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Introduction

Caffeine is a stimulant which is commonly found in tea, coffee and some carbonated or sports drinks. Its action on the central nervous system can help to relieve feelings of fatigue. When we consume caffeine in drinks and sometimes in foods, nearly all of the caffeine we ingest (99 peretic cent) is absorbed by the body and distributed to all our tissues and organs. The extent to which each person reacts to caffeine varies between individuals, and depends on the amount of caffeine consumed, and their sensitivity, or tolerance to its effects.(1) For example, one cup of average strength tea may be enough to cause sleeplessness in some people, but has little or no effect on others.(2) The amount of caffeine contained in different drinks can also vary quite widely, but some typical examples are shown in the box below.

Diuretic effects of caffeine

Caffeine is a weak diuretic, which means that it stimulates the body to produce more urine than it would if same volume of water was consumed. This can have a dehydrating effect on the body. For example, six cups of coffee per day can result in a 2.7 per cent decrease in total body water.(5) This amount of water loss means that the body is mildly dehydrated and some physiological and performance responses will be impaired.

The extent to which caffeine acts as a diuretic can vary between individuals, and regular caffeine drinkers appear to be less susceptible to the dehydrating effects.(6,7,8)

Caffeine content of drinks(3,4)

Doses of caffeine equivalent to the amount normally found in a standard servings of tea, coffee and carbonated soft drink do not appear to have diuretic effects on habitual caffeine drinkers.(9) However, since the process of drinking has been shown to alleviate the feeling of thirst, consumption of caffeine can not only cause more urine to be produced, but also reduces the desire to drink. This increases the likelihood that caffeine consumption will result in mild dehydration.(5)

Because of the potential dehydrating effect of caffeine, the Food Standards Agency recommends that caffeinated drinks should not be the only source of fluid intake.(10) Individuals who drink caffeinated beverages should also drink plenty water or other non-caffeinated drinks in order to help counteract any dehydrating effects of caffeine.

Other adverse effects of caffeine

Caffeine can have a number of other effects on the body, apart from its action as a diuretic. Some of the most important examples are outlined below.

Central nervous system
Caffeine is a central nervous system stimulant. It exerts its stimulatory effect by blocking the action of adenosine, which leads to an increase in the levels of several neurotransmitters in the brain including adrenaline, noradrenaline, tryptophan and dopamine. Caffeine also modulates the effects of GABA and serotonin.(11) GABA is the brain’s principle inhibitory neurotransmitter and regulates neuronal excitability and rapid changes in fear arousal, such as anxiety, panic, and the acute stress response.(12) Serotonin is another neurotransmitter which allows control of body functions such as appetite, sleep, memory and learning, temperature regulation, mood, cardiovascular function, muscle contraction and endocrine regulation.(13) It is unclear whether the central nervous system ever becomes completely tolerant of caffeine, and certainly caffeine dependence and withdrawal symptoms can occur.(14,15) Some of the specific adverse effects outlined below arise from the stimulatory action that caffeine has on the central nervous system.

Cardiovascular system
Caffeine stimulates the heart to beat faster and in some people this can cause uncomfortable palpitations and an irregular pulse.(2) Caffeine consumption has also been linked to an increased risk of cardiovascular disease.(16) One study found that for high caffeine consumers, ie individuals who drink more than four cups of coffee per day, the odds radio was 1.83. This means that high caffeine drinkers are 1.83 times more likely to suffer from coronary heart disease than non-caffeine drinkers. For moderate caffeine consumers (three or four cups per day) the odds ratio was 1.33. There was no significant association between caffeine and coronary heart disease in individuals who drank less than two cups per day (odds ratio close to 1). (17) These findings are, however, controversial since other studies have found that habitual moderate caffeine intake is not harmful to long-term cardiovascular health.(18,19,20)

There is, nevertheless, extensive evidence to show that caffeine causes an increase in blood pressure.(16) This increase can be quite sharp, for example, in people with normal blood pressure, drinking two or three cups of coffee can raise systolic pressure by 3 to 14 mmHg and diastolic pressure by 4 to 13 mmHg. Some research has found that people who regularly drink caffeine have a higher average blood pressure than those who drink none, whereas other habitual caffeine consumers develop a tolerance to it, and their blood pressure is not increased in the long-term. However, people who are prone to high blood pressure may be at particular risk of adverse effects from chronic caffeine consumption since their blood pressure remains reactive to the effects of caffeine in their diet.(21) Overall, caffeine is likely to have only a modest impact on blood pressure levels within the whole population, probably only increasing systolic and diastolic levels by 4 and 2 mmHg respectively. Nevertheless, at these levels caffeine consumption could account for approximately 14 per cent of premature deaths from coronary heart disease and 20 per cent of premature deaths from stroke.(16)

