Available online at:
https://cc.arcabc.ca/islandora/object/cc%3Apsycjournal

J Camosun Psyc Res. (2020). Vol. 2, pp. 7-16.
Submitted Fall 2019; accepted June 2020.

Does the method of consuming caffeine influence its
effects?
Authors: Kate Johnson*, Jayda Duthie, and Gabriel Mara
Supervising Instructor: Michael Pollock, Psyc 245 (“Drugs & Behavior”)
Department of Psychology, Camosun College, 3100 Foul Bay Road, Victoria, BC, Canada V8P 5J2
*Corresponding author email: johnsonandkate@gmail.com

ABSTRACT
Caffeine is an integral part of the daily lives of many undergraduate students and is often used
to increase cognitive wakefulness and as a tool to help the learner maintain their focus
throughout a long day of studying. The researchers sought to understand the objective
physiological and cognitive effects of the caffeine found in coffee, where they specifically
focused on heart and respiratory rates, cognitive wakefulness, and the possible moderating
influence of temperature and milk. Methodology: Both the correlational and the experimental
study utilized a longitudinal within subject design, and the experimental study utilized a doubleblind procedure. The correlational study showed the following results: (1) Heart rate and caffeine
consumption had statistically significant results in the pooled raw data but insignificant in the
pooled standardized data. (2) Respiratory rate and caffeine consumption had insignificant results.
(3) Milk percentage and cognitive wakefulness had insignificant results. (4) Temperature of
caffeine and cognitive wakefulness had insignificant results. Caffeine consumption and cognitive
wakefulness showed statistically significant results in the standardized pooled data. The
experimental design utilized a double-blind procedure to test if caffeine influences heart rate.
However, it showed statistically insignificant results. This research study provides a solid
foundation for other likeminded students to expand upon.
1. Introduction
1.1 Research Problem
Undergraduate students frequently utilize
caffeinated products for a multitude of
reasons. Caffeine is primarily used to
increase cognitive arousal and to assist the
individual in maintaining their focus.
According to a study by Mahoney et al.
(2019), among a sample of 1200 university
students 92% reported using caffeinated
products at least once in the past year, which
exemplifies caffeine’s popularity. Due to

caffeine’s prevalent use, the researchers
wanted to discern whether the stimulating
effect of caffeine is merely a placebo effect
or if there is merit to the continued use of
caffeine. Furthermore, if caffeine
consumption does have a significant effect,
the researchers wanted to discover what
method of consuming coffee acts the
quickest and provides the most effective
delivery system for caffeine. Should one
choose hot coffee or cold brew? Does
adding milk into one’s coffee lessen its
stimulating impact? Will drinking one’s
beverage quicker result in a faster onset of
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caffeine’s stimulating effects? As caffeine is
one of the most widely used drugs on the
legal market, these questions are important
ones that hold relevance to most who enjoy
it in any of its varied forms.
1.2 Literature Review
Contrary to public perception, consuming
a caffeinated beverage does not cause
statistically significant changes in vital
signs. Brothers et.al (2016) investigated the
effect of caffeine from both coffee and
energy drinks on heart rate, blood pressure
and QT interval (electrical activity of the
heart) in a population of fifteen
normotensive (normal blood pressure), nonsmoking young adults. Their study was
comprised of two components: During
component 1, participants were given
strictly controlled caffeine quantities that
were titrated to the individual based on body
weight. For component 2, caffeine was
administered in quantities that an individual
would normally consume (e.g., one serving
of coffee, or one can of energy drink.).
Brothers et. al, recognized that most people
will not carefully titrate the amount of
caffeine they consume to their body weight.
Instead, people are more likely to measure
their caffeine usage based on the serving
size of the beverage. Therefore, component
2 examined the effects of caffeine being
consumed in a practical dose. In this case,
the participants were given one serving of
coffee, and another set of participants were
given a whole can of energy drink to
consume. For both the caffeine-controlled
study and the normal use study, the results
did not find that caffeine had any
measurable effect on the heart rate, blood
pressure or the QT intervals.
Also contrary to what would be expected
based on pharmacokinetics, neither the
temperature of the beverage, the type of

