T he consumption of caffeine-containing drinks such as tea and coffee dates back at least 1,000 years in the East and the first recorded commercial cultivation was in the fifteenth century in Ethiopia. By the end of the seven-teenth century coffee drinking and coffee houses had spread throughout Europe, includ-ing in Britain where these establishments became important meeting places and centres of commerce, especially in the C ity of London, where some developed into famous institutions such as the Stock Exchange and Lloyd’s. Today coffee is big business with worldwide consumption averaging 3.6 million tonnes in the 1990s (International C offee Organisation, 1998). Europe consumes the most coffee, followed by N orth and C entral America. Two important twentieth century developments have been the manufacture of instant coffee and decaffeinated coffee (D ebry, 1994). Instant coffee is made by spray- or freeze-drying a coffee brew to remove the water, leaving a powder or granules which can be reconstituted. C ommercial decaffeination processes are based on extraction with water, carbon dioxide or organic solvents. Today the major U K coffee manufacturers use either the water or carbon dioxide methods and solvent decaffeination is used to a much lesser extent than in the past. T he types of solvent used for decaffeination are governed by both national and European legislation; they include methyl-ene chloride (dichloromethane) and ethyl acetate, which is a natural component of fruits and other foodstuffs. U nder European law, methylene chloride residue levels must be under two parts per million. All methods remove about 97-98 per cent of the caffeine at the green bean stage, before roasting, and therefore do not affect aroma development.
In Britain, coffee sales grew by 32 per cent between 1992 and 1997, overtaking tea in 1993, and now there is something of a coffee drinking revolution under way with U S-style coffee bars becoming a familiar sight on Britain’s high streets, and coffee carts and bars, offering an amazing choice of speciality cof-fees, springing up at stations, in shopping centres, and in bookshops. On average in Britain we drink three cups of tea and just under two cups of coffee daily, compared with eight to nine cups of coffee in Scandinavia and four cups in the U SA. Instant coffee still accounts for 85-90 per cent of all coffee drunk Nutrition & Food Science
Number 6 · November/December 1998 · pp. 314–319 © M CB University Press · ISSN 0034-6659
he a lt h
Juliet Gray
The author
Juliet Grayis a Consultant Nutritionist based in Guildford, Surrey, UK.
Abstract
in Britain and we are still fairly traditional, preferring to drink tea at breakfast and in the afternoon and coffee during the morning. In contrast, almost all the coffee drunk in Scandi-navian countries is brewed from the roasted and ground bean.
The com posit ion of coffee
Although there are about 50 different species of coffee, only two are used for world-wide commercial cultivation: varieties of Coffea arabica(arabicas) and ofCoffea canephora (robustas). T he composition of coffee is com-plex and depends on the species and variety of green coffee and on its commercial and domestic handling; it has been reviewed in detail by D ebry (1994). C offee contains more than two thousand different components, some in minute amounts, including carbohy-drates, fats, protein and other nitrogenous substances, notably caffeine, minerals, notably potassium, some B group vitamins, various acids which influence the organoleptic proper-ties of brewed coffee, and more than 700 volatile compounds, many of which also influ-ence coffee aroma and pigmentation.
C affe ine
C affeine is the major pharmacologically active ingredient in coffee. Since the discovery of its chemical structure in 1895 (see D ebry, 1994), caffeine has become one of the most extensive-ly studied food ingredients. T he chemical structure of caffeine (1,3,7-trimethylxanthine) is shown in F igure 1. It is one of a group of plant alkaloids and occurs naturally in the leaves, seeds and fruits of more than 60 plant species, including coffee, tea, cocoa and cola and is therefore present in the beverages made from these substances, as well as in chocolate. C affeine is present in various soft drinks
flavoured with extracts of the cola nut, but caffeine itself is also added to some drinks, especially to the so-called “energy drinks” which are marketed to have a stimulant effect. T hese “energy drinks” may also contain guarana, a naturally occurring plant product that contains guaranine, a substance identical to caffeine, with similar stimulant effects (see next section on the physiological effects of caffeine and coffee). If caffeine is present as a specific addition to a food or drink, then to conform with U K and European food labelling legislation it must be listed in the ingredients list, but naturally occurring caffeine is not listed. C affeine is also a component of certain pharmacological preparations, including various over-the-counter medications, such as some analgesics, where it is used to enhance the action of the painkiller, and in cold and flu remedies and diet aids. In addition, both caffeine and another dimethylxanthine deriva-tive theophylline exhibit bronchodilatory properties and are used in the treatment of neonatal apnoea.
