Methods

Twenty healthy young adults (10 women), aged 19 to 29 (22.1 ± 0.5) years, were recruited and paid for their participation. The mean height of the participants was 173.3 ± 2.0 cm, their body weight was 65.7 ± 2.4 kg, and their body mass index was 21.8 ± 0.6 kg m ⁻² . None of the subjects had any diseases or were taking any medication affecting cardiovascular or autonomic regulation. Based on a questionnaire (daily intake of coffee, energy drinks, and other caffeinated beverages), the estimated daily caffeine intake of study participants ranged from 1 to 4 drinks per day. All participants fasted for ≥12 hours and they were requested to avoid alcohol or caffeine for at least 24 hours before the test. The local ethics committee approved the study, and written informed consent was obtained from each subject.

All experiments took place in a quiet, temperature-controlled (22°C) laboratory and started between 08.00 a.m. and 09.00 a.m. Every subject attended 2 separate experimental sessions (each session separated at least by 2 days) according to a randomized cross-over design. On arrival at the laboratory, subjects were asked to empty their bladders if necessary and to sit in a comfortable armchair. Equipment for cardiovascular and cerebrovascular recordings was then attached. After a variable period for attainment of cardiovascular stability (usually 30 minutes), a baseline recording was made for 20 minutes. Then, the test subjects ingested, not blinded, either 355 ml of a degassed Red Bull drink containing caffeine (114 mg), taurine (1420 mg), and glucuronolactone (84.2 mg), sucrose, and glucose (39.1 g) or 355 ml of tap water at room temperature. Subjects were asked to ingest their drink in a convenient pace for over 4 minutes. After 80 minutes of postdrink cardiovascular recordings, a mental arithmetic task was performed for 5 minutes followed by 5 minutes recovery period.

Subjects subtracted continuously the number 6 or 7 (chosen at random) from a random 3 or 2 digit number for 5 minutes and were instructed to give the answer verbally. Each mental stress task comprised of 60 unique calculations, with 5 seconds interval between each calculation and was presented to the subjects on a monitor. Immediately after the mental task, subjects were asked to rate their perceived stress using a standard 5-point Likert scale.

Cardiovascular and electrocardiographic (cardiac intervals) recordings were performed using a Task Force Monitor (CNSystems, Medizintechnik, Graz, Austria) with data sampled at a rate of 1,000 Hz as described previously. Cerebral blood flow velocity was measured using transcranial Doppler ultrasonography (Doppler-Box, DWL, Sipplingen, Germany). The left and the right middle cerebral artery was insonated at a depth of 40 to 55 mm using a 2 MHz probe, and the probe was fixed in place with an adjustable headset. Beat-to-beat values of systolic, diastolic, and mean velocity were recorded and merged real time with the Task Force Monitor to allow synchronous cardiovascular and cerebrovascular recordings.

Beat-to-beat values of cardiac interval, systolic blood pressure (BP), diastolic BP, cerebral blood flow velocity, and stroke volume were averaged over the 20 minutes predrink baseline period and minute by minute starting 5 minutes before (postdrink baseline), during (mental task for 5 minutes) and after the mental stress (recovery for 5 minutes). Heart rate was calculated by the appropriate RR interval. Cardiac output was computed as the product of stroke volume and heart rate. Mean BP was calculated from diastolic BP and systolic BP, respectively: mean BP = diastolic BP + 1/3 (systolic BP − diastolic BP). Total peripheral resistance was calculated as mean BP divided by cardiac output. Double product was calculated as heart rate × systolic BP and provides valuable information for the oxygen consumption of the myocardium. Cerebrovascular resistance index was calculated as the mean BP at brain level (BP _{mean_brain} ) divided by mean cerebral blood flow velocity. BP _{mean_brain} was calculated as the difference between mean BP at heart level and the hydrostatic pressure (BP _hydro ) at the level of transcranial insonation (temporal bone window).

Data are expressed as means ± SEM. Statistical analysis was performed by 2-way analysis of variance for repeated measures with time (6 time points: postdrink baseline, mental task 1 to 5 minutes) and treatment (water or Red Bull) as within-subject factors using statistical software (Statistix version 8.0; Analytical Software, St. Paul, Minnesota). The effects of each drink over time were analyzed by comparing values at each time point over the mental task and recovery period with the averaged postdrink baseline values recorded during the 5 minutes immediately before the mental task ( Figures 1 to 4 ) using repeated measures analysis of variance with Dunnett’s multiple comparison post hoc testing. Changes between postdrink baseline, mental task, and recovery were evaluated using repeated measures analysis of variance with Newman-Keuls post hoc testing ( Figures 1 to 4 ). A Wilcoxon matched pairs test was used to elicit differences in mistakes and stress perception in response to the drink (GraphPad Prism version 5, GraphPad Software, Inc, San Diego, California). All reported p values were 2-sided and the level of statistical significance was set as p <0.05.

Left panel : Time course for changes in SBP (A) and DBP (B) starting 75 minutes (i.e., −5 minutes on the panel) after ingestion of Red Bull ( full circle ) or tap water ( open rhomb ). Right panel : Mean responses averaged over 5 minutes PDBL, 5 minutes MT, and 5 minutes RE relative to predrink baseline values and presented as a delta (i.e., average over the respective 5 minutes interval minus the average over the 20 minutes predrink baseline period) after tap water or Red Bull. *p <0.05, **p <0.01, and ***p <0.005 statistically significant differences over time from predrink baseline values ( left and right panels ). §p <0.05 and #p <0.01, statistically significant differences between PDBL, MT, and RE ( right panel ). All values are reported as means ± SE. DBP = diastolic blood pressure; MT = mental task; PDBL = postdrink baseline; RE = recovery period; SBP = systolic blood pressure.

