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Effect of Caffeine on Blood Pressure Experiment

Paper Type: Free Essay Subject: Biology
Wordcount: 3557 words Published: 22nd May 2018

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  • Isobel Burton

Explore the Effect of Drinking 171mg (approx.) of Caffeine (Coffee) a Day on the Systolic and Diastolic Blood Pressure of Healthy Women.

Contents (Jump to)


Research and Rationale

Effect of Coffee on Physiological Well-being of Humans

Impact of Coffee on Physiological Well-Being of the Heart


Results and Analysis




In this investigation into the effect that drinking 171mg of coffee a day has on the diastolic and systolic blood pressure of healthy women it became clear that the majority of women did not experience any increases of blood pressure over the week-long period. Within this, this study has found that there is little to no correlation between drinking three cups of coffee a day and an increase or decrease in blood pressure. It is therefore evident that 171mg a day is a safe amount of coffee to be consumed by healthy individuals and is in line with Dawber’s research that suggested no correlation between increase in hypertension and coffee consumed under 300mg a day (Dawber, 1974).

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Research and Rationale

My rationale in choosing the link between caffeine consumption and an increase in diastolic and systolic blood pressure stemmed from reading an article in the British Medical Journal that outlined the potentially dangerous link between caffeine and Coronary Heart Disease (CHD). Whereas Caffeine is not officially listed as a contributing factor in discerning those at high risk of developing Coronary Disease, a variety of studies have investigated the correlation between caffeine and several of the recognized factors that contribute to CHD. These contributing factors include: smoking, abdominal obesity, low fruit and vegetable intake, lack of exercise, stress and abnormal lipoprotein ratios. In this study it is essential to recognize the intrinsic link between high blood pressure and CHD as it is the major contributing factor in the number one cause of death in the UK with ’74,000 people dying of Coronary Heart Disease each year’ and ‘1 in 3 people suffering from high blood pressure’ (BBC, 2014). It is therefore essential that the positive and negative dietary effects of caffeine on CHD be ascertained. Dawber suggested that ‘there is no support for the hypothesis that coffee intake is either quantatively or qualitatively related to the initial development or manifestations of atheroscelerotic disease (Dawber, 1974)). However, coffee and its long term and short term effects have long been hailed as potentially dangerous for the heart.

Effect of Coffee on Physiological Well-being of Humans

Firstly, it is important to understand the physiological effect of coffee and it’s chemical nature. Coffee is a daily staple for many and it is an extremely popular beverage. This can be seen through the growing amount of coffee shops on UK High Streets. It is however important to note that Britain’s intake of coffee is relatively low in comparison to other countries in Europe

(Figure 1)

An ingredient in coffee called caffeine is a naturally occurring chemical stimulant called trimethlxanthine and this acts as a stimulant in the body (this is an alkaloid) and antagonises the A1 and A2 adenosine receptors.

Figure 2.

In the cells caffeine stops the development of a chemical named phosphodiesterase that breaks up the second chemical messenger cyclic adenosine monophosphate inside cells. These second messengers are molecules that transmit messages acquired at receptors that exist on the surface of the cell to specific molecules in the cytosol or nucleus.

Hormones and neurotransmitters bind to the cells through receptors on the cell surface which creates a chemical reaction called an ‘enzyme cascade’. This culminates in the creation of second messenger chemicals that transmit to the nucleus. It is clear that this system is sophisticated and the main benefit is that an extra-cellular signal can be significantly enhanced in the practice and thus have a huge intracellular impact. It is therefore clear that when caffeine prevents the disintegration of cyclic adenosine monophosphate the effects of caffeine are prolonged and the body’s response is heightened and it is clear that the heart rate and force of muscle contractions in the heart is increased due to the release of noradrenalin and epinephrine.

Impact of Coffee on Physiological Well-Being of the Heart

In addition, another important issue is to discuss the supposed effect of caffeine on the physiological well being of the heart. It is imperative to address this before looking at the study on the effect of coffee on systolic and diastolic blood pressure and investigating whether caffeine consumption should be considered a major contributing factor in coronary heart disease.

Firstly, several studies including Thelle et al, have suggested that coffee consumption directly relates to an increase in cholesterol. Cholesterol is a lipid that is created in the body and transported by lipoproteins. High blood cholesterol in turn increases the risk of heart disease as it increases atheroma formation in the arterial walls and subsequently leads to hardening of the arteries, blockages or blood clots. In the study it was suggested that ‘coffee consumption was positively associated with levels of total cholesterol and triglycerides in both sexes and was inversely associated with levels of HDL cholesterol in women. The coffee-cholesterol relation remained strong and statistically significant’ (Thelle , 1983). Moreover, coffee has been suggested to raise the plasma levels of homocysteine. This is an amino acid that in high levels is an indicative precursor of Coronary Heart Disease. Homocysteine has been suggested to increase fatty deposits in the arteries, raising blood pressure and resulting in Coronary Heart Disease because of damage to the arterial walls. Furthermore, it is evident that the consumption of coffee by people who suffer from arrythmias or have tachycardia can be dangerous because the normal rhythm of heart rate has already been disrupted and coffee has been proven to result in possible tachycardic episodes and potential hart rate increases. Overall, research has shown that excessive coffee intake has been investigated (Jee, 1998)) and links have proven that that there is a strong link between coffee consumption and central, systolic and diastolic blood pressure increase. It is this that I will investigate in the following study.