caffeinated beverage, nor the length of time
it takes to consume it has any effect on
caffeine concentration in the blood plasma.
According to White et al. (2016), there is
actually little to no variation in the peak
concentration of caffeine in the participants’
blood plasma after consuming hot coffee,
cold coffee or chilled energy drink.
Furthermore, the peak effects of caffeine
manifest within 50 minutes postconsumption regardless of which of these
drinks were used. Their study contained 24
participants (n=12 men, n=12 women) and
measurements of caffeine plasma
concentrations were extracted from blood
samples taken prior to drink administration
and then at intervals of five minutes until
480 minutes post-administration. The drinks
taken included hot coffee, cold coffee and
sugar-free energy drink. These drinks were
consumed over both 20-minute periods and
2-minute periods, with the exclusion of hot
coffee, which was taken only over a 20minute period due to safety concerns.
However, although there were no significant
differences in caffeine area under the curve
from time zero to infinity, cold coffee
consumed in a 2-minute period exhibited the
highest caffeine plasma concentrations in the
participants caffeine concentration-time
profile.
There is a possibility that other factors,
such as the amount of milk added to the
caffeinated beverage, moderate the
immediate physiological effects of caffeine.
Quinlan, Lane, & Aspinall (1997) examined
the effect of adding milk into hot beverages,
which included tea, coffee and hot water
(which had 100mg/400ml caffeine added, so
the caffeine concentration was equivalent to
the coffee and tea). The dependent variables
measured were skin conductance and
temperature; heart rate; blood pressure;
salivary cortisol concentration subjective
anxiety levels and overall mood. The sample
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group consisted 16 individuals (n=8 men,
n=8 women), who had a median age of 35.5
at the time of this study. The participants
were non caffeine-naïve, who habitually
consumed their coffee with milk.
Furthermore, none of the participants
smoked and were otherwise healthy
individuals. When drinking coffee, the
participants showed acute spikes in their
heart rate and skin conductance. The
participants who consumed coffee with milk
added, had a heart rate spike of 9.4 bpm 10
minutes post consumption. When the
beverages without the presence of milk were
consumed, participants showed a spike of
11.7 bpm from their baseline heart rate. The
participants skin conductance levels had a
similar spike. Notably, the caffeine levels
found in the saliva of the participants
remained consistent regardless of the
presence of milk which indicates the
bioavailability of caffeine is not altered by
the presence of milk. However, the
researchers hypothesized that milk acts as a
mediating influence on the subjective
experience of drinking coffee, and the acute
physiological effects may be explained by a
sensory reaction to coffee’s natural
bitterness, although more research would be
required to substantiate that claim.
1.3 Hypotheses
During the literature review, it was noted
that the aforementioned studies did not
adequately control for the placebo effect,
which could have been a confounding
variable influencing the results of the
participants. Furthermore, drinking their
caffeinated beverages in a foreign
environment may have increased anxiety
levels in the participants, which could
potentially have influenced the outcome of
their trials. This led the researchers of this
paper to create the following five hypotheses

