OK I searched the archives and came up with zilch.
About 5 years ago I saw a doco about water and hydration. They mentioned the benefits of remaining hydrated the whole day and I have personally done my best to drink more water each day based on this and follow-up information. One of the things mentioned in this documentary was that tea has a diuretic affect on you - that is you lose as much water as you gain from drinking a cup of tea. This came up in conversation recently and I was told that I was wrong. That tea isn’t a diuretic. I checked the Straight Dope archives to no avail. I then Googled it and found out that it is indeed a diuretic but the ratio is 6 to 1 not 1 to 1 as I had thought. That is you need to drink 6 cups of tea (where a cup size isn’t really defined) before you have get a diuretic affect.
All well and good. But all the links I could dig up were on the websites of the major tea manufacturers. So I took them with a grain of salt. So the questions at the end of my rambling are:
Does tea have a diuretic affect?
If so what’s the ratio?
Do different teas have different affects (example Green tea vs traditional Earl Grey)?
If there is no diaertic affect why do I get thirsty after I drink tea? And why does it make me pea?
If the only liquid I drank over a 24 hour period was tea would I by dehydrated by the end?
There are many types of “tea”; some, but not all, are preparations of the tea shrub Camellia sinensis. There are many herbal teas that do not contain any diuretic ingredients at all.
Assuming you are talking about the traditional “Tea”, The most popular preparations vary considerably in diuretic effect (e.g., prepared green tea usually has less caffeine than Black tea). Furthermore, tea contains not only caffeine, but other xanthines, which vary in their diuretic (and stimulant) effects. Even given a specific type of tea, the diuretic effect will depend on the brewing method, brewing time, and whether additional ingredients are added to the beverage.
Because of the number of variables involved, there is certainly no fixed ratio or number of cups that would always produce a diuretic effect.
Furthermore, the experts are now saying that caffeine’s diuretic effect is overstated. I most recently read it in an article in National Geographic - the body simply doesn’t excrete as much water as it takes in from coffee and tea, so they’re not a problem that way. That said, coffee still does make me pee a lot. I drink a lot of water with it for that reason. But it’s not just running straight out your kidneys as soon as you drink it.
Yes, the diuretic effects of caffeine are exaggerated. Frankly, if you are already adequately hydrated, then water itself is a diuretic - drink an extra liter and you have to pee an extra liter. If you drink a lot of water (or a lot of coffee, tea, or anything with an osmolarity less than that of plasma) you will have to pee a lot.
Also, the whole “eight glasses of water a day” thing is not the result of any sort of medical study or research. How it got to be such a widely accepted “recommendation” is a mystery to me. And don’t get me started on ‘drinking more water prevents constipation’.