It is widely claimed that despite being made mostly of water, drinking coffee or tea will actually dehydrate you, because the caffeine is a a diuretic.
Here is an example from the SFGate news site
The diuretic effect of caffeine increases your urine output. The more you drink in a short amount of time, the more water you could lose, especially if you are not used to caffeine. If you are exercising or are in a hot environment where you are already losing extra water due to perspiration, taking caffeine could magnify the potential for dehydration. For approximately every 100 milligrams of caffeine you consume – for instance, the approximate amount in one cup of coffee or two cups of black tea – you should drink an additional cup of water to compensate for the drug’s diuretic effect.
Alcoholic and caffeinated beverages, such as coffee, teas, and colas, are not recommended for optimal hydration. These fluids tend to pull water from the body and promote dehydration.
Healthy Beginnings Magazine makes the same claim:
Many people simply do not take in this amount of water, preferring instead to get their intake of liquids through coffee, tea, sodas, beer or wine.
The problem is that most of these drinks have a diuretic effect which forces the body to eliminate more water than it is actually taking in – causing dehydration. In fact, some believe there is an epidemic of chronic dehydration in the US due to our caffeine and alcohol consumption.
Does this have any scientific basis?