for testing in a longitudinal correlational
study and a follow-up double-blind
experimental study.
Main effects of caffeine:
Hypothesis #1: If caffeine intake
increases, then heart rate will be unchanged.
Hypothesis #2: If caffeine intake
increases, then respiratory rate will be
unchanged.
Hypothesis #3: If caffeine intake
increases, then cognitive wakefulness will
be unchanged.
Moderators of caffeine effects:
Hypothesis #4: If the temperature of
coffee decreases, then cognitive wakefulness
will be unchanged.
Hypothesis #5: If the quantity of milk in a
caffeinated beverage increases, then
cognitive wakefulness will decrease.
2. Methods
2.1 Participants
Three participants were tested in these
studies, with ages ranging from 19 to 32
years old and with an average age of 24.5
years (n=1 men, n=2 women) The
participants were all students in Psychology
245 at Camosun College and grouped
together due to a mutual interest in the
effects of caffeine. All the participants were
regular caffeine users and the caffeine
amounts that the participants consumed fell
within their normal levels.
2.2 Correlational Study Methods
2.2.1 Materials
Each participant measured the
temperature of their coffee with an
Accutemp Compact Folding Thermometer
immediately prior to consumption. The
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participants used a French Press style coffee
maker with 400ml of 60.0 C water. They
documented cognitive wakefulness by using
the Simple Reaction Time box target test
from the “Psych Lab 101” mobile phone app
(Neurobehavioral Systems, Inc).
2.2.2 Procedure
Self-reported data was collected by each
participant on 12 consecutive days. Upon
first waking up, the participants recorded
their baseline heart and respiratory rates.
Once their physiological data was recorded,
the participants took the Simple Reaction
Time test (SRT) as a measure of their
cognitive arousal. The participants agreed to
record the first set of data between the hours
of 7:00 AM and 10:00 AM, prior to
consuming any caffeinated products.
First, the participants measured their
Heart Rate (HR) by manually counting their
radial pulse for 30-sec. They calculated their
Beats Per Minute by multiplying their
findings by 2. Second, they recorded their
Respiratory Rate (RR) by counting the
number of breaths they took for 30-sec and
then multiplying this number by two to get
the rate of breaths per minute. Third,
participants completed a Simple Reaction
Time test (SRT) on their mobile device to
measure cognitive arousal. All three of the
data points were recorded into their personal
journal which accompanied them throughout
the trial.
Upon completion of the SRT, the
participant prepared their morning coffee.
Prior to consumption, the participant
recorded the quantity of caffeine in each
serving of their coffee, the temperature of
the beverage, and the quantity of milk
added. All of this was added to their daily
journal. Once the coffee was prepared, the
participants had 30-min to consume their
beverage, and after one hour, participants
collected their post-consumption HR, HR

and SRT scores. This sequence was repeated
any time a participant drank coffee on a
given day. At the end of the day, the
participants calculated the change in their
HR, RR and SRT scores to find the
difference between baseline and postconsumption levels.
2.3 Experimental Study Methods
2.3.1 Materials
The researchers purchased two large bags
of Alex Campbell’s Arabica bean from
Thrifty Foods. Bag 1 contained caffeinated
coffee grounds and bag 2 contained
decaffeinated coffee grounds.
Bags 1&2 were subdivided into 36
individual clear plastic bags, which acted as
portion-controlled servings. Each serving
consisted of 15g (3tsp) of dry ground coffee.
Each serving of caffeinated coffee grounds
contained approximately 180 mg of caffeine,
and the decaffeinated coffee contained
approximately 21 mg of caffeine per
serving. Every day each participant used
one serving of coffee and 400 ml of boiling
water in a French Press, which they allowed
to sit for 2 minutes before compressing and
pouring into a cup for consumption.
2.3.2 Procedure
Six bags were comprised of caffeinated
coffee grounds and six bags were comprised
of decaffeinated coffee grounds. Each bag
was labeled 1-12 by another team member
and given to another participant. The
numbers associated with the caffeinated or
decaffeinated coffee were selected at
random, without a clear pattern to avoid the
participants inadvertently guessing which
control variable they were being exposed to.
Each group member privately noted on a
notepad which bags they filled with
caffeinated coffee and which bags they filled
with decaffeinated coffee prior to giving
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their labeled bags to another group member.
Each group member noted who received the
bags of coffee they filled. Pre-measuring
coffee grounds controlled for possible
placebo effects, as the participants were not
aware of whether they were consuming
caffeinated or decaffeinated coffee on a
given day.
Following this, the individuals took home
their premeasured bags of coffee and used
those bags to make their coffee every
morning. They used 350ml of 60.0 C water
in a French Press or drip style coffee
machine, and the individuals drank their
coffee with no milk, sugar, or other additives
to control for possible effects of the other
additives. Furthermore, the coffee was
consumed in the morning between the hours
of 7:00 AM to 10:00 AM, which controlled
for the individual expectations of increased
wakefulness later in the day. During the
Experimental study, the heart rate,
respiratory rate, and cognitive wakefulness
were all measured immediately prior to the
consumption of caffeine, and directly after
the coffee was consumed. The Correlational
study likewise studied these effects and the
same format was utilized to measure the
change of psychological and cognitive
functions.
The experimental design continued with
the same procedure from the correlational
method. Prior to drinking coffee in the
morning, each participant measured their
HR, RR, and conducted the SRT test. After
the participant finished their coffee, they
waited one hour before completing the same
procedure. The participant pool for both the
Correlational and Experimental studies
entirely consisted of the three researchers,
who all gave informed consent prior to
conducting the study on themselves.
Furthermore, on conclusion of the study, the
participants conducted an informal
debriefing with one another, and discussed