T he caffeine content of some common drinks, foods and medications is shown in Table I. T he actual amount of caffeine in a beverage is extremely variable and dependent on many factors. T hese include the size of the cup and the strength of the brew, which itself depends how finely the coffee is ground and on the method of preparation, for example
CH3
CH3 O
N O
H3C N
N N
1 2
6 3
5 4
8 9
7
Figure 1 The structure of caffeine (1,3,7-trimethylxanthine)
Table I The caffeine content of some common beverages and foods
mg
Average content in a standard 150ml cup
aRoast and ground coffee 80-90
aInstant coffee 60
aDecaffeinated coffee (roast and ground) 3
aDecaffeinated coffee (instant) 3
aTea 40
bHot chocolate drink 2-4
Average content of 330ml can
bStandard cola drink 40 (range 11-70)
Average content per 100ml
b“ Energy drinks” 11 (range 0.05-35)
Average content per 100g
bM ilk chocolate 18
bPlain chocolate 34
White chocolate Nil
Note:Some analgesics and various other over the counter medica-tions may also contain variable amounts of caffeine in the range of 10-100 mg per dose.
whether it is filtered, percolated, expresso, cafetière or instant, and because arabica beans have about half the caffeine content of robusta beans, on the variety and blend of the coffee as well. With tea, the variety and blend will also influence caffeine content.
The physiological effect s of caffeine and coffee
Following ingestion, caffeine is readily absorbed from the gut and does not accumu-late elsewhere in the body as it is rapidly metabolised by the liver and eliminated. It has an average half-life of approximately four hours, although there is a tremendous individ-ual variation in metabolism and estimates vary between two and ten hours (D ebry, 1994). Smokers metabolise caffeine more quickly than non-smokers, and metabolism is slowed down during pregnancy, especially during the latter stages.
It is well-documented that caffeine is a mild stimulant, acting on the central nervous system (C N S) and that it can increase metabolic rate. Its primary mode of action as a C N S stimulant is related to its role as an antagonist of adeno-sine receptors in the brain, again discussed in detail by D ebry (1994). Various studies have shown that the stimulant effects are beneficial and that the consumption of an amount of caffeine equivalent to that found in two cups of coffee can improve alertness and enhance concentration. For example, a cup of coffee after lunch has been shown to counteract the “post-lunch dip” in concentration (Smith et al.,1990). Research on night shift workers has clearly demonstrated a beneficial effect on alertness and performance of a variety of tasks and it is suggested that caffeinated coffee is beneficial to shift workers in helping to main-tain attention, thereby reducing the likelihood of industrial accidents (Smithet al., 1993; Walshet al., 1990). Studies investigating driver sleepiness (estimated to account for one in five road accidents) indicate that two cups of coffee and a ten minute nap are the most effective solutions (Reyner and H orne, 1997; 1998).
T here is also evidence that a moderate intake of caffeine may improve athletic perfor-mance, for example in swimmers (M acIntosh and Wright, 1995) and cyclists (Pasman et al., 1995), at levels of intake which do not adverse-ly affect urinary caffeine levels.
Some people who drink caffeine-containing beverages in the late evening find that their
sleep is delayed and there is evidence that there is wide variation in sleep responses to caffeine (D ews, 1982). Other research undertaken in groups of self-rated “poor” and “normal” sleepers suggests that a higher proportion of poor sleepers metabolised caffeine slowly (T iffinet al.,1995). C affeine tolerance appears to be a matter of individual sensitivity and decaffeinated beverages are an alternative for people who believe they are sensitive in this way.
C affeine is also a mild diuretic, acting on the kidney to increase blood flow and to decrease the reabsorption of sodium and water. It has been shown to increase the fre-quency of urination (C reighton and Stanton, 1990), but claims that caffeine has a dehydrat-ing effect appear to be unsubstantiated. Patients with kidney stones are routinely advised to increase fluid intake and recent research involving 81,000 women (C urhan et al.,1998) has shown that drinking coffee reduces the risk of kidney stones by 10 per cent for each 200ml cup consumed daily, confirm-ing the result of an earlier study in 45,000 men (C urhan et al.,1996).
Habit uat ion
Coffee and healt h
Over the years there have been numerous health scares concerning caffeine and coffee, but despite a vast amount of research, the evidence to support most of these is limited. F urthermore, the strong positive correlation between the consumption of coffee, tobacco and alcohol, and even inactivity, may some-times confuse the picture presented by epi-demiological studies and must be properly taken into account in their interpretation (D ebry, 1994). T his is particularly relevant in relation to the debate about coffee drinking and cardiovascular disease (C VD ) and its recognised end points of myocardial infarction and cardiac arrhythmias and the risk factors of hypertension and raised blood cholesterol. T he consensus of published literature provides little support for a strong association between coffee drinking and C VD and no support for the suggestion that caffeine or coffee are causal factors (G urr, 1997). M any physicians and their patients believe that cardiac arrhythmias (irregular heartbeats) are caused or exacerbat-ed by caffeine but a review by Lynn and Kissinger (1992) concluded that the frequency of arrhythmia was not significantly increased by caffeine and more recently N ewby and colleagues (1996) concluded that caffeine restriction has no role in management of patients with this disorder.
