This study was a double-blind, randomized, placebo-controlled, crossover study. No formal ethical approval was required by the Medical Ethical Committee of the University Medical Center Utrecht. The study was conducted according to the ICH Guidelines for “Good Clinical Practice” and the Declaration of Helsinki and its latest amendments. Written informed consent was obtained from the participants before taking part in the study.
Twenty-four adult healthy volunteers (12 males and 12 females) were recruited by means of public advertisements at and around Utrecht University campus. Subjects were included if they were aged between 21 and 35 years, were regular drivers (>5,000 km/year), had been in the possession of a drivers license for at least 3 years, had a normal body mass index (21 < BMI < 30; 55–85 kg), were non-smokers, had regular sleeping hours, and were otherwise healthy. Sleep disturbances were assessed with the SLEEP-50 questionnaire (Spoormaker et al. 2005). The Epworth Sleepiness Scale (ESS) was administered to assess general levels of daytime sleepiness (Johns 1991). Subjects with ESS scores above 10 were excluded from participation. Other inclusion criteria were moderate caffeine consumption (two to four glasses of caffeine-containing beverages per day) and infrequent energy drink consumers (<1 drink per month). On each visit, urine samples were collected to test for drugs of abuse (amphetamines (including 3,4-methylenedioxymethamphetamine), barbiturates, cannabinoids, benzodiazepines, cocaine, and opiates) and a pregnancy test in female subjects (β-human chorionic gonadotropin test). In addition, alcohol use was tested using the Dräger Alcotest 7410 Breath Analyzer. Alcohol consumption was not permitted from 24 h before the start of the test day and on test days. From awakening until the end of the tests, caffeinated beverages and smoking were not allowed.
The study comprised one training day and three test days. On the training day, participants were screened and familiarized with the test procedures. If subjects met all inclusion and exclusion criteria, a practice session in the STISIM driving simulator was performed. Thereafter, subjects completed the Simulator Sickness Questionnaire (Kennedy et al. 1993) to determine possible simulator sickness. Included subjects were randomly assigned to a treatment order comprising three conditions: (1) Red Bull® Energy Drink + break, (2) placebo + break, and (3) no break + no treatment condition (see Fig. 1).
On test days, drug or alcohol use, pregnancy, illness, and medication use were checked after arrival. In addition, quality of sleep was assessed using the 14-item Groningen Sleep Quality Scale (Mulder-Hajonides van der Meulen et al. 1980), in which 0 indicates high quality sleep and 14 indicates very poor sleep. When subjects met all criteria, they performed a two times 60-min driving session in the STISIM driving simulator. On two test days, a 15-min break was scheduled in which subjects received either of the double-blind treatments. After the break, another driving session of two times 60 min was performed. In the “no break” condition, participants drove for a total of 4 h without a break. In each condition, every 60-min driving session was followed by subjective assessments on driving quality, driving style, mental effort to perform the test, and sleepiness. Test sessions were performed either in the morning (0800–1300 hours) or in the afternoon (1300–1700 hours) in a balanced manner. Each subject started each test day at the same time.
Treatments were 250 ml of Red Bull® Energy Drink or placebo, administered during the break. Per 250-ml Red Bull® Energy Drink contains 21 g sucrose, 5 g glucose, 1 g taurine, 80 mg caffeine, 60 mg glucuronolactone, 50 mg inositol, and B vitamins (niacin, pantothenic acid, vitamin B6, vitamin B12). The placebo drink was Red Bull® Energy Drink without taurine, caffeine, glucuronolactone, inositol, and vitamin B complex. The blinded Red Bull® Energy Drink and placebo beverage samples were provided by Red Bull GmbH. Treatment appearance (bottle and color of the beverage) was double-blinded, and a nose clip was worn to enhance treatment blinding. Drinks were consumed within 5 min, starting from 5 min after onset of the break. Subject randomization was conducted at Utrecht University. The treatment code was revealed by Red Bull GmbH after the study was completed and data were analyzed.
STISIM highway driving test
Driving tests were performed using STISIM Drive™ (version M300, Systems Technology, Inc., Hawthorne, CA, USA). The simulator consists of a car unit with adjustable car seats and a dashboard and includes a steering wheel, turn sign indicators, gear lever, clutch, brake, and gas pedals for vehicle control. The system generates a realistic roadway scenery which is projected on a screen (2.10 × 1.58 m), placed 1.90 m in front of the center of the steering wheel. Speed and gear number are displayed on the dashboard and the screen. Auditory feedback is provided by speakers and included sound of the engine, braking, speeding in curves, and driving off-road. Whenever a collision occurs, a broken windshield and the sound of braking glass are presented. Subsequently, the car is placed back in the middle of the right traffic lane and the test continues.
A 100-km highway driving test scenario was developed by EyeOctopus B.V., tailored to Dutch traffic situations (e.g., Dutch traffic signs, vehicles, buildings, and sceneries). The test scenario aims to resemble the on-the-road driving test in real traffic. The scenario consists of a two-lane highway in each direction with a lane width of 3.5 m. The environment is monotonous and comprised trees, occasional bridges, and hills as well as other traffic (see Fig. 2).
Subjects were instructed to drive with a steady lateral position in the right (slower) traffic lane while maintaining a steady speed of 95 km/h. Overtaking maneuvers were allowed whenever a subject approached a slower moving car. These events were removed from the data before analysis. Weaving of the car, expressed by the SDLP (centimeters), was the primary outcome measure of this test (see Fig. 3).
SDLP has been used as primary parameter in standardized on-the-road tests. SDLP showed to be sensitive to dose-dependent impairment after administration of a variety of psychoactive drugs including hypnotics, antidepressants, and antihistamines (Verster and Mets 2009). On-the-road, SDLP increments corresponding to the most common legal limits for driving were +2.4 cm (0.05%), +4.1 cm (0.08%), and +5.3 cm (0.10%; Louwerens et al. 1987). Recent research showed that the highway driving test scenario and its primary parameter SDLP in the STISIM driving simulator also differentiate between impairment produced after consumption of different dosages of alcohol (Mets et al., submitted for publication).
The second outcome measure was the standard deviation of speed (kilometers per hour). Mean lateral position (centimeters) and mean speed (kilometers per hour) were control variables.
After each hour of driving, participants made subjective assessments on driving and sleepiness causing a 2-min interruption of the driving task. Subjects indicated their perceived driving quality on a visual analog scale, ranging from 0 (“I drove exceptionally poorly”) to 20 (“I drove exceptionally well”). In addition, mental effort exerted during the driving test was rated on an interval scale (15 cm) ranging from “almost no effort” to “very great effort”. Furthermore, subjects completed a driving style questionnaire (McCormick et al. 1987), which consisted of different bipolar differential scales (10 cm) including foolish-wise, unpredictable–predictable, dangerous-safe, tense-relaxed, inconsiderate–considerate, and irresponsible–responsible. Finally, the KSS was used to rate sleepiness on a scale of 1 to 9, ranging from 1 (very alert) to 9 (very sleepy, an effort to stay awake, fighting sleep; Åkerstedt and Gillberg 1990). Completion of the subjective assessments took approximately 2 min. Driving was resumed right thereafter.
Data were analyzed using ANOVA general linear model for repeated measures (two-tailed, p ≤ 0.05). For the STISIM driving simulator test, the primary parameter was the SDLP.