Anxiety and stress
Even relatively small amounts of caffeine can give some individuals, including school-aged children, a sense of anxiety.(2,22) Larger amounts of caffeine (in excess of 350mg or 3-4 cups of coffee) can cause increased stress levels.(23) Caffeine can cause anxiety symptoms in normal individuals, but people with a pre-existing anxiety disorder are particularly susceptible and may experience panic attacks.(24,25) Caffeine can also aggravate the symptoms of premenstrual syndrome.(26) In some individuals, chronic excessive caffeine consumption leads to the development of a condition known as caffeinism which is characterized by heightened anxiety, depression, psychophysiological disorders, and degraded performance.(27) Chronic users of caffeine who are caffeine-sensitive may experience symptoms of caffeinism even though their daily intake of caffeine is relatively low.(26)

Concentration and memory
Larger amounts of caffeine can cause lapses in concentration.(23) One study found the individuals who consumed 100mg of caffeine were able to recall fewer words from a list of words both after single and repeated attempts.(28) In another study, the responses of subjects who had higher caffeine doses were seriously impaired on a numerical Stroop test.(29) The effect of caffeine on memory is not straightforward, however, since it appears to have a different effect on men and women, depending on female hormone levels.(30) Other studies have found that caffeine did not produce any effects on memory,(31) or that memory performance was enhanced by caffeine.(32)

Sleep patterns
Caffeine can also cause difficulty sleeping and restlessness.(2,33) High caffeine intake of more than 600mg caffeine or approx 7 cups per day, is particularly likely to cause sleep problems and lead to feelings of tiredness in the morning.(34,33) One study found that people who drank 240mg of caffeine (approx 4-5 cups of coffee) per day were 40 per cent more likely to suffer from insomnia compared to people who drank no caffeine.(35) The rate of caffeine metabolism varies between individuals and this can determine the extent to which caffeine disturbs sleep.(36)

Exercise and physical performance
Caffeine ingestion prior to exercise can increase sweating rates and water loss,(37) although it does not appear to hinder hydration or exercise performance in habitual caffeine drinkers.(38,39) Large doses of caffeine (4-5 cups per day) can also cause muscular tremor.(2,35)

Urinary tract
Drinking a lot of caffeine can cause some people to experience symptoms similar to those of urinary tract infection.(40) Caffeine can also increase the risk of developing bladder cancer.(41) One study found that non-smokers who drank more than seven cups of coffee per week were twice as likely to develop bladder cancer than those who did not.(42)

Headaches
It is well-recognised that excessive caffeine consumption, can cause chronic daily headaches and migraine.(43,44) For example, drinking 4-5 cups of coffee or tea per day increases the chance of getting a headache by 30 per cent.(35) Children and adolescents can also suffer caffeine-induced headache as a result of the consumption of caffeine in cola drinks, particularly if they consume more than 1.5 litres per day (or approx. 200mg of caffeine daily).(44) Headaches and migraines can also arise as a result of caffeine withdrawal and this may be one of the causes of weekend migraine attacks, which are suffered by some individuals.(45)

Glaucoma
The consumption of caffeine can increase intraocular pressure and this is a particular concern for patients with normotensive glaucoma and ocular hypertension. Individuals who drank more than 200mg caffeine per day were found to have a higher intraocular pressure than those who drank less than 200mg daily. It is, therefore, recommended that individuals with either of these conditions should not drink more than 180mg of caffeine per day.(46,47)

Osteoporosis
Consumption of caffeine causes the body to excrete more calcium and other minerals, such as magnesium, sodium and chloride. The body does not adapt to the effect of caffeine on the urinary tract and so the continued, uncompensated loss of calcium, presents a risk factor for the development of osteoporosis, particularly among older people.(48) However, studies have yielded conflicting results regarding the effect of caffeine and bone loss. For example, caffeine consumption at levels in excess of 300mg per day has been shown to accelerate bone loss from the spine in elderly postmenopausal women.(49) Whereas, other studies have failed to find any association between caffeine and bone loss.(50) Individual variation in terms of susceptibility to osteoporosis may partly explain these findings. Until more is known about this condition and the interaction with caffeine and dietary factors, sensible guidance would be to recommend that older people consume adequate dietary calcium and moderate their caffeine intake, to less than 300mg per day.(50)

Pregnancy
There is evidence to show that reproductive-aged women and children should moderate their caffeine intake. Caffeine consumption during pregnancy can increase the risk of having a miscarriage, stillbirth(51) or having a baby of low birth weight. The Food Standards Agency advises pregnant women not to drink more than 300mg of caffeine a day (3-4 cups of coffee).(52)

