What is caffeine?
People have enjoyed caffeinated beverages for many years
Caffeine is a naturally occurring substance found in the leaves, seeds or fruits of at
least 100 different species worldwide and is part of a group of compounds known as
methylxanthines. The most commonly known sources of caffeine are coffee, cocoa
beans, cola nuts and tea leaves. Caffeine is also added to specifically formulated
‘energy drinks’ and pharmaceutical products such as cold and flu remedies.
Coffee and tea also contain other dimethylxanthines; theophylline which has similar
properties to caffeine and theobromine whose pharmacological actions is far less
potent than caffeine and theophylline.
The amount of caffeine present in products depends on the type of the product, the
serving size and the preparation method. For example a 190ml cup of tea contains
50mg of caffeine, one third less than the same amount of an instant cup of coffee
(75mg). Table 1 gives an indication of the amount of caffeine found in other drinks
compared to tea:
Table 1
Type of Product Caffeine (mg/ serving)
Tea All types 50mg/ 190ml serving1
Coffee Brewed (filter or percolated) 100-115mg/ 190ml serving1
Instant 75mg/ 190ml serving1
Cola drinks Standard and Sugar Free 11-70mg/ 330 ml can2
‘Energy’ drinks All types 28-87mg/ 250ml serving2
Chocolate Bar 5.5-35.5mg/ 50g bar2
On average we consume 3.98mg of caffeine /kg body weight per day ie 239mg/ day
for a 60kg person3.
What is a safe intake of caffeine?
Up to 300mg/day (6 cups of tea) is considered moderate, with no evidence of harmful
effects in the vast majority of the adult population. Some individuals are sensitive to
caffeine and will feel effects at smaller doses than other individuals who are less
sensitive. For this reason, these individuals may need to limit their caffeine intake.
Metabolism and Clearance
Caffeine does not accumulate in the body over a course of time and is normally
excreted within several hours of consumption. The rate of caffeine elimination varies
between individuals and this maybe as a result of genetic factors affecting the
enzymes involved in the metabolism, or due to certain lifestyle factors eg smoking.
Children also metabolise caffeine at a quicker rate. Generally caffeine absorption is
complete within about one hour after ingestion and the plasma concentration peaks
2
after about 60-90 minutes. The half-life of caffeine in the plasma is about 2.5 – 4.5
hours in healthy adults.4
Caffeine Tolerance
A number of different factors affect individual tolerance to caffeine, including the
amount ingested, the frequency of caffeine consumption and individual metabolism. It
is widely recognised that gradual tolerance develops with prolonged caffeine use.
Physiological Effects
Caffeine is a pharmacologically active substance, and depending on the dose, has a
number of actions:-
• Central Nervous System Stimulant. A moderate caffeine intake can cause mild
stimulation that maybe beneficial in terms of increased alertness, concentration,
improved performance and decreased fatigue.5-10 However, higher intakes may
affect sleep, cause nervousness and an irregular heartbeat.
• Weak Bronchodilator. As a result, interest has been shown in its potential role as
an asthma treatment. A number of studies have explored the effects of caffeine in
asthma and the conclusions from a Cochrane Review suggest that caffeine
appears to improve airways function modestly in people with asthma for up to four
hours after consumption.11
• Diuretic. The diuretic action of caffeine may be due to an increase in renal blood
flow, leading to an increased glomerular filtration rate (GFR), or due to a
decreased reabsorption of sodium in the renal tubules. The diuretic effect of
caffeine is dependent on the amount consumed and duration of intake eg the
caffeine in tea does not have a diuretic effect unless the amount of tea consumed
at one sitting contains more than 250-300mg of caffeine, equivalent to between 5
and 6 cups of tea.12-17
In fact, due to the volume of fluid that is drunk whilst enjoying a cup of tea, the
British Dietetic Association advises that tea can contribute towards the dailyrecommended
fluid intake of 1.5 to 2 litres.
• Cardiac Muscle Stimulant. Moderate caffeine consumption does not increase
cardiac arrhythmias.18
If regular caffeine consumption is stopped abruptly, symptoms such as headaches,
irritability and fatigue may occur. These effects are usually temporary, disappearing
after a day or so and can be avoided if caffeine cessation is gradual.
Caffeine and Health
The role of caffeine in the development of certain diseases and conditions has been
the subject of extensive research in recent years.
• Cancer.
A number of studies investigating the impact of caffeine in the development of
cancer have failed to establish a relationship.19-22 In fact, tea is one of the richest
sources of flavonoids, a powerful group of antioxidants. The role of antioxidants in
the prevention of free radical damage has led to suggestions that tea maybe anti
3
carcinogenic.23 Despite interesting preliminary research, further work is required to
prove its beneficial effect in this area.
• Heart Disease.
A number of studies have investigated the relationship between caffeine and heart
disease and results from these and epidemiological studies have led to the
conclusion that the ingestion of moderate amounts of caffeine is not associated
with any increased risk of heart disease.24-28 The Committee on Medical Aspects of
Food Policy concluded that ‘there is little evidence that caffeine itself has any
relation with CHD risk’ in the 1994 Nutritional Aspects of Cardiovascular Disease
report.29
• Parkinson’s Disease.
Observational studies have suggested that caffeine may play a role in protecting
against Parkinson’s disease,30-31 although further research is required to determine
the exact mechanism.
• Relief of headaches.
In a study of 301 regular headache sufferers, researchers found that a combination
of ibuprofen and caffeine was better than either drug alone in relieving pain.32
Although a caffeine ‘pill’ was used in this trial, the researchers believed that
caffeinated beverages would work just as well. However, they did warn that
chronic headache sufferers should avoid caffeine because it might exacerbate
symptoms. More work is required in this field before firm conclusions about
caffeine and pain relief can be drawn.
• Pregnancy
Caffeine crosses the placenta and achieves blood and tissue concentrations in the
foetus that are similar to maternal concentrations. For this reason recent advice
published by the Food Standards Agency33 recommends that pregnant women
should limit their intake of caffeine consumption to less than 300mg/ day
(equivalent to 6 cups of tea/ day). At this level there is little evidence to suggest
that the health of the unborn child or mother is affected.
In Summary…
Despite recent publicity about caffeine, the fact remains that the consumption of
caffeine at intakes of 300mg/ day has no adverse effects in the vast majority of the
adult population. For this reason an average intake of three to four cups of tea34 a day
is well within the level considered safe.
Caffeine enters breastmilk in small amounts (about 1% of the mother's
plasma level) but it does accumulate in smaller babies. Six to eight
cups of coffee a day can result in infant wakefulness and
hyperactivity. Smoking augments this effect. Of course the dose of
caffeine from one cup of tea a day is nothing like the dose from
several cups of coffee a day, but it is best to keep the dose the baby
gets as low as possible
References:-
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34. National Drinks Survey, April 2001
