Caffeine is a naturally occurring chemical stimulant also called trimethylxanthine. Its
molecular shape is similar to adenosines – a chemical that can be found in all human cells. For
the brain, it works as a central nervous system depressant. In standard conditions, the chemical
prompts sleep and limits arousal by slowing down nerve function. Caffeine can bind to the
brains adenosine receptors, however it doesnt decrease cells activity, like adenosine does.
As caffeine takes up adenosine receptors, the brain can no longer sense adenosine, which makes
the cellular activity rise instead of going down . This results in an increased blood pressure,
as the high cellular activity causes a greater demand for blood flow . The Food and Drug
Administration (FDA) describes caffeine as both a drug and a food additive. In nature, caffeine
can be found in around 60 plant species, such as cocoa beans, kola nuts, tea leaves and coffee
beans. It is added to various soft drinks and is also a component of pharmacological
preparations and over-the-counter medicines including analgesics, diet-aids, and cold/flu
It is commonly known that one can benefit from regular intake of caffeine in many ways, for example improving their mental alertness, sports performance, slowing down their mental decline and more, however it can cause a lot of harm. Among many, it may induce high blood pressure (also known as hypertension). High blood pressure is a condition that causes a person to have their arteries walls to receive too much pressure continuously – 140/90mmHg or more .
In my investigation I am going to investigate the short term effect of caffeine intake on blood pressure in a group of regular coffee drinkers.
Furthermore, coffee is the most common source of alkanoid for the majority of my society – it is consumed regularly by 75% of it .
Research Question: What is the influence of caffeine on teenagers blood pressure? Hypothesis
Results of my biological investigation should be the following:
– Caffeinated coffee increases the blood pressure more than decaffeinated coffee.
Independent Variable: Presence of caffeine in a beverage
Dependent Variable: Blood pressure, measured by the sphygmomanometer in mmHg [± 0.5mmHg]
I chose this topic, because I am a regular coffee drinker myself. I have been drinking coffee for
over two years and I have not noticed any negative effects of it. However I keep hearing very
mixed opinions about the possible outcomes of drinking coffee. The thing I would hear the most
often, was always coffees negative effect on blood pressure. That is why in my experiment I
would like to find out if that really is the case.
Amount of coffee – Every participant is going to get 10 grams [± 0.001g] of coffee,
measured by the electronic mass balance. If the amounts differed between participants the results could be significantly altered too.
Amount of water – Every beverage is going to be 200 mL [±0.5mL], measured by a scale. If there was too little water, coffee would not fully dissolve, hence influencing participants overall caffeine intake and blood pressure levels.
Beverages temperature – Every beverage is going to brewed using boiling water and then cooled down to be slightly above the rooms temperature (up to 40°C [±1]). If the drinks were too hot participants could hurt themselves trying to drink them.
Participants age – I am aiming at finding the effect of caffeine on teenagers, therefore I will be taking measurements people aged 16-19. However, there could be some age- specific differences in the human blood pressure levels, which could result in various results without any real correlation.
Coffee Brand – Both of the coffees used, caffeinated and decaffeinated, were from the brand Nestle. Other brands may possess more caffeine or other substances which may affect blood pressure. Thus, the results of the experiment may only apply to this specific coffee mix.
Measured but not controlled:
Dates of taking the measurements – I am going to meet with participants on the dates
that suit them best, therefore I cannot control the amount of time between sessions. There could be some differences in the atmospheric pressure, which can affect participants blood pressure.
Participants sex – The effect of caffeine intake on blood pressure may differ among sexes, however due to a limited sample pool I am not able to create a division between males and females.
Temperature of the room where the measurements are taken The temperature of the surrounding could have an effect on the samples blood pressure, thus making the readings unreliable.
Materials: Thermal plastic cups, coffee Nestle, decaffeinated coffee Nestle, sphygmomanometer [±0.5mmHg], kettle, electronic mass balance [±0.001g], 250mL beaker [±10mL], thermometer [±1°C]
1. Find 20 people, who consume caffeine regularly, and assign them numbers for the purpose of confidentiality.
2. Debrief the sample about the purpose of the experiment, the way it is going to be carried out, their rights of withdrawal and confidentiality.
3. Ask each person to give their written consent for participation.
4. Take 3 persons at a time – They cant have drunk coffee that day before the
5. Prepare 3 beverages of either regular or a decaffeinated coffee using the procedure:
measure 10g of the substance, using the electric mass balance, and mix it with 200mL
of water, measured using the beaker, in a thermal plastic cup.
6. Measure their blood pressure, using the sphygmomanometer, and write it down next to
7. Give each participant the beverage, without them knowing what type of coffee it is.
Make sure its temperature does not exceed 40°C, by using a thermometer [±1°C].
8. Wait 40 minutes.
9. Measure their blood pressure again.
10. On a different day ask the person to drink the coffee (the other type) again, along the
11. Repeat until all 20 people have already been tested after drinking the regular and
? No participants were hurt throughout the experiment.
? All participants are regular coffee drinkers, to avoid any sort of a caffeine intake shock
which may occur once a person is not accustomed to the substance.
