The overall consensus of published studies in the medical literature
is that drinking coffee, whether caffeinated or not, and other
caffeinated beverages will not have a significant dehydrating effect,
if any at all. One study did indicate that mild dehydration may be
experienced with high coffee consumption.
My search strategy was to search the bibliographical database Medline
at Pubmed on the web site of the National Library of Medicine
(http://www.ncbi.nlm.nih.gov/entrez/query.fcgi) As search terms I
used coffee, caffeine, caffeic acid and chlorogenic acid (two
other major constituents of coffee), and combined each of them with
the terms dehydration, hydration, fluid, diuretic. I did a
separate search for each pair of terms. I was surprised to find very
few studies that had the specific objective of researching the effects
of coffee/caffeine on fluid balance and hydration. I first summarize
my findings overall, and then list the articles I found below,
together with my own summary of each one and a URL to the more
extensive Medline summary (in one case to the full text) of the
Firstly, coffee can have a diuretic effect and reducing caffeine
intake was found to decrease urinary frequency, urgency and leakage in
people with urinary symptoms (see ref. 6). However, whether the
diuresis caused by coffee leads to dehydration is another question. A
study (see ref 1) of volunteers given 6 cups of coffee found that they
did show a average decrease in total body water of 2.6%. Therefore a
relatively high level of coffee consumption may cause a mild degree of
dehydration. However, another study (see ref 4) found that drinking
various combinations of water and caffeinated and non-caffeinated
beverages gave no significant differences in the effect of various
combinations of beverages on hydration status of healthy adult males.
A review of the literature on the effects of caffeine on athletic
performance concluded There is no evidence that caffeine ingestion
before exercise leads to dehydration, ion imbalance, or any other
adverse effects. (see ref 3). Another review (ref 2, with which you
are already familiar), looked at 10 published studies, which
investigated caffeine doses of 100-680 mg, concluded that caffeine is
mildly diuretic, but not more so than water. Caffeine was not found to
be associated with significant levels of dehydration, and any
dehydrating effects it did have were smaller in people who had
developed tolerance through regular consumption. One further
interesting fact is that the diuretic effect of caffeine was found to
be lower in conditions of physical exercise than at rest (see ref 5).
The authors suggested that the diuretic effect of caffeine might be
counteracted by the increase in levels of plasma cathecholamines
(norepinephrine, adrenalin, dopamine) during exercise. Similarly to
the studies quoted so far, the authors concluded concluded that
caffeine consumed in sports drinks during moderate endurance exercise
apparently does not compromise bodily hydration status.
One last piece of circumstantial evidence comes from a study into the
prevention and treatment of urinary stones (see ref 7). Maintaining
adequate hydration is a key element in prevention. The authors found
Hydration has proved effective
"fluids" may consist of water (any
kind), coffee (caffeinated or decaffeinated), tea, beer and wine,
thus providing further indirect confirmation that coffee does not
markedly affect hydration.
The studies I found pertained only to coffee and caffeine. Therefore,
the other major constituents of coffee, chlorogenic acid and caffeic
acid, do not seem to have a role in the effects of coffee on hydration
1. Annals of Nutrition and Metabolism 1997; Vol. 41(no. 1):29-36
Coffee consumption and total body water homeostasis as measured by
fluid balance and bioelectrical impedance analysis.
Neuhauser-Berthold, Beine S, Verwied SC, Luhrmann PM.
Institute of Nutritional Sciences, Justus Liebig University, Giessen,
The authors put 12 healthy volunteers onto a standardized diet in
which mineral water was the only liquid. On the 6th day, they replaced
some of the water with 6 cups of coffee containing 642 mg caffeine.
The result was an increase in the amount of urine excreted over 24
hours. The average increase was 753 ml (+/- 532 ml). At the same
time, there was a decrease in body weight and total body water was
found to have decreased by an average of 2.7%. This study therefore
shows that a relatively high level of coffee consumption does lead to
a degree of dehydration.
2. International Journal of Sport Nutrition, Exercise and Metabolism
2002 Jun; Vol 12 (no. 2):189-206
Caffeine, body fluid-electrolyte balance, and exercise performance.
Departments of Kinesiology, Nutritional Sciences, and Physiology &
Neurobiology, University of Connecticut, Storrs, CT 06269-1110, USA.
