… the suggestion that caffeine administration is used primarily for withdrawal reversal is equivocal for several reasons. First, there are positive effects of moderate caffeine use which may be reinforcing on their own (Smith, 2002). Second, there are infrequent caffeine users who do not exhibit withdrawal symptoms when abstaining from caffeine. Although non-habitual caffeine consumers are rarely studied, the studies that have been done suggest that caffeine administration can stimulate mood and cognitive performance even in moderate and irregular caffeine users (Childs and de Wit, 2006; Haskell et al., 2005) and that a primary reason for infrequent use of caffeine may be the negative physiological and psychological symptoms that can occur (Stern et al., 1989). Third, not all habitual caffeine consumers exhibit withdrawal symptoms (Hughes et al., 1998), thus counteracting withdrawal symptoms as a motive for caffeine use would not apply to these individuals. It is also possible that there are multiple reasons for caffeine self-administration. In some individuals, it may be primarily for withdrawal reversal, but in others, it may be for the mild stimulant effects, and in some cases, it may be both. Most epidemiological studies suggest that there is a broad range of sensitivity to caffeine within the population, which may be associated with variability in caffeine self-administration, caffeine tolerance, and caffeine withdrawal and could be mediated, at least in part, by genetic polymorphisms that relate to enzymatic breakdown of caffeine or adenosine receptor function (Cornelis et al., 2007; Retey et al., 2007). These biological differences could mediate susceptibility to withdrawal and/or positive effects of caffeine and, therefore, may underlie motivation for caffeine administration…
… There are also potential health risks and benefits of habitual caffeine use. … Regular, high levels of caffeine consumption (> 450 mg/day) have been shown to increase the risk of cardiovascular disease (CVD) in some studies (Greenland, 1993; Panagiotakos et al., 2003). However, there are an equal number of studies that show no relationship between caffeine consumption and CVD (Hart and Smith, 1997; Stensvold et al., 1996). High levels of caffeine use are also associated with calcium excretion and bone loss, which may contribute to osteoporosis (Barger-Lux et al., 1990; Bergman et al., 2000), however, caffeine intake appears to interact with calcium intake, such that the only group in which caffeine consumption increases bone loss is those with low calcium intake (Harris and Dawson-Hughes, 1994; Barrett-Conner et al., 1994). Finally, although the data differ among studies, most agree that high levels of caffeine consumption in women trying to conceive can be associated with low rates of conception (Christianson et al., 1989; Jensen et al., 1998; Stanton and Gray, 1995; Williams et al., 1990) and higher rates of spontaneous abortion (Dlugosz et al., 1996; Fernandes et al., 1998; Srisuphan and Bracken, 1986). As with the other papers reviewed, there is a wide range of doses at which these effects are reported as well as some studies showing no effect of caffeine on fertility (Alderete et al., 1995; Olsen, 1991; Watkinson and Fried, 1985), but the general consensus is that high levels of caffeine consumption may have adverse effects on fertility and the recommendation is for women who are trying to become pregnant to limit caffeine to <300 mg/day (Nawrot et al., 2003).
Despite the potential health risks of caffeine consumption, there are also some reported health benefits. One of the best characterized benefits of coffee consumption is a reduction in the risk of type 2 diabetes mellitus (Salazar-Martinez et al., 2004; Tuomilehto et al., 2004; van Dam and Hu, 2005). Although the mechanism is not known, it appears to be related to coffee consumption and not to caffeine consumption, as decaffeinated coffee provides similar benefits and tea does not (Salazar-Martinez et al., 2004; van Dam and Feskens, 2002). There is also evidence that the thermogenic effects of caffeine can increase energy expenditure (Astrup et al., 1990; Dulloo et al., 1989), and, perhaps, reduce weight gain over time (Lopez-Garcia et al., 2006). Caffeine also appears to improve sports performance (Jones, 2008), including perceived exertion (Hudson et al., 2008) and endurance (Hogervorst et al., 2008). Finally, there is some evidence of an inverse relationship between caffeine consumption and colorectal cancer (Giovannucci, 1998; Tavani and La Vecchia, 2004) and Parkinson’s Disease (Herman et al., 2002; Ross et al., 2000), but the mechanisms for this apparent protection remain unknown. …