ABSTRACT
Introduction 115
Caffeine Acts by Blocking Adenosine Receptors 115
Effects of Caffeine on the HPA Axis during Rest 116
Caffeine and Catecholamines 117
Caffeine’s Cardiovascular Effects during Rest 118
Caffeine’s Effects on Blood Pressure 119
Caffeine’s Effects on the Heart 120
Caffeine and HPA Responses to Stress 120
Caffeine’s Effects on Cardiovascular Responses during Stress 121
Mental and Cognitive Stressors 121
Exercise, Psychomotor Stress, and Cold Pressor Challenge 122
Workplace and Related Settings outside the Laboratory 123
Individual Differences in the Effects of Caffeine 123
Other Individual Differences 125
Concluding Remarks and Future Directions 125
References 126
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INTRODUCTION
Caffeine is one of the world’s most widely used drugs. Surveys in the U.S. have shown that approximately 80% of adults regularly consume caffeine, averaging two to three cups of coffee a day (Bonham and Leaverton, 1979). Caffeine activates the central nervous system (CNS) (Rall, 1980; Nehlig et al., 1992), leading to behavioral, autonomic, and endocrine responses. Caffeine’s effects on peripheral functions are widespread and are mediated by direct tissue effects along with hormonal and autonomic outputs. Caffeine increases circulating catecholamines and free fatty acids (Robertson et al., 1978; Pincomb et al., 1988). It also increases blood pressure (BP), both at rest and during behavioral stress, and in the lab and the workplace (Lane and Williams, 1985; Sung et al., 1990; James, 1993). The combined BP effect of caffeine plus stress is usually additive (Lane and Williams, 1985; Lovallo et al., 1991; Shepard et al., 2000). Limited evidence from animal models suggests that caffeine may have pathogenic effects when combined with stress (Henry and Stephens, 1980). Such evidence in humans is lacking, although evidence suggests that persons at risk for hypertension may have greater cardiovascular responses and stress endocrine changes when exposed to stress following ingestion of caffeine, both in the lab and in daily life (Shepard et al., 2000). Caffeine interacts with CNS functions, it elevates stress hormone secretion, and it has direct effects on the heart and blood vessels. These points of action suggest that caffeine may be capable of influencing stress responses. In this chapter, we review review evidence of caffeine’s effects on adrenal and sympathetic functions during rest and in response to acute stressful challenges.
