Among the most common preconceived ideas about tea, dehydration is undoubtedly one of the most important. Why such a preconceived idea when tea is composed mostly of water? Which molecule would be involved? How exactly does it work during consumption?
Caffeine, a historical diuretic molecule
One of the major organs in the control of water levels in the body is the kidney. It filters and secretes the excess water in the body depending on hormones and the detected plasma concentration. The phenomenon of production and excretion of urine is called diuresis. We speak of a diuretic effect when a molecule induces the secretion of urine.
The caffeine contained in coffee and tea can bind to adenosine receptors located in different parts of the kidney (distal nephron and proximal tubule, i.e. production and excretion of urine respectively). It will inhibit the activation of these receptors which prevents sodium absorption at low doses. At high doses (≥500 mg), the binding of caffeine to these receptors will induce a diuretic effect, preventing water reabsorption and a secretion of urine by the kidneys. Thus, since the first studies more than 80 years ago, researchers have wondered whether tea induces diuresis, and by extension whether the amounts of caffeine in tea that are ingested are sufficient to induce dehydration.
A case study of black tea
The first study that investigated the direct effects of tea consumption in practice (Western/English style brewing, 24cl including 2cl of milk, one black tea bag from an unknown national company) came out in 2011. This study was based on different protocols: people drank tea or only water. There were no dietary restrictions. Tea drinkers drank 6 mugs per day (252 mg caffeine) or 4 mugs per day (168 mg caffeine). 6 measurements of urine volumes, plasma volumes, mineral salt, creatinine and protein concentrations were performed over a 12-hour period.
The authors found no change in mineral salts, urine excretion and plasma volume. The consumption of black tea does not seem to have a diuretic effect in the body.
However, it is important to note that the study does not take into account the liabilities of each person included in the study (daily tea drinkers or not, quantity, etc.) and that the number of people included is small (21 people).
Caffeine and diuresis: also a matter of habit?
In 2014, Killer et al. conducted a study on daily coffee drinkers that aimed to show that people who drank coffee regularly did not experience the diuretic effect of caffeine (urine volume, concentration of excreted mineral salts). To start their study, the authors compiled a number of data in the scientific literature: follow-ups of people who ingested caffeine tablets, coffee drinkers, energy drinks and tea. In 4 of these studies, the people evaluated were either daily caffeine consumers who stopped consuming caffeine for several days to several weeks or people who consumed little caffeine. For caffeine doses ranging from 250 to 680 mg of caffeine, the studies concluded that there was a diuretic effect induced by caffeine and caffeinated drinks. On the other hand, 6 out of 7 studies including people consuming caffeine daily (114 to 700mg of caffeine) did not conclude to a diuretic effect of caffeine and caffeinated drinks.
These studies would suggest that a person drinking caffeinated beverages on a daily basis would develop an addiction to caffeine, limiting or preventing the diuretic effect of caffeine.
Although still marginalized, studies on caffeine and coffee and tea consumption point out several things. Caffeine would only have a "dehydrating" effect at doses higher than 500 mg (well above the standards recommended by the health authorities). Regular consumption of tea below the maximum consumption limits of caffeine would not cause dehydration. Finally, the diuretic effect of caffeine could be attenuated if the person consumes tea daily and without interruption of its consumption. Studies on tea are still few and are mainly based on the most common infusion methods in the West (tea bags < 2g of tea, addition of milk or not, high volume of water) and not on reinfusion methods like Gong Fu Cha.