The primary aim of my investigation is to explore the link between caffeine consumption and systolic and diastolic blood pressure through ingestion of 3 cups of instant coffee daily. This study took place over 7 days and I have used a sample of 15 women. These women are all aged between 25-40. All of these women are from a White British ethnic group and none are on any medications that would affect their blood pressure. They are all also within a healthy body mass index range. It was important to complete a risk assessment to discuss the study with each woman individually and bear in mind their heath status before agreeing to their participation in the study.

Figure 3. Risk Assessment For Investigation



I decided that this was an appropriate age group because it focused the study to how caffeine affects those not in the risk age range for CHD.

It was less of a risk to use women who were younger than the most common age range for developing hypertension and CHD.


18.5- 25 BMI

I decided it was important to use women within a healthy BMI because I didn’t want the factors of obesity or malnourishment to become factors in my results.

It was less of a risk to use women who were not in a dangerous weight zone in terms of their BMI because of the health complications of being over or under weight.


Not discussed

I did not discuss lifestyle because they knew me and therefore It would have been unprofessional of me to discuss their private lifestyle choices. I also did not want to include this in the study as the study is investigating the effect of caffeine and not smoker status or alcohol intake. Although these are contributing factors to high blood pressure.

I discussed with the women to make sure that they read the safety leaflet I gave them complete with emergency numbers.

Health Status and medications

Discussed conditions that would affect results

None of the women had any medical conditions that would effect the study.

I advised all the women that regardless of their health status, if they experienced anything that worried them they should contact their doctors immediately and cease any involvement in the experiment.

Leaflet on possible side effects.

Given to all

This was in order to explain the details of the experiment with any guidelines and rules.

I advised all women to read the instructions thoroughly and to abstain from ingesting any other caffeine in the form of tea, chocolate, energy drinks etc. as this could raise the levels of caffeine to be potentially dangerous for the women.

Social Status

Not discussed


I advised all women to seek medical help immediately in the case that any issues should arise.

Economic Status

Not discussed


I advised all women to seek medical help immediately in the case that any issues should arise.

I decided to keep the group of women as similar as possible in age, weight and health status in order that the effects of the caffeine were more even and gave more reliable results. Interestingly, I did not ask the women their social situation or status and I did not request their smoker status as the aim of my investigation is to discuss whether caffeine affects the blood pressure of these women and not to investigate the effects of smoking coupled with caffeine.

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In addition I requested that the women remove any form of caffeine from their diets over the 7 day period (fig 4.). This was so that I was able to see the total amount of caffeine consumed by these women in a daily period. In this case I was able to work out that they would be consuming around 57mg of coffee per 8floz cup. This would mean that the women were effectively drinking around 171mg of coffee a day. This amount was chosen because in Heckman’s study (Heckman, 2013) he suggested that ‘300-400mg of caffeine can be ingested daily without any adverse effects’. Of course, this figure relates to healthy adults in a low-risk category such as my participants. I therefore thought it would be interesting to only give my participants 171mg (approx.) of caffeine a day in order to monitor whether or not their blood pressure was effected by a smaller dose than the guideline daily amount of caffeine. In addition, I didn’t want to put additional risks on the experiment by increasing the level of caffeine consumption beyond the guideline daily amount. It is important here to note that three cups of coffee would be a moderate daily amount to drink and therefore if these women’s blood pressure would be effected by a moderate amount then it would be a significant challenge to the claim that 300-400mg a day is safe. In addition, by choosing a moderate level of caffeine ingestion, it was also my aim to differ from Jee et al’s preeminent journal on the effect of excessive caffeine intake on men and women. Jee claimed that excessive caffeine intake sparked a significant rise in blood pressure (ambulatory) of both systolic and diastolic nature, therefore it would be scientifically interesting to investigate whether a lower level of caffeine would have the same effect (Jee, 1998). Out of the 15 women, three were given a placebo pot of coffee that had been filled with 100% decaffeinated coffee. The women were not made aware of this and this was done in order to limit the ‘white coat ‘ blood pressure effect whereby participants blood pressure will go up naturally because they are nervous or they expect it to or because they believe coffee will make it increase.

