There’s a common belief that drinking caffeinated drinks doesn’t hydrate you as well as water, or other caffeine-free beverages. This is because caffeine is known to have a diuretic effect, so it makes sense to assume that a caffeinated drink would have the same impact.
However, evidence to suggest this is the case is, at best, scant, especially in the 40-or-so milligram dose you’re likely to find in a typical up of Green Tea. For example, a study conducted at Loughborough University found that, only when individuals abstained from caffeine for a number of days and were then given a dose equivalent to about 5-8 cups of tea (250-300mg), there was a temporary increase in their fluid output (they peed a bit more than usual). Doses equivalent to a normal cup of green tea were, on the other hand, found to have no diuretic effect, and they found that regular caffeine consumers became resistant to caffeine’s diuretic effects altogether!