Caffeine Enhances the Physiological Response to Occupational ...

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closely to those reported by the Cooper Clinic (Blair, Goodyear, Gibbons, .... Thorsen, and the data collection assistance of Gary L. Edwards, Kathryn B.

HEALTH PSYCHOLOGY, 1987, 6(2), 101-112 Copyright © 1987, Lawrence Erlbaum Associates, Inc.

Caffeine Enhances

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Physiological Response to Stress i n M e d i c a l

the Occupational

Students

Gwendolyn A . Pincomb and William R. Lovallo Department o f Psychiatry University o f Oklahoma Health Sciences Center and Research Service Oklahoma City VA Medical Center

Richard B. Passey Department o f Pathology University o f Oklahoma Health Sciences Center

Daniel J . Brackett and Michael F. W i l s o n Department o f Medicine University o f Oklahoma Health Sciences Center and Research Service Oklahoma City VA Medical Center

Caffeine (3.3 mg/kg) was tested against a placebo in 20 male medical students during periods of low (no exams) versus high (final exams) work stress. On each of 8 test days, heart rate and blood pressure were measured at baseline and over a 40-min postdrug interval; immediately afterward, blood was drawn to test plasma Cortisol and serum lipid concentrations. Exams increased heart rate (p < .005) and systolic blood pressure (p < .02). Caffeine decreased heart rate (p < .0001) and increased systolic blood pressure (p < .005), diastolic blood pressure (p < .0001), plasma Cortisol levels (p < .01), and serum cholesterol levels (p < .02). Caffeine effects were additive with those of exams, and together they increased the number of men showing systolic blood pressures in the borderline hypertensive range. Thus, caffeine use during periods of increased occupational stress may enhance the cumulative stress response. Key words: caffeine, cardiovascular, Cortisol, lipids, occupational stress Requests for reprints should be sent to Gwendolyn A. Pincomb, Behavioral Sciences Laboratories (151 A), Veterans Administration Medical Center, 921 Northeast 13th Street, Oklahoma City, OK 73104. 101

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PINCOMB, LOVALLO, PASSEY, BRACKETT, WILSON

Caffeine is recognized as the most widely used drug in the Western world. A 1979 study estimated that 80% of the U.S. civilian population above the age of 20 years drank coffee, and although the majority of these individuals drank one to three cups a day, the mean for all coffee drinkers was 3.2 cups, or 272 mg of caffeine per day (Bonham & Leaverton, 1979). Personal characteristics and environmental factors influence coffee consumption. Heavy usage (more than five cups or 425 mg of caffeine a day) was found to be more prevalent in middle-aged adults, in men, and in employed individuals, particularly members of two occupational groups: managers/ administrators and craftsmen/kindred workers (Bonham & Leaverton, 1979). Increases in coffee consumption were also found to be systematically related to periods of heightened occupational demands among naval company commanders (Conway, Vickers, Ward, & Rahe, 1979). Given such ubiquitous use of caffeine by individuals who are exposed to substantial occupational stress, investigation of its cardiovascular and related effects is warranted, particularly during exposure to naturally occurring stressful events. The purpose of the present study is to examine (a) the cardiovascular, endocrine, and lipid responses to caffeine use in the work setting and (b) the influence of caffeine on responses to systematically repeated variations in occupational stress by testing medical students during final-exam weeks and during weeks with no exams. Responses to caffeine and occupational challenge potentially act via common organ systems, thereby introducing the possibility of cumulative (additive) and synergistic (interactive) relationships. Occupational challenge has been shown to increase heart rate, blood pressure, and concentrations of plasma free fatty acids, plasma Cortisol, total serum cholesterol, and low density lipoprotein cholesterol subfractions (Bogdonoff, Estes, Harlan, Trout, & Kershner, 1960; Frankenhaeuser, 1981; Grundy & Griffin, 1959; Sapira, Pittenger, Gerende, Small, & Cerul, 1965). Placebo-controlled studies have shown that caffeine decreased heart rate, increased blood pressure, and increased plasma levels of catecholamines and free fatty acids (Bellet, Kershbaum, & Aspe, 1965; Pincomb et al., 1985; Robertson et al., 1978). In combination with psychological stress, caffeine has been shown to produce an additive increase in systolic and diastolic pressure in male college students during performance of a challenging mental arithmetic task (Lane, 1983). In addition, caffeine administered in moderate amounts to male mice during exposure to population crowding and frequent confrontations in cramped tunnels to reach food and water produced additive increases in plasma renin, systolic blood pressure, adrenal weight, and levels of plasma corticosterone and blood urea nitrogen (Henry & Stephens, 1980). We tested medical students in a small classroom located near their customary work area while they were engaged in quiet study during periods of low (no exams) and high (final exams) work stress. The onset of final examina-

CAFFEINE AND OCCUPATIONAL STRESS

103

This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.

tions week provided a naturally occurring increment in work pressure when performance was being evaluated critically and the outcome could influence career progress. Preparation for these examinations was expected to result in longer working hours and decreased leisure time.