Left panel : Time course for changes in HR (A) and CBFV (B) starting 75 minutes (i.e., −5 minutes on the panel) after ingestion of Red Bull ( full circle ) or tap water ( open rhomb ). Right panel : Mean responses averaged over 5 minutes PDBL, 5 minutes MT, and 5 minutes RE relative to predrink baseline values and presented as a delta (i.e., average over the respective 5 minutes interval minus the average over the 20 minutes predrink baseline period) after tap water or Red Bull. *p <0.05, **p <0.01, and ***p <0.005 statistically significant differences over time from predrink baseline values ( left and right panels ). §p <0.05 and ‡p <0.005, statistically significant differences between PDBL, MT, and RE ( right panel ). All values are reported as means ± SE. CBFV = cerebral blood flow velocity; HR = heart rate; MT = mental task; PDBL = postdrink baseline; RE = recovery period.

Left panel : Time course for changes in SV (A) , CO (B) , and DP (C) starting 75 minutes (i.e., −5 minutes on the panel) after ingestion of Red Bull ( full circle ) or tap water ( open rhomb ). Right panel : Mean responses averaged over 5 minutes PDBL, 5 minutes MT, and 5 minutes RE relative to predrink baseline values and presented as a delta (i.e., average over the respective 5 minutes interval minus the average over the 20 minutes predrink baseline period) after tap water or Red Bull. *p <0.05, **p <0.01, and ***p <0.005 statistically significant differences over time from predrink baseline values ( left and right panels ). §p <0.05, #p <0.01, and ‡p <0.005, statistically significant differences between PDBL, MT, and RE ( right panel ). All values are reported as means ± SE. CO = cardiac output; DP = double product; MT = mental task; PDBL = postdrink baseline; RE = recovery period; SV = stroke volume.

Left panel : Time course for changes in TPR (A) and CVRI (B) starting 75 minutes (i.e., −5 minutes on the panel) after ingestion of Red Bull ( full circle ) or tap water ( open rhomb ). Right panel : Mean responses averaged over 5 minutes PDBL, 5 minutes MT, and 5 minutes RE relative to predrink baseline values and presented as a delta (i.e., average over the respective 5 minutes interval minus the average over the 20 minutes predrink baseline period) after tap water or Red Bull. *p <0.05, **p <0.01, and ***p <0.005 statistically significant differences over time from predrink baseline values ( left and right panels ). §p <0.05, #p <0.01, and ‡p <0.005, statistically significant differences between PDBL, MT, and RE ( right panel ). All values are reported as means ± SE. CVRI = cerebrovascular resistance index; MT = mental task; PDBL = postdrink baseline; RE = recovery period; TPR = total peripheral resistance.

Results

Predrink hemodynamic values were similar between the drinks, whereas Red Bull elevated significantly systolic and diastolic BP, heart rate, cardiac output, double product, cerebrovascular resistance, and decreased cerebral blood flow velocity over the postdrink period ( Table 1 and Supplementary Tables 1 and 2 ).

Table 1

Individual cardiovascular pre- and post-drink parameters measured in 20 (first 10 are males and second 10 are females) young and healthy human subjects using a randomized cross-over study design

Treatment	Water	Water	Red Bull	Red Bull	Water	Water	Red Bull	Red Bull	Water	Water	Red Bull	Red Bull
Condition	Pre-drink	Post-drink	Pre-drink	Post-drink	Pre-drink	Post-drink	Pre-drink	Post-drink	Pre-drink	Post-drink	Pre-drink	Post-drink
Case	SBP [mmHg]	SBP [mmHg]	SBP [mmHg]	SBP [mmHg]	DBP [mmHg]	DBP [mmHg]	DBP [mmHg]	DBP [mmHg]	HR [bpm]	HR [bpm]	HR [bpm]	HR [bpm]
1 (20 years)	130	130	120	130	80	82	70	77	62	61	64	67
2 (23 years)	115	122	108	119	74	77	69	78	63	66	67	72
3 (29 years)	129	127	122	132	85	81	80	90	70	68	68	70
4 (20 years)	113	117	113	121	76	77	78	84	58	58	65	72
5 (24 years)	133	145	146	155	79	81	81	79	67	62	63	59
6 (22 years)	114	115	110	129	73	75	71	79	56	74	65	77
7 (22 years)	122	119	116	120	85	87	82	86	95	76	81	86
8 (19 years)	143	134	133	137	86	82	80	81	69	70	64	82
9 (23 years)	117	119	121	122	74	76	81	79	58	65	58	67
10 (24 years)	101	116	111	115	69	80	71	76	58	60	62	66
11 (23 years)	115	119	120	135	76	75	78	85	53	53	59	66
12 (20 years)	104	106	99	105	66	70	63	64	63	53	55	58
13 (22 years)	114	115	111	113	68	72	71	72	50	65	47	59
14 (23 years)	113	117	124	135	74	76	80	88	61	62	66	68
15 (20 years)	116	124	117	128	79	80	81	93	69	78	71	89
16 (20 years)	112	115	107	111	67	70	66	68	60	61	56	61
17 (21 years)	105	95	109	114	65	59	69	72	73	65	69	71
18 (22 years)	112	106	120	125	77	75	87	86	64	66	68	69
19 (21 years)	120	108	116	116	76	64	67	70	64	55	68	77
20 (23 years)	111	110	120	120	74	72	86	84	80	72	74	89
Average	117	118	117	124*	75	76	76	80*	65	64	65	71 *
SEM	2	2	2	3	1	1	1	2	2	2	2	2

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