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When blood reaches the kidneys, it is filtered into nephrons. This filtrate will eventually become urine after some things are added and other things are removed. How much water is removed from the filtrate determines how much we pee. If more is reabsorbed, then we pee less. If less is reabsorbed, then we pee more. ADH's action is to increase the reabsorption of water (therefore, less pee). However, if ADH is blocked (which is what caffeine does), then less water is reabsorbed. Therefore you pee more.

So that is a "interesting finding". The study being referred to does not seem to be cited. However, from the description, it sounds like the study design is flawed. The basic premise sounds valid. Compare chronic caffeine users to chronic non-caffeine users. And see how much urine they each produced.

However, the design flaw is that they did not start with two similar groups and introduce a difference. They have one group of chronic caffeine users and one group of chronic non-caffeine users. Then they gave caffeine to one group (the caffeine users) and nothing to another group (the non-caffeine users). Therefore, they cannot really prove anything. To prove that caffeine does not make you pee more, you'd have to set up a study where the only difference between two groups is if they took caffeine or not. Like a bunch of well-hydrated non-caffeine users are split into two equal groups. One group is given caffeine and another is given a placebo. The average urine outputs of the groups are compared. If they output is the same, then you could conclude that caffeine does not make you pee more (as in the only difference between the groups does not cause a difference in the outcome).

Thinking physiologically, the reason the flawed study got that result is that the body naturally works toward an equilibrium state (homeostasis). Caffeine is not a strong diuretic. It is not powerful enough to push your body too far from equilibrium. Chronic caffeine users may pee out all the excess water (ie. water that would dilute sodium concentration to lower than normal) they have. However, chronic non-caffeine users also output that water because they too need to be in the normal range of sodium concentrations. Since both groups are in a steady state, the amount of pee will be proportional to the amount of free water one takes in. Chronic caffeine users adapt to being on the more concentrated end of the range of normal and thus don't feel the need to drink much more free water than non-caffeine users also at the normal range. Basically, the difference is an internal mechanism that causes urination as opposed to an external mechanism (ie. caffeine). All such this study proved is that non-caffeine users don't need caffeine to be able to pee.

I was toldby my doctor to increase caffeine consumption, (that truly made my day!) because it would help stimulate my appetite. I can say it helped . I'm not sure many people would want that side affect though;-)

My understanding of coffee is that there is much much more at work than just the caffeine. There are dozens of other chemical compounds that all have varying effects on body functions and organs, including pretty important ones like the thyroid and the hypothalamus. To generically state that caffeine is all good therefore coffee is too is at best overly simplistic, and in all likelihood misleading and downright dishonest.