? The coffees used in the experiment, are standard coffees that anybody can buy in a
? Every participant was informed about their rights and the purpose of experiment.
? The temperature of the beverages was up to 40°C, therefore no participant could harm
themselves by taking part in the experiment.
In my experiment there were no environmental issues.
Caffeine may have affected distinct individuals differently. Certain participants appeared more stressed and active after indulging the caffeinated beverages, while others were more sleepy and calm. There was also a number of participants who did not exhibit any conspicuous reactions to the beverage.
Table 1. Systolic and diastolic blood pressure measured in mmHg [±0.5mmHg].
Participant Before After Before
1 71 81 121
2 79 67 125
3 59 60 103
4 81 82 122
5 91 79 151
6 83 90 126
7 72 74 114
8 82 70 127
9 80 83 113
10 83 85 130
11 71 67 112
12 79 63 128
13 80 67 127
14 63 71 101
15 77 89 122
16 74 78 119
17 63 70 109
18 88 95 119
19 70 80 119
20 60 60 108
Mean 75.3 75.6 120.0 SD 9.06 10.20 11.10
120 65 121.0 71.7 10.50 10.80
After Before After
82 124 113 68 103 102 63 105 100 87 134 135 73 121 132 72 126 122 89 134 142 85 133 133 75 114 114 76 136 128 62 97 110 57 136 110 58 98 101 52 98 95
66 102 103 74 114 118 80 116 134 93 96 129 73 121 132 74 109 111 73.0 116.0 118.0 11.10 14.30 14.20
Mean Calculated using the formula: ?? = ??
?? sum of all results
? number of distinct pieces of data
Example of calculation:
?? = 75.3
SD – Standard Deviation is a measure of average variance among
the pieces of data and the mean. It is important to calculate it to see whether the pieces of data
have similar values (small SD) or if they are extremely different (great SD) .
Standard Deviation was calculated using this formula:
? ? ?????? ?? ???? ??????
?? ? ?h? ???? ?? ?h? ?1
? ? ???h ?? ?h? ?????? ?? ?h? ????
? ? ????? “sum of”
? ? ?h? ???????? ?????????? ?? ? ??????
? = ??(? ? ??)2 ??1
Example of calculation:
(71?75.3)2 +(79?75.3)2 +(59?75.3)2 +(81?75.3)2 +(91?75.3)2 +(83?75.3)2 +(72?75.3)2 ?+(60 ? 75.3)2(80 ? 75.3)2 + (63 ? 75.3)2 + (82 ? 75.3)2 + (77 ? 75.3)2 + (74 ? 75.3)2 + (63 ? 75.3)2 +
? = (88?75.3)2 +(70?75.3)2 +(71?75.9)2 +(79?75.3)2 19
? = 9.0
Figure 1. Blood pressure values.
140 120 100
60 73 40
DD2 DS1 DS2
CD1 CD2 CS1
CD1 – Caffeinated coffee, diastolic pressure before drinking the beverage
CD2 – Caffeinated coffee, diastolic pressure 40min after drinking the beverage
CS1 – Caffeinated coffee, systolic pressure before drinking the beverage
CS2 – Caffeinated coffee, systolic pressure 40min after drinking the beverage
DD1 – Decaffeinated coffee, diastolic pressure before drinking the beverage
DD2 – Decaffeinated coffee, diastolic pressure 40min after drinking the beverage
DS1- Decaffeinated coffee, systolic pressure before drinking the beverage
DS2 – Decaffeinated coffee, systolic pressure 40min after drinking the beverage
In each case, there has been an increase in blood pressure. It can be noticed that in the case of
caffeinated coffee there has been a mean increase in diastolic blood pressure of 0.3 mmHg and
1 mmHg in systolic blood pressure, 40min after drinking the beverage. Whats surprising, in
case of decaffeinated coffee the mean increases in both types of blood pressure have been
greater. The mean increase in diastolic blood pressure was 1.3 mmHg, while the mean increase
in the systolic blood pressure was 2 mmHg. Moreover, the standard deviation values, in every
trial, are extremely similar before and after drinking a beverage. In the case of caffeinated
coffee, the difference in the standard deviation values of the diastolic blood pressure, before
and after drinking the beverage, was 1.14, while in the systolic blood pressure it was just 0.20.
When it comes to the decaffeinated coffee, the difference in the standard deviation values of
the diastolic blood pressure, before and after drinking the beverage, was 0.30, and in the
systolic blood pressure it was 0.10. Which means that theres very little variance among the
average results, and thus the values obtained are similar.
Blood pressure [±0.05 mmHg]
Figure 2. Percentage increase in blood pressure.