The author reviewed 10 published studies to find whether caffeine can
cause dehydration. The overall consensus from the studies is that
caffeine is mildly diuretic, but only to a similar extent to the
diuretic effect of water. The studies, which compared the effects of
100-680 mg caffeine with those of water or placebo seldom found a
statistically significant effect of caffeine on the volume of excreted
urine, and did not associate caffeine with a physiologically
significant level of dehydration. It also appeared that individuals
who have developed a tolerance to caffeine through regular consumption
are less likely to show any effects of caffeine on body hydration.
The author concluded that moderate consumption of caffeince will not
adverely affect the fluid-electrolyte balance of athletes and other
sports enthusiasts, while people whose lifestyles are more sedentary
will have an even smaller risk because they lose less fluid through
sweating than athletes.
members of the general public should be a less risk than athletes
because their fluid losses via sweating are smaller.
3. Sports Medicine 2001; Vol 31 (no. 11):785-807
Caffeine and exercise: metabolism, endurance and performance.
Human Biology and Nutritional Sciences, University of Guelph, Ontario,
This is another review of the literature on the effects of caffeine on
athletic performance. The author concludes There is no evidence that
caffeine ingestion before exercise leads to dehydration, ion
imbalance, or any other adverse effects.
4. Journal of the American College of Nutrition, Vol. 19, No. 5,
The Effect of Caffeinated, Non-Caffeinated, Caloric and Non-Caloric
Beverages on Hydration
Ann C. Grandjean, EdD, FACN, CNS, Kristin J. Reimers, RD, MS, Karen E.
Bannick, MA and Mary C. Haven, MS
The Center for Human Nutrition, (A.C.G., K.J.R.), Omaha, Nebraska
School of Allied Health Professions, University of Nebraska Medical
Center (M.C.H.), Omaha, Nebraska
Bannick Consulting, Isle, Minnesota (K.E.B.)
This was a study in 18 healthy males aged 24-39. They were made to
drink various combinations of water and caffeinated and
non-caffeinated beverages. The authors concluded This preliminary
study found no significant differences in the effect of various
combinations of beverages on hydration status of healthy adult males.
Advising people to disregard caffeinated beverages as part of the
daily fluid intake is not substantiated by the results of this study.
Full text at: http://www.jacn.org/cgi/content/full/19/5/591
5. International Journal of Sports Medicine 1997 Jan; Vol. 18 (no
Caffeine vs caffeine-free sports drinks: effects on urine production
at rest and during prolonged exercise.
Wemple RD, Lamb DR, McKeever KH.
Exercise Physiology Laboratory, Ohio State University, Columbus, USA.
The authors compared the effects of caffeinated and non-caffeinated
sports drinks on urine production and water balance during rest and
during exercise. They did find a statistically significant difference
in the amount of urine produced at rest, with greater quantities being
produced after consumption of caffeine. However, there was no
significant difference between the two groups during exercise. The
authors suggest that the diuretic effect of caffeine might be
counteracted during exercise by the increased levels of plasma
catecholamines (norepinephrine, adrenalin, dopamine) that result from
physical activity. They conclude caffeine consumed in sports drinks
during moderate endurance exercise apparently does not compromise
bodily hydration status.
6. British Journal of Nursing 2002 Apr 25-May 8; Vol.11(no 8):560-565
Caffeine reduction education to improve urinary symptoms.
Bryant CM, Dowell CJ, Fairbrother G.
Prince Henry and Prince of Wales Hospitals, Australia.
The objective of this study was to see whether reducing caffeine
consumption would have an effect in 95 patients with urinary symptoms.
They found that reducing caffeine intake did reduce urinary
frequency, urgency and leakage.
7. Journal of Nephrology 2000 Nov-Dec; Vol. 13 Supplement 3: S65-70
Medical prevention and treatment of urinary stones.
Colussi G, De Ferrari ME, Brunati C, Civati G.
Niguarda-Ca' Granda Hospital, Milan, Italy.
The authors review strategies for the prevention and treatment of
urinary stones. Adequate hydration is one of the key elements of
prevention. Hydration has proved effective in clinical trials and
population-based observational studies; "fluids" may consist of water
(any kind), coffee (caffeinated or decaffeinated), tea, beer and wine;
grapefruit juice appears to have an unexplained ill effect. This is
therefore indirect evidence that coffee, whether caffeinated or
decaffeinated, does not affect the maintenance of optimal levels of