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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.

The Cleveland Clinic makes a similar claim, also including alcohol (which is covered by a related question here:

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?

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There is no convincing evidence that consumption of caffeine or tea leads to dehydration (fluid loss) based on available research. Studies cited below indicate (a) that caffeine is a not a very efficient diuretic, (b) that coffee/tea's diuretic effects are similar to that of water and (c) that moderate caffeine consumption does not lead to dehydration.

The Institute of Medicine of the National Academy of Sciences conducted an extensive analysis on the research concerning hydration and released fluid intake recommendations in their 2004 report. The dietary reference intakes column mentions that "All sources can contribute to total water needs: beverages including tea, coffee, juices and soda".

Further research findings include:

  1. A 2003 review by R. J. Maughan et.al. on Caffeine ingestion and fluid balance. It concludes that there is no published support for the view that "consumption of caffeine-containing beverages leads to fluid loss." "Therefore, there would appear to be no clear basis for refraining from caffeine containing drinks in situations where fluid balance might be compromised."

  2. A review of 10 studies by Lawrence Armstrong from the University of Connecticut concludes that caffeine is a mild diuretic at most, with 12 out of 15 comparisons showing that people urinated the same amount, regardless of whether the water they drank contained added caffeine or not.

  3. Results from a randomized controlled trial in 2011 show that black tea is not significantly different from water in the maintenance of normal hydration in human subjects. It was concluded that black tea, in the amounts studied, offered similar hydrating properties to water.

  4. Research by Sophie Killer in 2014 at Birmingham University in the UK, measured the volume of urine in subjects, tested their blood for signs of kidney function, and calculated the total amount of water in the body. The results showed that men who drank four cups of coffee a day were not any more dehydrated than those who drank water alone.

  5. Advising people to disregard caffeinated beverages as part of the daily fluid intake is not substantiated by the results of a study by Grandjean AC et.al. in 2000. However, this study was supported by a grant from The Coca-Cola Company.

  • 2
    Wait you are quoting a "hydration expert" who works for Coke-Cola? – Sam I Am Aug 21 '15 at 20:04
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    Ms. Ann C. Grandjean serves as Assistant Professor in the Sports Medicine Program at the University of Nebraska Medical Center and is a Graduate Faculty Member at the University of Nebraska. She is an acknowledged expert in sports nutrition authoring articles on nutrition/sports performance, a consultant to the U.S. Olympic Committee and the Philadelphia Flyers and also a Fellow of the American College of Nutrition and the American College of Sports Medicine. She also serves as Director of SmartMouth Technologies which develops Web-based marketing and CRM strategies for the grocery industry. – pericles316 Aug 22 '15 at 5:28
  • ...who is paid to be a Coke-Cola mouthpiece. I'm not interested in her degrees or jobs anyways, that's an appeal to authority. – Sam I Am Aug 23 '15 at 19:56
  • Research by R. J. Maughan and Sophie C. Killer still adds to the evidence that 'coffee provides similar hydrating qualities to water' even if the claimed 'Coke mouthpiece' is discredited. – pericles316 Aug 24 '15 at 4:03
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    Can you provide evidence for your quote that she is a paid mouthpiece and does not follow the code of ethics such as 'practitioners promote and endorse products "only in a manner that is not false and misleading.' I could only find that her study was supported by a grant from the Coca Cola company. Furthermore many companies resort to this kind of practice which is discussed here-uk.businessinsider.com/… – pericles316 Aug 26 '15 at 4:58

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