Investigation Of The Effects Of Caffeine On
Physiological Functions In The Human Body
Keywords
Caffeine, blood pressure, heart rate, peak expiratory flow, alertness.
Introduction & Background
The drinking of caffeine-containing beverages (such as coffee and tea) is widespread in the
Western world. The caffeine is responsible for giving the drinks a mild central stimulant
effect. Caffeine itself is one of the pharmacologically active members of the methylxanthine
family (also including theophylline and theobrominei).
Sources
Sources of caffeine include (as mentioned above) beverages such as tea, coffee, soft drinks
(especially colas) etc. Foodstuffs which contain caffeine include chocolate.
Pharmacological Effects
The pharmacological effects of this family of drugs have been described by Rang, Dale and
Ritter thus: ii
Stimulation of the CNS
Diuresis
Stimulation of cardiac muscle
Relaxation of smooth muscle (especially the muscle of the bronchi)
The muscle effects (ie cardiac muscle stimulation and smooth muscle relaxation) are
analagous to those of -adrenoceptors stimulation. This suggests a mechanism -
methylxanthines inhibit phosphodiesterase (responsible for metabolism of cyclic AMP).
Hence, cAMP levels within the cell increase, producing the observed effects.
Methylxanthines also antagonise many of the effects of adenosine, acting on both and 2
receptors, and it is possible, but unproven, that some of the effects result from this
mechanism.
The diuresis is indirectly a result of same mechanism - caused by vasodilation of the afferent
glomerular artieriole. The result of this is an increase in glomerular filtration rate and hence
of urine production.
Pharmacokinetics of Caffeine
In humans, absorption of caffeine is complete, and comparatively fast. Within approximately
45 minutes of being administered, 99% has been absorbed iii. The pharmacokinetics do not
seem to differ according to route of administration.
The time to peak plasma caffeine concentrations varies widely, from as little as 15 minutes to
as much as 2 hours (following oral administration). The peak concentrations are typically 8 -
10 mg / l for doses of 5-8mg/kg. iv
Elimination is by first-order kinetics. The half life of caffeine varies from 2.5 to 4.5 hours in
humans who received a 4 mg/kg caffeine load v. Liver metabolism is the rate-determining
step for clearance - hepatic impairment can increase the half life to a matter of days.
Literature Review
The literature concerning caffeine is large, and often contradictory. There have been
numerous landmark studies examining caffeine from a pharmacological perspective, and their
findings have already been discussed briefly.
Of particular relevance to this study is the literature concerning the physiological and (to an
extent) psychological effects of caffeine upon the body. This project is examining a number
of such variables, and therefore from the literature was drawn exisiting evidence concerning
these variables.
Pulse
A recent study by Quinlan et al was performed, comparing various physiological variables in
subjects drinking tea and coffee.vi They found that in the coffee-drinking subjects (as
compared to a no-drink control) there was a transient but significant increase in pulse rate of
10.5 bpm as compared to a baseline of 69 bpm. The pulse rate proceeded to normalise within
10-20 minutes. In addition, the study examined the effects of milk in the coffee, finding that
the effects were somewhat depressed (ie the increase in pulse was not so great, although still
significant) when milk was included.
Blood Pressure
The same study also examined blood pressure. The findings were that both systolic and
diastolic blood pressure were increased significantly from the baseline values (by 6 and 5
mmHg respectively) during the first (drinking) phase (within 3 minutes of being given the
coffee to drink), which had reduced to 2.8 and 2.1 mmHg respectively during the third phase
(samples taken 30-57 minutes after drinking).
Also of interest is a study by Lane et al vii which demonstrates that cardiovascular effects of
caffeine (and specifically blood pressure alterations) are produced directly, rather than via
other mediators (such as a stressful lifestyle etc).
Reaction Time
A study performed by Lieberman et al in 1987 examined the impact of caffeine upon some
psychological functions viii. Included among these was a four choice reaction time test,
similar to the one used in our study. The findings were that there was a significant increase in
test performance in subjects who has been given caffeine at the all doses (from 32-256mg).
