One of the reasons that many people advocate drinking lots of water and think that they would not otherwise get enough fluids is that they fear that other beverages ‘don’t count’ towards your fluid requirements. Fizzy drinks, coffee and tea are not supposed to count towards the liquid your body needs because they contain caffeine, and since caffeine is a diuretic, these beverages must dehydrate you.
Diuretics are substances that promote the formation of urine within the kidneys and thus lead to more peeing. Water, cranberry juice and alcohol are weak diuretics. Caffeine is thought to be a diuretic, but the idea that caffeinated drinks have a strong diuretic effect is not well-supported by science. A study back in 1928 showed that caffeine acted as a diuretic in patients at rest, but this landmark study, upon which many doctors and dieticians have based their recommendations, involved only three subjects watched over a period of a few hours. Since 1928, more and more studies have begun to suggest that caffeine is only a mild diuretic that poses no harm to your health or even to how well you perform during exercise. In a series of experiments by A. C. Grandjean and colleagues in eighteen healthy adults, caffeinated beverages did not have any effect on changes in body weight, changes in urine or blood concentration, and changes in the concentration of electrolytes in the body. These scientists concluded that ‘advising people to disregard caffeinated beverages as part of the daily fluid intake is not substantiated by the results’ of this study. Three review articles have been published that summarize all of the studies looking at the diuretic effects of caffeinated beverages, and all three conclude that caffeinated beverages and water affect the body water balance similarly during exercise.
Did you get that? Even during exercise, when your body most needs fluids, caffeinated drinks work just as well as water. In twenty-three experiments testing caffeine doses and whether they made people pee more, no difference in urinating was reported in seventeen of the studies, which included consumption of up to 553 milligrams of caffeine (that’s around five cups of coffee).
Most of the studies looked only at how much peeing you did and at other measures of body fluid concentration over the course of a few hours. But others did look at caffeine consumption over a long period. Human water balance is always going to be challenged by periods of diuresis (or things prompting you to make more urine) over the course of a given day. But what if you drink caffeine every day? Will you be chronically dehydrated? The answer, again, is no (if you are healthy). Only two studies in the reviews looked at the chronic effects of drinking caffeinated beverages; they, too, found no indication of dehydration by any measure used to look at body chemistry and hydration status.
Caffeinated beverages might make you pee a bit more, and certain fizzy drinks may contain more sugar than you would want to take in, but they really are a reasonable source of the fluid your body needs. Remember, your body is very good at keeping the fluid it needs. Even if caffeinated drinks are not your favourite choice for other reasons, you should not avoid them because you think they will dehydrate you.