methylxanthine-containing foods and drugs for 5 days and who were then were given 642 mg of caffeine in the form of coffee, 24-hour urine output increased by 0.75 ± 0.53 L, a 41 percent increase (Neuhauser-Berthold et al., 1997). Given that the study design did not evaluate habitual intake, it is difficult to determine the extent to which this large amount of caffeine would impact total water needs on a chronic basis.
In an earlier study, the effect of caffeine intake on urinary output was evaluated in eight men who were asked to consume four cups of coffee or six cups of tea/day (providing approximately 240 mg of caffeine/day) for 5 days prior to data collection and then to abstain from caffeine 24 hours prior to data collection (Passmore et al., 1987). The subjects were then given various doses of caffeine (45, 90, 180, or 360 mg) on the study day. Cumulative urine volume 3 hours after consuming the test dose was increased significantly only at the 360-mg dose of caffeine. This is equivalent to four cups of regular brewed coffee (USDA/ARS, 2002).
Caffeine can induce hemodynamic effects not directly related to fluid balance. The acute pressor effects (e.g., vasoconstriction, palpitations) of caffeine consumption are well documented; however, in a review of the relevant literature, there was no clear epidemiologic evidence that habitual caffeine consumption leads to hypertension (Nurminen et al., 1999).
In aggregate, available data suggest that higher doses of caffeine (above 180 mg/day) have been shown to increase urinary output, perhaps transiently, and that this diuretic effect occurs within a short time period (Passmore et al., 1987). Whether or not caffeine ingestion at high amounts leads to a total body water deficit is uncertain (IOM, 2001a), although some have tried to develop a predictive model of water needs based on the limited data available (Stookey, 1999). Hence, unless additional evidence becomes available indicating cumulative total water deficits in individuals with habitual intakes of significant amounts of caffeine, caffeinated beverages appear to contribute to the daily total water intake similar to that contributed by noncaffeinated beverages.
Similar to caffeine, the diuretic effect of alcohol is mediated by the suppression of arginine vasopressin (Stookey, 1999). Increased diuresis was reported during the initial 3 hours of consuming a beverage in which alcohol (ethanol) was present (consumed at level of 1.2 g/kg of body weight in a solution of fruit juice) in healthy,