XII. Possible Harmful Effects of Caffeine at the Individual or Social LevelAbuse or Misuse
Negative social consequences of coffee drinking are not claimed, but DSM-IV (1994) lists caffeine intoxication, caffeine-induced anxiety, and sleep disorders as caffeine-induced disorders.
Despite its wide availability, caffeine intoxication occurs rarely. The lethal dose has been estimated to be in the range of 10 g (Ritchie, 1975), which would correspond to about 100 strong coffees. Provided adequate emergency measures are taken, patients appear to survive levels up to 1 mM or even slightly above, but still higher levels are fatal (Rivenes et al., 1997). Among the 3749 cases of "caffeine exposure", registered during 1 year by the American Association of Poison Control Centers, there were only three fatalities (Litovitz et al., 1987).
Although caffeine overdoses can induce anxiety, there is little and in part controversial evidence as to whether coffee might play a significant role in this disorder (see above Section IVB). No significant association between anxiety and coffee or tea consumption was seen in a US nationwide sample of 3854 subjects (Eaton and McLeod, 1984) or in an English sample of 9003 individuals (Warburton and Thompson, 1994). The same negative result holds also for depression (Warburton and Thompson, 1994), confirming the results of an earlier larger study (Jacobsen and Hansen, 1988). One possible explanation for this failure to find relationships between coffee drinking and anxiety may be that anxious subjects avoid coffee. In fact, avoidance of coffee by anxious subjects has been reported repeatedly over the last decades (Boulenger et al., 1984; Uhde et al., 1984; Lee et al., 1985). A review on putative correlations between sleep disorders or insomnia and caffeine consumption would yield a similarly controversial picture, as discussed above in the chapter on tolerance for the sleep-disturbing effects of caffeine. As in the case of anxiety, it appears that by far the most consumers of coffee adapt their intake both with respect to time of day and dosage so as to avoid acute sleep disturbance or chronic insomnia.
When people are interviewed about psychoactive substance use disorders, seven criteria are used: 1) tolerance; 2) withdrawal; 3) substance often taken in larger amounts or over a longer period than intended; 4) persistent desire or unsuccessful efforts to cut down or control use; 5) a great deal of time spent in activities necessary to obtain, use, or recover from the effects of the substance; 6) important social, occupational, or recreational activities given up or reduced because of substance use; 7) use continued despite knowledge of a persistent or recurrent physical or psychological problem that is likely to have been caused or exacerbated by substance use. Because coffee or caffeine-containing nutrients or drinks are widely available and culturally accepted, their consumption does not usually have negative social consequences. Indeed, in the studies on caffeine dependence, criteria 3, 5, and 6 are usually excluded. Especially in the US there is no doubt that many individuals reduce or try to reduce their caffeine intake due to perceived health problems (see Hughes and Oliveto, 1997). Indeed, not less than 14% of all erstwhile consumers in Vermont had stopped the intake of all caffeine-containing beverages largely for this reason (Hughes and Oliveto, 1997). This relates to criterion 7 if these individuals have difficulties in reducing intake. One interesting question is therefore if caffeine poses a real health hazard or if the negative association between health and caffeine is a perceived one.
Considering the individual consequences, caffeine-induced dysphoria and nervousness could negatively influence the relationship of some individuals in the society. However, this aspect of caffeine consumption does not seem very pertinent.
The possibility that caffeine consumption may pose major health risks has been widely discussed (see James, 1991). Caffeine does raise mean arterial blood pressure by a few millimeters of mercury; this has been suggested to pose a health risk by some (James, 1991), but not by others (Tuomilehto and Pietinen, 1991). More recently, greater concern has been voiced about the ability of caffeine to raise plasma cholesterol (Thelle et al., 1983, 1987). It is now known that the increase in plasma cholesterol is due to two diterpenes: cafestol and kahweol (see Urgert and Katan, 1997). These compounds are largely eliminated when coffee is prepared by filtration or percolation or from instant coffee. By contrast, boiled coffee and Turkish coffee, and to a lesser extent espresso and mocha coffee, do contain these diterpenes and have been shown to raise cholesterol levels by some 0.1 to 0.5 mM during prolonged use (see Urgert and Katan, 1997). The rather low intake of these brews suggest that coffee contribution to overall cardiovascular risk is small (Myers and Basinski, 1992; Greenland, 1993; Kawachi et al., 1994; Willett et al., 1996), even though it has been calculated that the large-scale switch from boiled to filtered coffee might have contributed to a third to half of the 10% reduction in serum cholesterol noted in Scandinavia since 1970 (Johansson et al., 1996b; Pietinen et al., 1996).
Another potential factor in predicting cardiovascular risk is plasma homocysteine. It was recently shown that, although coffee drinking per se has a limited effect on this variable, combined smoking and high coffee drinking was associated with an increased number of subjects with very high plasma homocysteine levels (Nygård et al., 1998). It is, however, too early to decide on the importance of these findings, particularly because the relevant intervention studies have not been performed.
There are several reports showing that very high doses of caffeine can have mutagenic or carcinogenic effects (see Mohr et al., 1993). This has raised concerns about cancer risks following normal caffeine consumption, but a careful consideration of the evidence "provides further reassuring information on the absence of any meaningful association of coffee with most common cancers" (La Vecchia, 1993).
Although there is a public perception (especially in the US) that caffeine is detrimental to one's health, this has a surprisingly weak basis in reality. On the other hand, health problems from other causes might provide an incentive to cease caffeine consumption, especially in the form of coffee. If this is true, then ex-caffeine consumers may constitute a subgroup with more health problems than the average population. This could be a concern in the interpretation of some epidemiological studies.