Lactation
Caffeine is secreted into breast milk, and so it is important that nursing mothers do not drink more than 50-100mg of caffeine per day (about 1-2 cups of tea or coffee). If more caffeine is ingested, the level in the breast milk may be high enough to cause restlessness in the baby.(2) Children, especially infants (0-1 year) should not be given drinks containing caffeine.(2)

Older people
Many over-the-counter medications contain caffeine. In older people this additional source of caffeine found in medication can lead to sleep problems. Older people, should be encouraged to always read the label on medication and, where possible, select drugs that are caffeine-free.(53) Hospitalized elderly patients appear to be particularly susceptible to a caffeine-related decline in sleep quality. This may be because they have less control over their environment and fluid consumption than individuals who live in the community.(54) Coffee consumption has been found to reduce cognitive decline in elderly men,(55) and has been associated with a lower risk of Alzheimer’s disease. However, the results are not clear and other studies have reported that caffeine intake was not associated with cognitive change.(56) In addition, coffee contains other substances such as magnesium and phenolic acids which may provide a protective effect against oxidative damage to neurons in the brain. Given that caffeine consumption is associated with adverse side-effects, a high level of consumption is not recommended.(55)

Health effects of caffeine – conclusions

For healthy adults, moderate daily caffeine intake (up to 400 mg/day, approximately four cups of coffee, or the equivalent of 6mg per kilogram of body weight in a person weighing 65kg ) does not appear to have any major adverse effects.(57,1) Nevertheless, because some of these findings are controversial, and caffeine consumption may be harmful to susceptible individuals. Strategies are needed to encourage a reduction in dietary caffeine, with recommendations to moderate caffeine intake, and to substitute other drinks which do not contain caffeine, such as water.

Other adverse effects of tea and coffee

Tea and coffee both contain tannin although the amount contained in different teas and coffees varies. In some people tannin can cause abdominal discomfort and this is thought to be due to the action of tannins on the surface protein of the stomach and to the stimulation of excess hydrochloric acid secretion. Tea and coffee have no appreciable nutritional value apart from any milk or sugar added to it.(2)

Tea and coffee also contain compounds, called polyphenols which impair the bodies ability to absorb iron. Iron is needed by red blood cells in order to transport oxygen around the body. This is important particularly for young women, pregnant women and toddlers who are most at risk of iron deficiency anaemia. In order to minimise the adverse effect on iron absorption, it is best to avoid drinking tea and coffee with meals or within 30 minutes after a meal.(10)

Energy drinks

“Energy drinks” are designed to increase stamina and improve physical performance for a short period of time. The have caused concern in the past because of their caffeine content. The amount of caffeine in an energy drink varies by brand, but has been estimated to be between 240-320ml/l. A typical 250ml can of energy drink would give a caffeine content of about 80ml per can, which is similar to a cup of coffee. In addition to caffeine, energy drinks usually contain taurine (an amino acid, one of the building blocks of proteins) and glucuronolactone (a carbohydrate).(58) Problems can arise when energy drinks are consumed in large quantities, mixed with alcohol, or taken after exercise to quench the thirst.

In 1999 the European Commission Scientific Committee on Food evaluated the safety of caffeine, taurine and glucuronolactone in energy drinks. It concluded that since the caffeine content of energy drinks was comparable to that of tea and coffee, and assuming that the energy drinks replaced other sources of caffeine, there was no concern for non-pregnant adults. In children, such energy drinks could constitute an increase in daily caffeine exposure, resulting in transient behavioural changes, such as increased arousal, irritability, nervousness or anxiety. Pregnant women should not exceed an intake of 300mg per day of caffeine, from any source. The Committee reviewed additional information on energy drinks in March 2003 and whilst their earlier opinion on caffeine remained unchanged, there were additional concerns that the diuretic effect of caffeine and taurine could be further enhanced by the ingestion of alcohol. This coupled with loss of body fluids due to sweating during exercise, could, theoretically, result in dehydration.(11)

Sensible advice for those consuming energy drinks is:

 •  Always read the label. Drinks containing caffeine in excess of 150 milligrams (mg) per litre have to carry the declaration, ‘High caffeine content’, together with the amount of caffeine expressed in mg per 100ml.(59)

 •  Do not drink excessive amounts of energy drink.

 •  Do not mix the energy drink with alcohol – this enhances the diuretic effect of the energy drink.

 •  If you engage in intense physical activity or exercise, drink enough water to ensure that you are properly re-hydrated.

Last updated: March 2007

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