Percentage Increase in blood pressure
Type of blood pressure and presence of caffeine
CD DD CS DS
CD Mean percentage increase in the diastolic blood pressure, 40 minutes after drinking the
DD Mean percentage increase in the diastolic blood pressure, 40 minutes after drinking the
CS Mean percentage increase in the systolic blood pressure, 40 minutes after drinking the
DS Mean percentage increase in the systolic blood pressure, 40 minutes after drinking the
The mean percentage increase in case of the diastolic heart pressure was 0.4 in case of
caffeinated coffee and 1.8 in case of decaffeinated coffee. Furthermore, the value of the mean
percentage increase in case of the systolic heart pressure was 0.8 in case of caffeinated coffee
and 1.7 in case of decaffeinated coffee. Thus, it can be noticed that the mean percentage
increase in case of the decaffeinated beverages was significantly higher, than those of the
T-test is any statistical hypothesis which is used when comparing two sets of normally
distributed data. It shows the degree of overlap between the sets. The more overlap there is, the
more likely the pieces of data are the same. Where a small value of t indicates that there is little
to none difference between the sets. The value of t is compared against the appropriate number
in the table of critical values. In order to search for the right place in the table, the degrees of
freedom (DF) are calculated. If t is greater or equal to the critical value, then you can assume
that there is a statistical difference between the pieces of data. Furthermore, in biology the t
value at 0.05 is taken .
?? = ?1 + ?2 ? 2
?? – number of terms in the set number 1 ?? number of terms in the set number 2 Figure 3. T-test for the diastolic blood pressure for the diastolic blood pressure.
Calculation of the value of t:
? = 3.15? 0.8 +89 ?65 20
20 ? = 5.61
Figure 4. T test for the systolic blood pressure.
Calculation of the value of t:
0.9 ? 3.2 + 142
?118 20 20
? = 2.11
Calculation of the degrees of freedom (DF)
?? = 20 + 20 ? 2 = 38
Figure 5. T-test values .
The value of t at 38 degrees of freedom is 2.0244 (fig 5.). Both of the results calculated
and 2.11) are greater than this value. Thus, there is a statistically significant
difference between the increase in blood pressure, both diastolic and systolic. However, it does
not confirm the hypothesis directly as the increases were significantly higher in the case of
decaffeinated beverages than in the caffeinated ones which is exactly opposite to what the
Ultimately, it can be concluded that caffeine increases blood pressure in teenagers. However,
the results of my experiment suggest that caffeinated coffee does not increase blood pressure,
in teenagers, more than decaffeinated coffee does. Furthermore, it is the decaffeinated coffee
that caused the significant blood pressure increase, determined through the t-test. This is
exactly opposite to what the hypothesis claims. Even though placebo, which is the only factor
that could have caused the increase in blood pressure for the decaffeinated beverages, is known
to an extremely powerful effect, it was present in both types of beverages as the sample was
not informed which one they were receiving.
After conducting further research on decaffeinated coffee, I have found out that decaffeinated
coffee does seem to have an effect on human blood pressure, in adults. This research was
conducted by Smits et al. in 1989. In the study blood pressure was measured, over a period of
a total of 12 weeks, when participants were given caffeinated or decaffeinated coffee daily (6
weeks each type). Its results indicate that there is only a slight difference between the effects
of the two types of coffee . However, this research differs slightly from mine. Not only is
the sample different, but also the amount of coffee and number of measurements. On the other
hand, the phenomenon of caffeine tolerance might have occurred. This effect claims that ones
body can become to numb to the effect of this substance. Only people who either never ingested
caffeine or have abstained from it for a long period of time, can experience its full effects .
Thus, regular coffee drinkers may have already developed immunity to coffee and thats why,
in the experiment conducted by Smits et al. there is little difference between the two conditions.
The other reference literature chosen does not provide data on the decaffeinated coffee intake,
however it is a meta-analysis of a large number of studies on caffeine, conducted by Mesas et
al. in 2011 . In 5 trials, the intake of 200-300mg of caffeine (equivalent of 10g of Nestle
coffee) resulted in a mean increase of 8.1mmHg in systolic blood pressure, and 5.7 mmHg in
diastolic blood pressure, while in my experiment it was 1 mmHg and 0.3 mmHg respectively.
Further, the measurements were taken in the first hour after the beverage intake, which is
similar to my experiments procedure. The greatest difference however, was that the
participants knew that they are getting a caffeinated beverage each time, and thus the placebo
effect could be stronger since the sample is expecting to have their blood pressure increased.
The conspicuous difference, which is present between the results that the literature suggests
and the ones I have obtained, could be the effect of the presence of certain limitations that flaw
the reliability of my results:
Table 2. Limitations and improvements for the experiment.
Participants were not isolated from the surrounding between measurements, which could have resulted in external factors, such as stress, affect their blood pressure. Thus, participants should be isolated from the environment between the
Barometric pressure fluctuates among distinct days, further it is a common factor affecting peoples blood pressures . Therefore, all measurements should have been taken on the
Not All Qualitative Data
A number of other factors could have played a role on human blood pressure fluctuations, such as stress. Thus, an interview should be conducted before each measurement in order to
No Division Between Males
access its reliability.
The effects of caffeine may vary between sexes. It is an important factor to consider, when trying to determine the effect of caffeine on human blood pressure. Thus, there should be a division between males and females, when it
comes to analysing the results.
Diet Was Not Recorded
Besides not drinking caffeinated drinks, no dietary restrictions were placed upon participants. As mentioned in the introduction, caffeine is present in a great deal of foods, some of each could have been eaten by the participants on the day when their measurements were taken. Therefore, diet of each participants, on the testing day, should be recorded and