Someone claimed coffee is a Diuretic and that drinking it dehydrates you.
My impression was it may be a diuretic, but the effect is not more than the water required to brew, so drinking a cup is still a net positive of hydration.
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You are right that the effect is negligible. A single cup of coffee normally does not have enough caffeine to have any measurable diuretic effect at all, and it does contain about a cup of water. So it will be hydrating. You need to drink about 3 cups of coffee before there is any measurable effect and by then you have drunk more then enough water to hydrate you. It is more complex then this though as the diuretic will cause your kidneys to attempt to lower the water content in the blood bellow its normal levels. So it is not as simple as x amount of diuretic cause you to produce y amount of urine and it is therefore possible that coffee will dehydrate you, especially if it is strong. On the other hand as your body gets used to caffeine the diuretic effect goes away. That means that regular coffee consumption can not dehydrate you at all.
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