Figure 4. Banned List of Foods


Pain Relief


Energy Water

Fizzy Drinks

Breath Fresheners

Energy Drinks


Ice Cream

Green Tea

Weight Loss Pills


Monitoring Blood Pressure

In order to complete the experiment on the effect of caffeine on blood pressure it was imperative to learn to understand blood pressure readings and how to take blood pressure and to teach the participants in my study to do so. The participants were being asked to record their blood pressure nine times daily; once before and twice after each cup of coffee. Therefore, it was essential for my participants to be able to accurately record their own ambulatory blood pressure. This was done through the following instructions that I issued to all participants. It was also important to have the correct equipment for my experiment and I was loaned 15 blood pressure monitors by the local military doctor for this investigation. All of the heart rate monitors were checked prior to commencing the investigation and I gave each of the participants one for the duration alongside a briefing sheet.

Figure 5: How to Use a Blood Pressure Machine: Instructions

Systolic Blood Pressure

Diastolic Blood Pressure

The number at the top of the blood pressure monitor is the systolic blood pressure. This is always the higher of the numbers because it records the arterial pressure during heart muscle contractions (beats).

The number at the bottom measures the diastolic blood pressure. Diastolic blood pressure records the pressure in the arteries between heartbeats when the heart fills with blood to pump around the body.

Recording Blood Pressure

  • Make sure that you are seated properly with your back straight and legs uncrossed.
  • Be still and refrain from drinking or smoking while recording your blood pressure.
  • Place the cuff around the top part of your arm ensuring that the tube is positioned centrally on the inner arm as in the picture above.
  • Press the button on the monitor and allow the cuff to inflate.
  • The cuff will inflate and then beep and release when the blood pressure has been taken.
  • Write down the results from the screen and record them on the sheet given.

Results and Analysis

It is evident that the majority of women did not experience any increases of blood pressure over the week-long period. Within this, it was clear that there was little to no correlation between cups of coffee consumed and an increase or decrease in blood pressure. Moreover, the participants did not experience a decrease in blood pressure either. It is therefore evident that my results seem to indicate that 171mg a day is a safe amount of coffee to be consumed by healthy individuals and is in line with Dawber’s research that suggested no correlation between increase in hypertension and coffee consumed under 300mg a day. It was clear that overall the results seemed to indicate no positive or negative correlation. The subjects who ingested the placebo coffee’s (participants 1,2 and 3) systolic and diastolic blood pressure remained relatively constant throughout the trial and did not differ in any meaningful way from the other subjects.

Figure 6. See Appendices for Results


It was however, important to note that one participant (participant 11) seemed to have a naturally high blood pressure that was seemingly exacerbated by the caffeine. In the latter part of the trial participant 11’s systolic and diastolic blood pressure seemed to increase following her third cup of coffee. Her systolic recorded a high of 160 on day 6. This was concerning, however the effects of the caffeine were not conclusive. It would have been interesting to explore the alternative factors that may have facilitated this increase in blood pressure and it is evident that this is a significant limitation of my study. After all, stress is a major factor in hypertension and this was not investigated in this study. In addition, a variety of other factors could have caused the later day increase in hypertension. Excessive exercise, smoking, drinking or stress could all have been contributing factors in the raising blood pressure. It is also possible that she may have taken her blood pressure readings incorrectly or recorded them as such.

It is evident that not taking into account the social, economic and smoker status of my partipants was a major limitation to my study. Although my aim was to discuss the effects of caffeine on systolic and diastolic blood pressure, it is evident that without taking into account the stress levels and lifestyle choices of the participants you will have significant variation in your results and that any increases or decreases in blood pressure will be deemed as inconclusively linked to caffeine consumption as a result. The fact that participant 11’s blood pressure was elevated to up to 160 systolic after the third cup of coffee for three consecutive days is suggestive that there was some influence of caffeine on her blood pressure. However, due to the limitations of this study it is inconclusively caused primarily by caffeine. Therefore it is my recommendation that coffee is safe to consume for healthy adults up to 400mg a day.


Bertrand. C.A. 1978. ‘No Relation Between Coffee and Blood Pressure’. New Engl. J.med, 299, 315-316.

Dawber,T. 1974. ‘Coffee and Cardiovascular Disease’. New Eng. J. Med. 291. 871-74

Hartley P. 1999. ‘Hypertension Risk Status and Effect of Caffeine on Blood Pressure’. Am.Heart Assc. Journals

Heckman. M.A. 2010. ‘Caffeine (1, 3, 7-trimethylxanthine) in Foods: A Comprehensive Review on Consumption, Functionality, Safety, and Regulatory Matters. Journal of Food Science, 75:R77–R87

Jee. S. 1998. ‘The Effect of Chronic Coffee Drinking on Blood Pressure: A Meta-Analysis’ . Am. Heart Assc Journals. 33. P.647-652

Thelle D.S. 1995. ‘Does Caffeine Raise Blood Serum Cholesterol’, J Med 1983; 308: 1454-1457


www.bbc.co.uk. (BBC News)

Date Accessed 12/12/14

www.bhf.org.uk, (British Heart Foundation)

Date Accessed 12/12/14

www.heart.org. (American Heart Foundation).

Date Accessed 12/12/2014



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