Although there have been small individual studies which appear to show that caffeine may contribute to increases in blood pressure, the majority of studies, including a recent analysis of data from the M ultiple Risk Factor Inter-vention Trial (Stamleret al., 1997), indicate that regular moderate coffee consumption has little influence on blood pressure. It is also clear that decaffeinated, instant, expresso and filtered coffee have no influence on blood cholesterol, but boiled coffee, which is decant-ed without filtering, and cafetière coffee have been reported to increase it, because of the presence of two lipid-soluble substances, cafestol and kahweol (U rgertet al., 1996). H owever, the cafetière study has been criti-cised (G urr, 1997) on the grounds that the changes observed over the timescale of the study could be explained by seasonal fluctua-tions in blood cholesterol and because the maximum effect was observed after consump-tion of large amounts of very strong coffee (more than five cups per day), equivalent to 10-13 cups of normal strength coffee. Overall,
the view that coffee drinking, as practised in the U K is not a risk factor for coronary heart disease (C H D ) is confirmed by a report of the D epartment of H ealth C OM A C ardiovascular Review G roup (1994) as well as by the British H eart Foundation (1996), which concluded that moderate coffee consumption (up to six cups per day) would not significantly increase the risk of C H D or stroke in normal healthy people.
Similarly, there is no conclusive evidence that coffee drinking or caffeine is a risk for the development of human cancer. In fact there is epidemiological evidence from several studies suggesting that coffee consumption may be protective aginst colorectal cancer ( Baron et al.,1994; Olsen and Kronberg, 1993; Tavani et al., 1997). T he mechanism of this effect is as yet unexplained but could, at least in part, reside in the presence of antioxidant substances in coffee (see section on antioxi-dants).
G uidelines on diet, nutrition and cancer, issued by the American C ancer Society (1991) found no evidence to recommend against moderate coffee intake. Subsequently, numer-ous papers have been published showing no causal link between coffee consumption and development of cancer at any site, including a large study of nearly 43,000 people in N orway, where coffee consumption is high (Stensvold and Jacobsen, 1994). A comprehensive review of diet and cancer by the World C ancer Research F und (1997) also concluded that there was no significant relationship with regular coffee and/or tea drinking and cancer at any site and the report of the D epartment of H ealth C OM A (1998) Working G roup on D iet and C ancer observed only a weak associa-tion between high levels of coffee consumpassocia-tion and increased risk of pancreatic cancer.
C affeine and coffee drinking have also been implicated in osteoporosis. It is well docu-mented that there are a variety of aetiological factors contributing to the development of this condition over a long period of time, including oestrogen deficiency, inadequate physical activity, smoking, and nutritional factors (British N utrition Foundation, 1989).
and discrepancies in caffeine measurement (Lloyd et al.,1997). Overall, it can be conclud-ed that moderate caffeine consumption is not an important factor for osteoporosis provided that people, especially women, consume a healthy diet containing adequate amounts of calcium. T his is emphasised by a study of 980 postmenopausal women among whom lifetime caffeinated coffee intake of two cups daily was only associated with decreased bone density in those who did not consume milk on a daily basis (Barrett-C onnor et al.,1994). In fact drinking coffee can be a useful way of intro-ducing milk into the diet, especially among young women who, in the U K, have signifi-cantly lower calcium intakes than older women (G regory et al., 1990).
N umerous studies have examined the association between coffee and/or caffeine consumption and various reproductive factors. Although some negative factors have been reported, there is no conclusive evidence that either coffee or caffeine have significant effects on conception, spontaneous abortion, or fetal and subsequent infant development (G urr, 1994). T he C entre for Pregnancy N utrition states that “information available at present suggests that five cups of ordinary strength coffee per day (ten cups of tea or cocoa or 12 glasses of cola) is a safe upper limit for preg-nant women”. C affeine can diffuse into breast milk, but one cup of coffee does not present a significant dose of caffeine to the infant (see G urr, 1994).
Ant ioxidant s in coffee
C offee, in common with many other plant materials, contains a wide array of substances with antioxidant properties. T here is mounting evidence that antioxidants are important in protecting against a range of diseases, includ-ing cardiovascular disorders, cancer and other degenerative and age-related conditions such as cataract and Alzheimer’s disease (Seis and Krinsky, 1995). C offee contains a number of the polyphenolic compounds called flavonoids, which possess these properties, and recent research suggests that other substances with antioxidant activity may be generated during the roasting process (N icoli et al., 1997). Although this is a new area of research, requir-ing further substantiation, it does suggest that coffee may contribute to antioxidant protec-tion in the body.
What is an accept able level of caffeine consum pt ion?
Research continues and should be subjected to careful and critical evaluation, but it can be concluded from this brief overview that a moderate intake of caffeine-containing bever-ages does not pose a risk to health and may even offer some benefits. A small number of people may be especially sensitive to the effects of caffeine on the central nervous system and they need to be aware of their own tolerance level and to drink decaffeinated beverages if necessary. T he scientific studies on the physio-logical and health effects of caffeine have generally used between 250mg and 600mg caffeine as representative of people’s average daily intake. A moderate intake is accepted to be about 300 to 400mg per day, i.e. four or five cups of average strength coffee daily, although it is important to take account of caffeine from other sources as well. H owever, an intake of 500 to 600mg daily, equivalent to six cups of strong coffee, appears to be without adverse effects in most people.
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