Don’t drink coffee or tea to quench your thirst, my grandmother always said: “Because it will dehydrate you.” The caffeine in these drinks causes your kidneys to excrete extra water: caffeine has a so-called diuretic effect. In net terms, it makes you more thirsty, a doctor also suggests The Guardian. But is that actually true?
Coffee contains average about 90 milligrams of caffeine per cup, tea about 40 mg. More than cola (35 mg), but energy drinks contain 80 to 200 mg per portion. Caffeine does indeed have a diuretic – i.e. diuretic – effect a British study in it Journal of Human Nutrition and Dietetics. But: only if you quickly drink two to three cups of coffee in a row, or five to eight cups of tea, and if you have not had caffeine for a while. In other words, if your body is used to caffeine, then there is no diuretic effect.
How does that effect actually work? For a long time it was thought that the hormone ADH had something to do with it. ADH stands for anti-diuretic hormone. It actually prevents fluid retention: it ensures that water from the ‘pre-urine’ in the kidneys flows back into the blood. This is part of normal moisture regulation: you do not lose a lot of water unnecessarily. The idea was that caffeine inhibits the action of ADH, and therefore causes you to urinate too much water. But already in 1990 showed Swiss research that caffeine has absolutely nothing to do with the effect – or presence – of ADH.
Other mechanisms appear to be more important for the diuretic effect of caffeine. For example, caffeine blocks certain receptors in the kidney cells that normally help with water recovery. But in people who drink caffeine every day, the body compensates do this by creating more of those receptors.
Female cycle
In addition, it is of course true that coffee and tea largely consist of water. That water more than compensates for any diuretic effect. This was also evident from a British study that investigated which is better for your fluid balance: drinking water or black tea. It turned out to make no difference at all. The same goes for coffee, according to another British study: “These data suggest that coffee, when consumed in moderation by caffeine-accustomed men, has the same hydrating properties as water.” Too bad the British only examined men. They may have wanted to rule out effects of the female cycle.
Are there any? Yes, thus an American study in it Journal of Caffeine Research. Women experience greater fluctuations in their blood pressure after caffeine use than men. This is probably related to sex hormones such as estradiol. And blood pressure plays a role in urine production. But a recent Polish study who directly compared the effect of caffeine on urine output between men and women, found no significant difference. That study did find that mainly men experience positive effects of caffeine, and mainly women experience negative ones.
In short, it does not matter for your fluid balance whether you drink coffee, tea, or water. At least, for moderate and regular caffeine users. But caffeine does of course have other negative effects: it increases anxiety blood pressure and heart rateyou keep awake longer in the evening and can headache, stomach problems and cause unrest.
So, above all, drink water to quench your thirst. In that respect, Grandma was right.
NEW: Give this item as a gift
As an NRC subscriber you can subscribe every month 10 articles give as a gift to someone without an NRC subscription. The recipient can read the article directly, without a paywall.