the results and subjective experiences of
each participant throughout the experiment.
3. Results
3.1 Correlational Study Results
3.1.1 Main Effects of Caffeine
Using raw data pooled across participants
we found a statistically significant
correlation (r = 0.61) between caffeine
intake and heart rate (see Figure 1), although
this relationship was not statistically
significant when using data standardized for
each participant (see Table 1). No
statistically significant relationship was
found between caffeine and respiratory rate,
although unfortunately we were only able to
test this hypothesis with one participant.
However, we did find a statistically
significant correlation between caffeine
intake and cognitive wakefulness using
pooled data across participants in either its
raw (r = 0.48) or standardized (r = 0.43)
form.
3.1.2 Moderators of Caffeine Effects
Using the pooled raw data there was a
statistically significant correlation between
the temperature of coffee and the level of
cognitive wakefulness seen (r = 0.45),
although no statistically significant
correlation was seen when using data
standardized for each participant (r = 0.21).
Using either the pooled raw or standardized
data, there was no statistically significant
correlation was found between the quantity
of milk in the caffeinated beverages
consumed and the level of cognitive
wakefulness seen.
3.2 Experimental Study Results
The results arising from the experimental
manipulation of caffeine amount consumed
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showed a statistically insignificant change in
HR (p = 0.138; see Figure 2).
4. Discussion

4.1 Hypothesis #1 - As caffeine intake
increases, the heart rate will remain the
same.

The main purpose of this study was to
discover the physiological effects of
drinking caffeinated beverages.
Furthermore, the researchers sought to
understand any moderating influences of
caffeine, including additives and
temperature, to discover the most effective
method of caffeine consumption. The data
indicates that there is a mild positive
correlation between caffeine intake and
increased heart rate, respiratory rate and
cognitive wakefulness. The data from the
experimental portion of the study was not
statistically significant.

During the correlational study, there was
a statistically significant positive correlation
in the raw pooled data (r = -0.51). One of
the participants showed a mild negative
correlation between heart rate and caffeine
consumption, which differed from the
positive correlation shown by the other two
participants. The standardized data was not
statistically significant and represented a
very low correlation (r = 0.09). These
findings are perplexing and have several
possible explanations: there may have been
collection problems for the HR data, and it is
possible the data were not synthesized
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properly. It is also possible that the HR data
should have been collected at multiple
points after caffeine consumption. Quinlan
et.al ( 1997) indicated that there was a minor
increase in HR almost immediately postconsumption, which rapidly disappeared.
Therefore, it is possible collecting the HR
data one-hour post consumption could have
missed the acute spike in HR. In the doubleblind experimental study there was not a
significant change in HR after coffee was
consumed. The methods for physiological
data collection in the experimental study
were identical to those in the correlational
study and, for the reasons listed above, it is
likely the researchers missed the acute spike
in HR, if one occurred at all.
4.2 Hypothesis #2 - As caffeine intake
increases, the respiratory rate will remain
the same.
Although the results were not statistically
significant, there was a minor correlation (r
= 0.51) exhibiting a relationship between
caffeine consumption and increased
respiratory rate. However, the sample size
was only a single individual, and this may
indicate a sensitivity to caffeine or
experimenter bias causing these results. For
further investigation, the participant pool
should be larger, as the results of a single
individual cannot be generalized to a
population.
4.3 Hypothesis #3 - If caffeine intake
increases, cognitive wakefulness will
increase.
The testing of this hypothesis resulted in
one of the highest raw pooled correlations
for all the participants (r = 0.43). However,
it should be noted that two participants
showed very low correlations (r = 0.27 and r
= 0.12) respectively, while one participant

