In a study3 investigating the effect of coffee drinking on urine excretion, the authors observed a significant increase in the volume of urine excreted among those who drank coffee, compared to those who drank the same volume of water. A 41% increase in 24-hour urine excretion was observed when the participants were given six cups of coffee (642mg of caffeine). However, the subjects abstained from foods and beverages containing caffeine for five days prior to treatment, effectively making them ‘caffeine naive’. In these circumstances, a more pronounced effect of caffeine-containing drinks, like coffee, is to be expected.
Another study4 investigated the effect of several caffeinated beverages on hydration. There were no significant differences between the outcomes of various hydration indicators, including urinary excretion and weight change, showing that hydration status was not altered. The study design was robust and numerous indicators were taken into account.
A further literature review5 concluded that the acute ingestion of caffeine in large doses (at least 250-300mg, equivalent to the amount found in 2-3 cups of coffee or 5-8 cups of tea) results in a short-term stimulation of urine output in individuals who have been deprived of caffeine for a period of days or weeks. Tolerance to the mild, short-term diuretic effects of caffeine develops, however, and these effects are diminished in individuals who regularly consume tea or coffee. Doses of caffeine equivalent to the amount found in standard servings of tea, coffee and carbonated soft drinks appear to have no diuretic effect.
A more recent study6 of 59 subjects looked at the influence of caffeine on possible fluid imbalance under three conditions. The participants consumed 3mg caffeine per kg/body weight on days 1 to 6. On days 7 to 11, the treatment phase, subjects consumed either 0mg, 3mg or 6mg caffeine per kg/body weight. There was no evidence of dehydration in the healthy, active males taking part in the study across the conditions.
A double-blind, randomised, crossover trial published in 20137 investigated the effects of a moderate does of caffeine (5mg/kg/bw/d) on total body water, extracellular water, and intracellular water in 30 non-smoking males aged between 20 – 39 years. These authors found that a moderate amount of caffeine, equivalent to 5 espresso cups of coffee or 7 servings of tea, does not alter total body water and fluid distribution in healthy men, regardless of body composition, physical activity, or daily water ingestion.
A further study published in 2014 directly assessed the effects of moderate coffee consumption compared to consumption of equal volumes of water in a group of 50 free-living male participants. This group used a variety of well established hydration measures including body mass and total body water, as well as blood and urine analysis. The researchers found no significant differences in total body water or any of the blood measures of hydration status between those who drank coffee or those who drank water, nor did they find any differences in 24-hour urine volume or urine concentration between the two groups8.
Caffeinated beverages and fluid intake
On the initiative of a group of scientists, a Beverage Guidance Panel was set up in US, with the objective of systematically reviewing the literature9 on beverages, and providing guidance to consumers. As far as caffeinated beverages are concerned, the Panel states that caffeine consumption of up to 500mg/day (the equivalent amount found in approximately 5 regular cups of coffee) does not cause dehydration.
A paper1 reporting on a conference in North America advises consumers that drinking a variety of caffeinated beverages, including coffee, can contribute to meeting the body’s requirement for fluids.