METHOD Healthy male volunteers were recruited from the first year medical school class. Demographic characteristics are shown in Table 1. Informed consent was obtained f r o m each recruit after the nature of the procedures had been fully explained. Screening ruled out those who reported infrequent caffeine use, caffeine intolerance, chronic health problems, or regular use of medications. The final pool included 20 men who agreed to abstain from food and fluids (10 hr to 12 hr), caffeine, alcohol and nicotine (12 hr), and other drugs (24 hr) prior to testing. The number of participants was determined by space and equipment limitations in the testing room. Female medical students were not included in order to avoid the possible confounding influences of hormonal fluctuations. Design specifications called for one caffeine and one placebo test to be held during the no-exam week and again during the final-exam week. This schedule was repeated over two semesters, for a total of eight sessions, to test the reproducibility of drug and exam effects. A double-blinded procedure, counterbalanced for drug order, was followed to administer premixed drinks containing 6 oz of unsweetened grapefruit juice plus either caffeine (3.3. mg/kg) sodium benzoate or sodium benzoate alone (placebo). A previous study in our laboratory confirmed that participants were unable to discriminate the placebo versus the caffeinated beverages (Pincomb et al., 1985). Daily procedure required that participants report to the testing room (10 students/day) between 6:30 a.m. and 7:00 a.m. After being seated at work taTABLE1 Descriptive Data for Students Showing Ranges, Means, and Standard Errors Variable

Unit

Range

M

SEM

Age

year

Height Weight Alcohol use Caffeine use Smokinga

cm kg

19 to 31 175 to 196 66 to 95 OtolO 50 to 1,582

23.0 182.9 78.8 3.4 300.0

0.5 1.3 2.0 0.7 77.0

drink/week mg/day

Note, n = 20. aOnly two students smoked regularly (average use==20 cigarettes/day).

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PINCOMB, LOVALLO, PASSEY, BRACKETT, WILSON

bles, they were asked to complete a battery of questionnaires and spend the remainder of the session quietly studying course work. The questionnaires included statements concerning general health, compliance with dietary restrictions, caffeine use from all dietary and medicinal sources, perceived stress, and dysphoric moods (anxiety, depression, and hostility) using the Multiple Affect Adjective Checklist (Zuckerman & Lubin, 1965). Two participants were assigned to one of five experimenters, each of whom operated a Critikon automated blood pressure monitor. Following an initial 20-min acclimation period, a baseline average of three readings for each cardiovascular variable (heart rate, systolic blood pressure, and diastolic blood pressure) was obtained. The participants were then asked to consume the caffeinated or placebo drink in 3 min or less. Subsequently, averages of three readings for each cardiovascular variable were obtained at postdrink intervals of 10,20, 30, and 40 min. Immediately following the final cardiovascular measurements, blood was drawn to test serum lipid levels and plasma caffeine, Cortisol, and free fatty acid concentrations.

RESULTS All 20 students completed testing for the first semester. However, 2 men terminated testing during the second semester, and data for these men were retained in analyses based on only the first semester or based on the combined averages from both semesters.1 Plasma samples obtained at 40 min postdrink during the second semester were tested for caffeine concentrations to confirm compliance with dietary restrictions and to confirm that sufficient caffeine was absorbed on drug days to detect its possible influence on dependent variables. Mean caffeine levels—plus or minus standard error of the mean (SEM) — were minimal on placebo days (0.6 mcg/ml ± 0.2 mcg/ml) and higher on drug days (4.4 mcg/ml ± 1.2 mcg/ml). Changes in psychological variables were analyzed using scores derived from self-report questionnaires, and these were tested in a series of analyses of variance with repeated measures during no exams and final exams. Significant increases during exams were seen in mean scores for perceived stress and for the measures of dysphoric moods: anxiety depression and hostility, all Fs(l,19) >13.7,/?s < .005. As shown in Figure 1, caffeine independently produced progressive eleva1A subsequent review of data from the two students who completed only the first semester revealed that neither of them showed a response to caffeine or to exams. Thus, if any bias existed from their inclusion in the analyses of covariance given here, the bias would most likely have been in the conservative direction and have resulted in a slight underestimate of the drug and exam effects that would have been obtained with only 18 students.

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CAFFEINE AND OCCUPATIONAL STRESS

105

DRUG D < .0001 FIGURE 1 Effect of caffeine on blood pressure and heart rate shown as the percentage differences from placebo values at postdrink intervals of 0, 10, 20, 30, and 40 min. Vertical bars denote SEM.

x si

MINUTES POSTDRINK

tions in systolic blood pressure, F ( l , 18) = 11.96, p < .005, and diastolic blood pressure, F ( l , 18) = 53.10, p < .0001, and a steady depression of heart rate, F ( l , 18) = 27.97, p < .0001.2 The presence of exam stress independently produced increases in systolic blood pressure, F ( l , 19) = 6.94, p < .02, and heart rate, F ( l , 19) = 14.46,/? < .005, as shown in Figure 2. Diastolic blood pressure was not altered by exams, F ( l , 19) = 0.38, p > .50. Biochemical variables, determined from blood drawn at 40 min postdrink, were tested in a series o f analyses o f variance using combined averages over both semesters with repeated measures for 2 Exam Conditions x 2 Drug Conditions. There were no interactions seen between drug and exam conditions. Table 2 shows that caffeine independently increased plasma Cortisol levels and produced a small rise in serum total cholesterol levels. Caffeine did not significantly alter concentrations of plasma free fatty acids, serum total 2There were no baseline differences in any of the cardiovascular variables on caffeine versus placebo days for either semester, nor were there any main effects for drug order. Therefore, the main effects due to drug and exam conditions and their interactions were tested in a series of analyses of covariance using combined averages from both semesters with repeated measures for 2 Exam Conditions x 2 Drug Conditions x 4 Postdrink Intervals. Baseline averages obtained during no exams were used throughout the analyses as covariates to control for the influence of initial values. No significant interactions were found between drug and exam conditions.

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PINCOMB, LOVALLO, PASSEY, BRACKETT, WILSON

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EXAMS p

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