mood and cardiovascular effects produced by caffeine. Ingestion of caffeine had no effect on initial mood but it did improve the encoding of new information and counteracted the fatigue that developed over the test session, resulting in greater subjective alertness at the end of the session in the caffeine condition. Another approach to understanding the C N S mechanisms underlying the effects of caffeine has been to combine pharmacological challenges with administration of caffeine. If has been demonstrated that caffeine is especially beneficial when alertness is reduced. Alertness can be reduced by changing a number of the neurotransmitter systems. For example, by using clonidine, a drug which reduces the turnover of central noradrenaline, it is possible to mimic sleep deprivation in a period of a few hours. Smith et al. (unpublished) conducted a study combining caffeine/placebo and clonidine/placebo conditions. Caffeine was found to reverse the effects of clonidine but produce few effects when the volunteers were alert. However, some effects of caffeine (e.g. the beneficial effect on encoding of new information; the cardiovascular effects) were not related to changes in the noradrenergic system. Indeed, Rusted and Smith (unpublished) have shown that the encoding of new information reflects the cholinergic system and there is evidence from other studies that caffeine also influences this neurotransmitter.

Conclusions The published literature on the effects of caffeine on behavior shows that it increases alertness, reduces fatigue and leads to improved performance of vigilance tasks and simple tasks requiring sustained response. Our research shows that these effects are most apparent when alertness is reduced by other factors (e.g. sleep deprivation, working at night, prolonged work, consumption of lunch and minor illness). Our recent research suggests that these effects can be obtained with realistic doses of caffeine (and normal consumption regimes) and that the performance improvements can be observed using simulations of real-life activities (e.g. driving) and by assessing changes over the working day. The positive effects of caffeine can be observed in very low/non-consumers which suggests that withdrawal docs not underlie the effects. This conclusion is supported by studies which have shown similar effects of caffeine in withdrawn and non-withdrawn volunteers. The mechanisms underlying these effects arc now being identified. Caffeine improves the encoding of new stimuli and this may reflect cholinergic changes. In addition, it



36 improves performance when central noradrenaline is reduced which may be the mechanism underlying the large effects of caffeine in low alertness situations. Effects on more complex tasks are less clear and probably depend on complex interactions between caffeine and other variables (e.g. personality and time of day). Indeed, the combined effects of caffeine and other variables is clearly an area which requires further study. Negative effects of caffeine have been found with extremely high levels of consumption and in certain sensitive individuals (e.g. patients with anxiety disorders). These effects must, however, be distinguished from the behavioral changes observed when caffeine is consumed in moderation by the majority of the population. Generally, consumption of caffeine is well controlled and the pattern of consumption suggests that individuals ingest caffeine to help restore levels of reduced alertness to a more optimum level. Similarly, consumption is reduced when high alertness is undesirable, such as when the person is trying to sleep.

Acknowledgements Research described in the article was support by M A F F / L I N K AFQ39, the Institute for Scientific Information on Coffee, and E S R C R O P A R022250090. Carolyn Brice was supported by an E S R C studentship.

References

1. Smith, A. P. (in press). Food Toxicology. 2. Smith, A. P.; Sturgess, W.; Gallagher, J.; Brice, C.; Collison, C.; Rich, N.; Hayward, R. MAFF/LINK report, 1997, AFQ 39. 3. Smith, A. P.; Sturgess, W.; Gallagher, J. (in press). Human Psychopharmacology. 4. Smith, A. P. In Caffeine and behavior: Current views and research trends. B. S. Gupta, U. Gupta (eds.) CRC Press. (1999), 161-178. 5. Warburton, D.M. Psychopharmacology.1995,119,66-70. 6. Smith, A.P.; Maben,A.; Brockman,P. Appetite, 1994, 22, 57-65. 7. Smith, A.P.; Rich, N.; Gallagher, J.; Turner, E. MAFF/LINK Report, 1997, AFQ 39. 8. Horne,J.A.; Reyner, L.A. Psychophysiology. 1996, 33, 306-309. 9. Smith, A. P.; Gallagher, J.; Rich, N.; Turner, E. MAFF/LINK Report, 1997, AFQ 39. 10. Smith, A. P.; Clark, R.; Gallagher, J. (in press). Physiology and Behavior.


Behavioral Effects of Caffeine - American Chemical Society

Recommend Documents