The Science behind Hydration
The popular myth is that coffee and tea don't count towards your daily hydration requirements because they dehydrate you. This groundless belief coupled to the also false requirement of Eight 8-ounce glasses of water a day rule may cause to over-hydrate, which is not good for you either (See: Water Intoxication).
Actually you obtain about 20% of your daily fluid requirements from the food you eat, and the remaining 80% from the fluids that you drink. And by fluids, we mean precisely that: all non-alcoholic beverages.
Alcohol is a diuretic
Alcohol is a diuretic, a substance that increases the output of urine by the body.
Calculations indicate that for each unit of alcohol taken in (1 unit = about 8 g alcohol, roughly 1 ⁄ 3 oz.), about 80 ml (2.9 oz.) of extra water is lost from the body.
One unit is the alcohol is far less than you would imagine:
- One 25ml single measure of whisky.
- One third of a pint of beer
- Half a standard glass of red wine
Recommended Daily Water intake
The current daily water intake recommendations of the National Academy of Medicine (NAM) for adults aged +19 are:
- 91 ounces of total water for women (2.7 liters)
- 125 ounces of total water for men (3.7 liters)
So, in addition to the water content of your solid foods, you would have to drink about 80% of that amount as actual fluids (water, soft drinks, sodas, milk, fruit juice, coffee, tea or other herbal infusions).
Is caffeine a diuretic?
Caffeine is an alkaloid found in many beverages and food (cola sodas, coffe, tea, South American mate, cocoa, energy drinks, chocolate).
According to several studies, caffeine has a well-known mild diuretic effect first described in 1949 by James Davis and Nathan Shock ( 1 ).
However there seems to be several factors that influence its diuretic effect such as habituation and a minimum threshold.
Maughan (2003) ( 2 ) sets the diuretic threshold at 250 - 300 mg of caffeine, which is "equivalent to the amount found in 2-3 cups of coffee or 5-8 cups of tea".
The U.S. Food and Drug Administration (FDA) ( 3 ) sets a level for healthy adults of 400 milligrams a day (about four or five cups of coffee) as one that is not dangerous or has negative effects. There is no level for children.
The FDA also considers caffeine as "generally recognized as safe" (GRAS) for use in cola-type beverages up to a level of 0.02%, or 200 parts per million. So a 12 oz can of cola soda has not more than 71 mg of caffeine.
Regarding higher doses of caffeine found in some dietary supplements or energy drinks, the FDA describes its negative side effects as follows ( 4 ):
- 1200 mg : Toxic effects such as tachycardia, ventricular arrhythmia, and seizures.
- 10000 to 14000 mg: life-threatening dose. But lower doses can be life-threatening in children or other sensitive populations.
The following table shows the average (values can vary quite a bit) caffeine content in one cup or glass of some common caffeinated beverages:
Caffeine per cup or glass
Studies on caffeine threshold
Seal et al. (2017) ( 5 ) had a two groups of healthy adults drink coffee with a "low caffeine" and a "high caffeine" content and they found that those drinking 6 mg per kg of body weight experienced a greater diuretic effect than those drinking 3 mg per kg.
They concluded that "3 mg kg-1 [of caffeine] do[es] not disturb fluid balance in healthy casual coffee drinking adults at rest."
Based on the average weight of the group this sets the threshold at 238 mg per day for a "non-diuretic" effect.
Bhalla and Gupta (2018) ( 6 ) find that the safe level for healthy adults is 300 mg of caffeine daily.
Wikoff (2017) ( 7 ) agrees, but with a slightly higher level (400 mg/day) and recommend an intake of 300 mg/day in pregnant women and around 2.5 mg/kg day in teens an children
Habituation: Regular consumption key to lower diuretic effect
The first study on diuresis an caffeine back in 1928 by Eddy Downs ( 8 ) found that caffeine does have a diuretic effect but that regular caffeine consumption could lead to a tolerance developing against this effect.
Izzo (1983) ( 9 ) and Maughan (2003) ( 2 ) both agree with Downs, finding that even though caffeine has a diuretic action it is much lower in individuals who consume coffee or tea on a regular basis: "previous caffeine exposure may decrease its diuretic effectiveness" ( 9 ) and "A profound tolerance to the diuretic and other effects of caffeine develops, however, and the actions are much diminished in individuals who regularly consume tea or coffee." ( 2 ).
Caffeine does not affect hydration
No scientific backing to the "no caffeine" recommendation
Maughan (2003) summarizes this very clearly: "The most... 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."(2)
This same finding is echoed by Killer et al. (2014) ( 13 ) who debunks the notion that coffee causes dehydration and its consumption should be avoided or reduced to maintain an optimal fluid balance.
Killer's team concluded that despite a higher excretion of sodium through urine in the coffee drinkers: "there were no significant differences [...] of hydration status between trials. suggest[ing] that coffee, when consumed in moderation by caffeine habituated males provides similar hydrating qualities to water."