Alertness
The same study also found significantly increased scores on a modified form of the
Wilkinson auditory vigilance test, which can be used as a measure of alertness.
Peak Expiratory Flow Rate (PEFR)
It would seem that there are no major studies on caffeine which involve PEFR. However, the
information gleaned from the pharmacological effects as outlined above would seem to
suggest that an increase might be expected, as a result of the bronchodilatory actions of the
substance.
Hypotheses
The following physiological variables will be significantly increased by administration of
caffeine:
v Pulse
v Systolic blood pressure
v Diastolic blood pressure
v Peak expiratory flow rate
Choice reaction time will be significantly decreased following administration of caffeine
Perceived alertness will be significantly increased following administration of caffeine
Aims
To carry out a clinical trial to determine the effects of caffeine, a known stimulant, on the
human cardiovascular, respiratory and nervous systems.
Setting
The experiments will be conducted at the University of Birmingham Medical School.
Sample
The sample will consist of 10 students undertaking the Integrated Health Sciences
intercalated BMedSci degree course. They consist of third and fourth year medical and dental
students. Each case will act as their own control, owing to the crossover nature of the study.
This is further explained in ‘Design & Method’ section.
Design & Method
This will be a double blind randomised clinical trial, and due to a limited sample population,
a crossover study design will be employed. Selection criteria includes willingness to
voluntarily participate in the study and to comply with the 12 hour caffeine abstinence period
prior to the beginning of each trial session. Exclusion criteria are limited to those persons
with allergies or intolerance to caffeine or those who may have an unstable pre-existing
condition that may specifically influence the data being collected (e.g. asthma suffers may
participate only if their respiratory state and medication intake is consistent at each trial).
Owing to the crossover design, the trial will be carried out on two separate occasions two
weeks apart.
On the first occasion half of the sample population (5 people) will be randomly selected and
given a measured dose of caffeine (normal coffee), the other half of the sample will be given
a placebo (decaffeinated coffee). On the second occasion the groups will be swapped over,
receiving whichever type of coffee they did not have on the first occasion.
Data on heart rate, blood pressure, peak expiratory flow rate, alertness and choice reaction
times will be recorded once prior to the single dose of caffeine and again at three timed
intervals after the dose. All measurements will be carried out by two impartial observers (one
assigned to each group).
Measurement
The five variables will be measured as follows:
1. Heart rate will be measured for 30 seconds and then doubled to yield a rate per minute.
2. Blood pressure will be measured using a standard sphygnomanometer in millimetres of
mercury (mmHg).
3. Peak expiratory flow rate will be assessed (in litres per minute) using standard peak flow
meters with disposable mouth pieces.
4. Feelings of alertness will be measured in two ways: firstly with a self-rating score and
secondly using a timed playing-card sorting technique.
5. Choice reaction rate will be assessed by the time taken to sort a well shuffled pack of
standard playing cards into the four different suits.
Analysis
Correlation between the time elapsed since caffeine challenge and each of the five measured
variables will be examined. Spearman’s rank correlation coefficient will be used to examine
correlation between time and variable. T-tests will be used to examine the mean values in
order to examine significance of any differences in the data.
References
iRang HP, Dale MM, Ritter JM. Pharmacology 3rd ed p359. London: Churchill Livingstone;
1995.
iiIbid p641
iiiMarks V, Kelly JF. Absorption of caffeine from tea, coffee and Coca-ColaThe Lancet
1973; 1:827.
ivBonati M, Latini R, Galetti F, Young JF et al. Caffeine disposition after oral doses. Clin
Pharm Ther 1982; 32:98-106.
vArnaud MJ. The pharmacology of caffeine. Progr Drug Res 1987;31:273-313.
viQuinlan P, Lane J, Aspinall L. Effects of hot tea, coffee and water ingestion on
physiological responses and mood: the role of caffeine, water and beverage type.
Psychopharmacology 1997; 134:164-173.
viiReference missing
viiiLieberman HR, Wurtman RJ, Emde GG, Roberts C, Coviella ILG. The effects of low
doses of caffeine on human performance and mood. Psychopharmacology 1987;
92:308-312.