Caffeine – friend or foe

Tea and Caffeine

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)

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
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 daily-recommended fluid intake of 1.5 to 2 litres. • Cardiac Muscle Stimulant. Moderate caffeine consumption does not increase
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- carcinogenic.23 Despite interesting preliminary research, further work is required to prove its beneficial effect in this area. 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 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. 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 wel . 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.
1. Gray J (1998). Caffeine, coffee and health. Nutrition and Food Science 6:314-
2. Ministry of Agriculture, Fisheries and Food (MAFF) (1998). Survey of caffeine and other methylxanthines in energy drinks and other caffeine containing products (updated). Food Surveillance Information Sheet No. 144 (No. 103 revised). London 3. Barone JJ, et al. (1996) Caffeine consumption. Food and Chemical Toxicology 34:119-129
4. Graham TE (1997) The possible actions of methylxanthines on various tissues. In Reily T., Orme M (eds). The clinical pharmacology of sports and exercise. Elsevier Science, Amsterdam. 257-270 5. Lieberman HR, et al (1987). The effects of low doses of caffeine on human performance and mood. Psychopharmacology 93:308-312
6. Jarvis M. (1993). Does caffeine intake enhance absolute levels of cognitive performance? Psychopharmacology 110:45-52
7. Hindmarch I, et al. (1998). The effects of black tea and other beverages on aspects of cognition and psychomotor performance. Psychopharmacology
139(3) :230-238
8. Smith AP, et al (1990-91). Effects of caffeine given before and after lunch on sustained attention. Neuropsychobiology 23(3): 160-163
9. Durlach PJ, et al (1998). The effects of a low dose of caffeine on cognitive performance. Psychopharmacology 140(1):116-119
10. Battig K. (1986) Effect of coffee on the speed of subject-paced information processing. Neuropsychobiology;16(2-3):126-30
11. Bara AI, Barley EA. (2001) Caffeine for asthma (Cochrane Review). In: The 12. Nussberger, J. et al. (1990) Caffeine-induced diuresis and atrial natriuretic peptides. Journal of cardiovascular Pharmacology, 15, 685-691
13. Neuhäuser-Berthold, M. et al. (1997) Coffee consumption and total body water homeostasis as measured by fluid balance and bioelectrical impedance
analysis. Annals of Nutrition & Metabolism, 41, 29-36
14. Martof, M.T. and Knox, D.K. (1997) The effect of xanthines on fluid balance. Clinical Nursing Research, 6:186-196
15. Stookey, J.D. (1999) The diuretic effects of alcohol and caffeine and total water intake misclassification. European Journal of Epidemiology, 15, 181-188
16. Passmore AP et al (1987) Renal and cardiovascular effects of caffeine: a dose response study. Clin. Sci. 72(6), 749-56
17. Grandjean AC et al (2000) The effect of caffeinated, non-caffeinated, caloric and non-caloric beverages on hydration. J. Am. Col . Nutr. 19(5), 591-600
18. Myers MG. (1991) Caffeine and cardiac arrhythmias. Annals of Int Med,114:
19. Rosenberg L. (1990). Coffee and tea consumption in relation to the risk of large bowel cancer. A Review of Epidemiological Studies. Cancer Letters 52:163-171
20. Jacobsen BK, et al (1986). Coffee drinking, Mortality and Cancer Incidence: Results from a Norwegian prospective study. Journal of the National Cancer
Institute, 76:823-831
21. Gordis, L (1990). Consumption of methylxanthine-containing beverages and risk of pancreatic cancer. Cancer Letters, 52:1-12
22. Lubin F, et al. (1990) Consumption of methylxanthine-containing beverages and the risk of breast cancer. Cancer Letters, 53:81-90
23. Huang MT, et al (1992). Phenolic compounds in food and cancer prevention. Phenoloc Compounds in Food and Their Effects on Health II Washington: American Chemical Society Symposium Series, 24. Grobbee, DE, et al (1990). Coffee, caffeine and cardiovascular disease in men. The New England Journal of Medicine 323:1026-1032
25. Bak AAA, et al (1991). Caffeine, blood pressure, and serum lipids. Am J Clin Nut, 53:971-975
26. Stamler J, et al (1997). Relation of body mass and alcohol, nutrient, fiber and caffeine intakes to blood pressure in the special intervention and usual care groups in the Multiple Risk Factor Intervention Trial. Am J Clin Nut, 65(Supp.):
27. Willett WC, et al (1996). Coffee consumption and coronary heart disease in women. A ten-year fol ow up. JAMA 275: 458-462
28. Woodward M, et al (1999). Coffee and tea consumption in the Scottish Heart Health Study fol ow-up: conflicting relations with coronary risk factors, coronary
disease and all-cause mortality. J. Epidemiology and Community Health, 53:
29. Nutritional Aspects of Cardiovascular Disease (1994). Report of the Cardiovascular Review Group Committee on Medical Aspects of Food Policy. Department of Health 30. Ross G et al (2000). Association of coffee and caffeine intake with the risk of Parkinson Disease. JAMA, 283:2674-2679
31. Ascherio A, et al (2001). Prospective study of caffeine consumption and risk of Parkinson’s disease in men and women. Ann Neurol, 50(1):56-63
32. Diamond S, et al (2000). The Use of Ibuprofen Plus Caffeine to Treat Tension- type Headache. Clinical Pharmacology and Therapeutics;68:312-319 33. Food Standards Agency (2001). Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment. Statement on the Reproductive Effects of Caffeine 34. National Drinks Survey, April 2001


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