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Caffeine is a known diuretic, stimulates urine production, and I have even seen "potential dehydration" as one of the arguments in the perennial "is coffee good or bad" debate. It also changes blood pressure, although I don't know if it is only through dehydration or if it has more direct effects on the cardiovascular system.

But coffee has many more active substances than just caffeine. What effect does the drinking of coffee have on fluid balance in the body? How does it influence urine excretion, the excretion of other fluid-influencing substances such as salt, is it related to blood pressure, does it change the levels of blood pressure regulating hormones (vasopressin, angiotensin, etc.)? Or are all such effects of coffee purely based on its caffeine content?

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    I addressed the role of caffeine on fluid balance in the body, as the other items are outside my expertise, I would suggest that the effect of caffeine on blood pressure could also make a good stand alone question. – JohnP Feb 19 '16 at 14:43
  • I second @JohnP 's suggestion that the effect of caffeine on blood pressure should be a separate question: these two questions would be easier to tackle individually, and people searching specifically for blood pressure and caffeine would be more likely to find it. – Lucky Feb 19 '16 at 15:08
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While coffee in large amounts can stimulate urine production, it's not enough to produce a dehydration effect, especially in people accustomed to drinking caffeine.

This recent study compared 50 male coffee drinkers in short trials both with and without caffeine, and concluded that in coffee accustomed males, coffee had much the same hydrating effects as drinking straight water.

A study review on 36 years worth of caffeine and tea studies also concluded that caffeine consumption does not lead to excess fluid loss.

The two relevant summaries:

METHOD:

A literature search was performed using the Medline database of articles published in the medical and scientific literature for the period of January 1966-March 2002. Subject headings and key words used in this search were: tea, coffee, caffeine, diuresis, fluid balance and water-electrolyte balance. A secondary search was performed using the bibliographies of publications identified in the initial search.

And from the same review:

CONCLUSION:

The most ecologically valid of the published studies offers no support for the suggestion that consumption of caffeine-containing beverages as part of a normal lifestyle leads to fluid loss in excess of the volume ingested or is associated with poor hydration status. Therefore, there would appear to be no clear basis for refraining from caffeine containing drinks in situations where fluid balance might be compromised.

Now to the caffeine itself: This study examined energy drinks, specifically caffeine and taurine, and concluded that the diuresis was largely controlled by the caffeine, as taurine by itself did not produce the same effects.

The diuretic effect of caffeine is noted in larger amounts, usually the amount found in 3-6 cups of coffee (Depending on how accustomed the individual is to the effect), or 8-10 cups of tea. While decaffeinated coffee still has some residual caffeine, it would not be enough to have the same effect as fully caffeinated drinks.

As far as the excretion of other substances, this study examined the effect of caffeine on excretion of certain elements (calcium, sodium, magnesium and potassium), with the conclusion that all but potassium had higher urinary output levels after caffeine consumption. I am uncertain how that would relate to cardiac function and blood pressure, however.

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