had an extremely high positive correlation (r
= 0.91) which was statistically significant.
4.4 Hypothesis #4 - Colder coffee will not
change the response to caffeine.
The results of testing this hypothesis
showed very little correlation between
temperature of coffee and cognitive arousal
or any other physiological metric. However,
the participants never consumed iced coffee,
which means there is a possibility the coffee
consumed was never cold enough to elicit
this phenomenon. The researchers in White
et al. (2016) kept their chilled beverages at
4C, while the researchers in Brothers et al.
(2019) never deviated below 50C.
Therefore, because the array of temperatures
of coffee was severely limited, subsequent
studies should examine the effect of iced
coffee >0.0C alongside hot coffee in order to
draw any conclusions about the correlation
between temperature and physiological
effect.
4.5 Hypothesis #5 - The consumption of
caffeinated beverages with milk decreases
the cognitive wakefulness-enhancing effects
of caffeine within a beverage.
Interestingly, the results of testing this
hypothesis varied greatly between
participant one and two. This study
examined the cognitive arousing effect of
caffeine, which was measured via a simple
reaction time test, courtesy of an app called
Psych Lab 101. Participant one found a
moderate negative correlation between
increased milk quantity in a caffeinated
beverage, and decreased reaction time
scores, which indicated a higher level of
cognitive arousal. However, participant two
had the opposite results, and had a
statistically significant, high positive
correlation between increased milk solute in
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coffee and increased reaction time scores,
which supports the initial hypothesis. If milk
causes caffeine to be absorbed slower, it
stands to reason the cognitive arousing
effects of caffeine would be equally
hampered. Unfortunately, the dataset is
incomplete, as participant three did not
conduct this correlational variable. The
differences between participants one and
two could be for a multitude of reasons, but
a possible explanation could be the types of
milk that were added to the beverages:
Participant one is lactose intolerant, and
thusly used non-dairy milk. The ‘milk’
participant one added to their coffee
included: almond, coconut, soy and rice
milk, which all have different fat and calorie
contents to conventional cow milk. Also, for
the duration of the baseline study,
participant number one had varying brands
and methods of consuming coffee, which
ranged from espresso shots to drip style
coffee, with varying amounts of ‘milk’
added. However, participant number two
consistently consumed Starbucks brand
Lattes with 2% cow milk added and had
very little variation outside of this beverage.
Subsequent studies should compare different
kinds of milk analogues to examine the
potential effects each one will have on
metabolizing caffeine. This limited data may
suggest that plant based milks are
metabolized faster than animal milks, which
suggests that people who avoid dairy may
feel stronger effects from coffee than those
who use traditional milk. Furthermore, other
studies may compare skim milk to cream or
whole milk, to look for a relationship
between increased milk fat, and longer
metabolism time.
4.6 Variations Between Participants
Notably across all of the variables,
participant number three consistently had the

lowest correlations, participant number two
had the highest correlations, and participant
number one had consistently moderate
correlations.
4.7 Limitations
This study would benefit from several
adjustments as there were several limiting
factors that may have affected the outcome
of this study. First, the quantity of caffeine
used may not have been strong enough to
elicit a strong response from the participant
group. Due to the participants habitual
caffeine use prior to the study, the quantity
of caffeine consumed may not have been
high enough to elicit a physiological effect.
Second, in subsequent studies, the
researchers should utilize a similar
methodology as Brothers, et.al (2017) study,
where the baseline physiological data was
recorded from seated participants who rested
for 28 minutes, after which point their
resting physiological data was recorded for
2-min. Post caffeine consumption the
physiological data was automatically
recorded at 30 intervals for 6 hours. This
controls for user error, inadvertent bias and
allows for an accurate representation of
potential physiological changes. Participants
in the current study were responsible for
manually recording their own radial pulse.
This method of data collection has several
potential issues: Firstly, the baseline HR was
recorded for 30s and the total was doubled
to get the BPM. This collection period is
significantly shorter than the procedures
conducted by Brothers et al. (2017) and
Quinlan et al. (1997) which took an average
of HR data for 2-3 minutes respectively.
Secondly, manually recording HR is a
procedural skill that is easily miscalculated
or misinterpreted. Furthermore, the
participants should not manually record their
vital signs, as the individual taking their own
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pulse allows for experimenter bias, which
may inadvertently affect the reading and
may produce inaccurate results.
In future research, the participant pool
should be larger so that the sample size is
more representative of the broader
population. Furthermore, the other studies
cited all contained samples ranging from 1524 participants (Brothers et al., 2017;
Quinlan et al., 1997; White et al., 2016).
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