Caffeine represents the most widely consumed psychoactive substance in the world, so understanding how this chemical affects an individual's physiology is essential to providing the best healthcare advice for the general public. Towards this goal, the response times of college students were studied before and after ingestion of water, Red Bull, or coffee. The task involved clicking a mouse button as fast as possible in response to a computer monitor screen changing color. Compared to water, response times improved by almost 6 and 13 seconds for Red Bull and coffee, respectively. Based on published information, which suggests the Red Bull and coffee ingestion would provide approximately 80 and 122 mg of caffeine, respectively, these results indicate a dose-dependent improvement in task performance as the caffeine dosage increased. Although between subjects variability was high, these results are remarkably consistent with previous findings. Basing dosage on subject weight may reduce the between subjects variability and is recommended for future studies.
Low doses of caffeine in non-caffeine users will produce a sensation of euphoria, increase alertness, and improve cognition, but at high doses nausea, anxiety, and trembling are not uncommon (Yang, Palmer, & de Wit, 2010). For example, low to moderate doses of caffeine injected into rats exposed to stress-inducing noise were protective against increased production of stress hormones, but high doses of caffeine did not (Patz, Day, Burow, & Campeau, 2006). Long-term, chronic consumption can lead to dependence, withdrawal, and an increased risk of cardiovascular disease, but the upside of chronic consumption is protection against neurodegenerative disease (Yang et al., 2010). Another advantage of caffeine consumption was revealed when 40 college aged women, habitually consuming less than 90 mg or more than 750 mg per day, with a mean weight of 58.2 kg, were given 2.5 or 5.0 mg per kg of caffeine (Jacobson & Thurman-Lacey, 1992). Only the low caffeine users suffered significantly in terms of hand dexterity and steadiness after acute caffeine ingestion. Despite the many benefits and hazards of consuming caffeine, it is the immediate sensation of alertness and improved cognitive performance that makes caffeine the most widely used psychoactive substance on the planet.
Ingested caffeine is rapidly absorbed through the gastrointestinal tract in a dosage-dependent manner and then metabolized by cytochrome P-450 in a rate-limiting manner (Yang et al., 2010). The immediate downstream demethylation metabolites of caffeine are paraxanthine, theobromine, and theophylline. Peak blood concentrations of caffeine are obtained within 1 to 2 hours after ingestion of 250 or 500 mg of anhydrous caffeine (Bruce, Scott, Lader, & Marks, 1986). The half-life for plasma caffeine is about 5 hours; however, peak plasma levels can occur in as little as 15 minutes (Brunye, Mahoney, Leiberman, & Taylor, 2010). Due to genetic variation in the CYP1A2 gene, which produces P-450, the metabolic clearance of caffeine from the blood can vary up to 40-fold between individuals (Yang et al., 2010). Within-individual variation depends on habitual caffeine use, smoking status, and drug use. The pharmacologic activity of caffeine depends on competitive binding to the A1 and A2A adenosine receptors. The dopaminergic system is the primary neurotransmitter system affected during acute ingestion, while chronic ingestion of caffeine will induce changes in the density of A1, muscarinic, nicotinic, and GABA receptors in the brain. Caffeine acts to block A1 and A2A receptor activity, thereby reducing dopaminergic inhibition of the motor system and improving psychomotor performance.
Empirical support for enhanced cognitive and motor activity has come from visual response tasks. For example, Kenemans and Lorist (1995) tested the visual response time for accepting or rejecting visual cues by pressing a button with either the left or right hand and discovered that caffeine increased reaction times from 404.6 to 382.9 ms (p < .0001). Electroencephalography (EEG) recordings attributed the improved performance to better selectivity of visual information processing, discrimination of visual stimuli, and motor processing. Similar findings were obtained when subjects were asked to respond to specific colors while EEG recordings were made (Ruijter, De Ruiter, & Snel, 2000). The authors of this study concluded that subjects who ingested caffeine experienced higher arousal levels, better processing of attended and unattended information, and improved motor responses. When low-caffeine users (< 42.5 mg/day) were given a placebo or 100, 200, or 400 mg of caffeine, caffeine significantly improved visual alerting and executive control networks, but slightly diminished orienting network performance (Brunye et al., 2010). The most improvement was observed for the first two traits at 200 mg caffeine.
Based on the research findings reviewed above, the quickness of a motor response to visual stimuli should be enhanced by acute caffeine consumption. Accordingly, the null hypothesis for this study will be that acute caffeine ingestion will have no effect on task performance. To test this hypothesis, a simple experiment was conducted that tested visual-motor response times to a computer screen changing color.
Methods and Materials
Study Design -- A quasi-experimental study design, without randomization of subjects or treatment group, was used for this study. The independent variable was the ingestion of a beverage between the pretest and posttest performance test. The independent variable consisted of a choice between 7.57 oz. water, 6.74 oz. Of coffee, or 8.4 oz. Of Red Bull. The predicted dosage of caffeine would be 0 for water, between 80 and 170 for coffee (Mayo Clinic Staff, 2011), and 80 mg for Red Bull (Red Bull GmbH, n.d.), although drip coffee would average somewhere around 122 mg for a 6.74 oz. serving (Caffeineinformer, 2014). The dependent variable was how vast a subject could click a mouse button in response to a visual stimulus.
Subjects -- Seven undergraduate students were recruited to take part in the study. All participants were required to provide informed consent before being allowed to participate. The study design was approved by the Institutional Review Board for human studies before the study commenced.
General Procedure -- Subjects were asked to sign on to the website www.humanbenchmark.com three times per week for three weeks. Once seated and attending to the screen, subjects would click the mouse button when the screen turned green. After ten repetitions of this task the website generated a mean response time in milliseconds. Subjects then drank water, coffee, or a can of Red Bull, waited 15 minutes, and then performed 10 more repetitions of the task. The total amount of time to complete this task was less than 30 minutes, so task fatigue should not have been a factor.
The seven subjects were divided into three groups, with one group containing three subjects. Each week of the trial the three groups would consume water, coffee, or Red Bull once each week, on separate days. Within a single day of the trial, each member of a group would consume the same beverage; however, a group would never drink the same beverage as the other two groups. This strategy resulted in each group ingesting all three beverages each week on three different days.
Data Analysis -- The mean response times for pretest and posttest were recorded and compared to determine the difference. The data was reported in terms of the response time difference (ms) and the standard error.
A comparison between the three independent variables, water, coffee, and Red Bull, revealed response times predicted the amount of caffeine consumed (Table 1; Fig. 1). The control condition of water ingestion produced close to a 3 ms improvement in response time, so subtracting this from the other two results produce an overall improvement in performance of almost 6.0 and 13 ms for Red Bull and coffee, respectively. The results between coffee and Red Bull may not be significant; however, the difference between coffee and water appears to be.
Magnitude of response difference between pretest and posttest trials.
Reflex Difference (ms)
Standard Error (ms)
Figure 1. Comparison of the means for the response times before and after ingestion of the respective beverage. Error bars represent standard error.
When the between subjects variability is examined (Table 2), it is remarkable that the results depicted in Table 1 and Figure 1 were obtained. Although some of the data appears to represent outliers, these were not eliminated from the data set before analysis. For example, water ingestion was associated with 30.7 ms decrease in response times for subject C, while subject D. experienced a 14.0 ms increase. Despite the large variability, the response times improved in a dose-dependent manner.