Tea and Hydration: What Science Says

20 mars

Among the most common misconceptions about tea, dehydration undoubtedly occupies pride of place on the podium. Why is this such a common misconception, when tea is composed mainly of water? What molecule is involved? And how exactly does it work when consumed?

Caffeine, a historic diuretic molecule

One of the major organs involved in controlling water levels in the body is the kidney. It filters and secretes excess water from the body, depending on hormones and the detected plasma concentration. The phenomenon of urine production and excretion is called diuresis. A diuretic effect occurs when a molecule induces urine secretion.

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. urine production and excretion respectively). It inhibits activation of these receptors, preventing sodium absorption at low doses. At high doses (≥500 mg), caffeine binding to these receptors induces a diuretic effect, preventing water reabsorption and urine secretion by the kidneys. Thus, since the first studies over 80 years ago, researchers have wondered whether tea induces diuresis, and by extension whether the quantities of tea caffeine ingested support the diuretic effect.

A case study of black tea

The first study to investigate the direct effects of tea consumption in practice (Western/English-style infusion, 24cl including 2cl milk, one bag of black tea 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 a day (252 mg caffeine) or 4 mugs a day (168 mg caffeine). 6 measurements of urine volumes, plasma volumes, mineral salts, creatinine and protein concentrations were taken over a 12-hour period.

The authors found no change in levels of mineral salts, urine excretion or plasma volume. Consumption of black tea does not therefore appear to have a diuretic effect on the body.

It is important to note, however, that the study does not take into account the liabilities of each individual 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 question of habit?

In 2014, Killer et al. carried out a study of daily coffee drinkers which aimed to show that people who drank coffee regularly did not experience the diuretic effect of caffeine (urine volume, concentration of mineral salts excreted). To begin their study, the authors compiled a number of data from the scientific literature: follow-ups of people who had ingested caffeine tablets, coffee drinkers, energy drinkers and tea drinkers. In 4 of these studies, the people evaluated were either daily caffeine consumers who had 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 caffeine and caffeinated beverages induced a diuretic effect. On the other hand, 6 out of 7 studies involving people consuming caffeine daily (114 to 700mg of caffeine) did not conclude to a diuretic effect of caffeine and caffeinated beverages.

These studies therefore suggest that a person drinking caffeinated beverages on a daily basis would develop an addiction to caffeine, limiting or even preventing caffeine's diuretic effect.

Although still marginalized, studies on caffeine and the consumption of coffee and tea highlight several points. Caffeine only has a “dehydrating” effect at doses above 500 mg (well above the standards recommended by health authorities). Regular tea consumption below the maximum caffeine consumption limits would not cause dehydration. Lastly, the diuretic effect of caffeine may be attenuated if tea is consumed daily without interruption. Studies on tea are still few and far between, and are mainly based on the infusion methods most widely used in the West (tea bags < 2g, milk added or not, high volume of water) and not on reinfusion methods such as Gong Fu Cha.