Zhang et al (2014) ( 10 ) ratifies this in his work which studied the effect of an ingestion of approx. 300 mg of caffeine on athletes during exercise and rest periods. The study concluded that caffeine had a "minor diuretic effect" which was slightly stronger in women than in men and that exercise reverted this diuretic effect.
Zhang's team added that "Concerns regarding unwanted fluid loss associated with caffeine consumption are unwarranted."
Bhalla and Gupta (2018) ( 6 ) also find that at 300 mg of caffeine intake daily did not cause "...any fluid electrolyte imbalance and that [it] did not lead to excessive fluid loss in healthy adults."
Tea and coffee actually hydrate you
Did you know that an average cup of tea or coffee is almost 99% water?
Fact: " The amount of soluble solids in brewed coffee varies quite a bit, from 1-2% " ( 11 ). So drinking tea or coffee is a great way to hydrate yourself, tea also has its fair share of antioxidants and other additional health benefits ( 15 ).
And if it does increase your loss of fluids due to its diuretic effect, the water intake that comes with the caffeine more than compensates the loss:
- Seal's research (5) indicated that ingesting the equivalent of (537 mg) led to a greater diuresis (an extra 257 ml). But 537 mg is roughly the caffeine content of 4.9 cups of coffee or 13 cups of tea, meaning that you'd intake between 980 and 2530 ml of water, and only loose 257 ml.
- Zhang (10) found that 300 mg resulted in an extra 109 ml of loss. But the water intake would be 544 to 1418 ml.
So drinking tea and coffee will not only not dehydrate you, you will have a net gain of fluid.
Other caffeinated beverages
What about Colas or Diet Colas?
Grandjean et al., (2000) ( 12 ) looked into how different beverages influenced hydration in healthy adult men: They drank carbonated, caffeinated caloric and non-caloric colas, soft drinks and coffee. The authors concluded that " 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.".
Tucker et al., (2015) ( 14 ) reached a similar conclusion comparing combinations of beverages: water, cola, diet cola (both of these have caffeine) and orange juice: " beverages of varying composition are equally effective in hydrating the body."
If you drink coffee or tea or other caffeinated drinks on a regular basis your body is most likely accustome to the extra caffeine and the diuretic effect is minimal.
All studies find that drinking tea, coffee or cola drinks will not cause excessive fluid loss or upset hydration. They confirm that water and the other caffeinated beverates are equal in their hydrating effects.
Cite this article:
A. Whittall. ©2018. Do I need eight glasses of water a day?. Patagonia Wellness, 11 Oct. 2018. http://www.patagoniawellness.com/health/water-coffee-or-tea.html
Subject: Do you really need to drink eight 8-oz. glasses of water each day? The facts and science behind maintaining an adequate and healthy level of hydration.
References and Further Reading
(1) Davis JO and Shock NW, (1949). The effect of theophylline ethylene diamine on renal function in control subjects and in patients with congestive heart failure. J Clin Investig 28: 1459-1468.
(2) R J Maughan, J Griffin, (2003). Caffeine ingestion and fluid balance: A review. Journal of Human Nutrition and Dietetics 16(6):411-20 December 2003 DOI: 10.1046/j.1365-277X.2003.00477.x
(3) Caffeine and Kids: FDA Takes a Closer Look. FDA, 2013
(4) Guidance for Industry: Highly Concentrated Caffeine in Dietary Supplements. FDA, April 2018
(5) Adam D. Seal et al., (2017). Coffee with High but Not Low Caffeine Content Augments Fluid and Electrolyte Excretion at Rest. Front. Nutr., 18 August 2017, https://doi.org/10.3389/fnut.2017.00040
(6) Ridhima Bhalla and Medah Gupta, (2018). Does moderate caffeine consumption causes diuresis? - A systematic review. Vol 1 No 1 (2018): International Journal of Recent Innovation in Food Science & Nutrition
(7) Wikoff D. et al., (2017). Systematic review of the potential adverse effects of caffeine consumption in healthy adults, pregnant women, adolescents, and childrenFood and developmental, bone & calcium, and acute effects. Food and Chemical Toxicology Volume 109, Part 1, November 2017, Pages 585-648
(8) Eddy N, Downs A, (1928). Tolerance and cross-tolerance in the human subject to the diuretic effect of caffeine, theobromine, and thoephylline. J Pharmacol Exper Ther 33: 167-174
(9) Izzo, Joseph L. et al. (1983) Age and prior caffeine use alter the cardiovascular and adrenomedullary responses to oral caffeine American Journal of Cardiology , Volume 52 , Issue 7 , 769 - 773
(10) Zhang, Yang et al. (2014). Caffeine and diuresis during rest and exercise: A meta-analysis. Journal of Science and Medicine in Sport. 18. 10.1016/j.jsams.2014.07.017
(11) David Masulli, (2018). The Role of TDS in Coffee & the Science Around your Brew. Hanna Instruments. 10.Jan.2018
(12) Grandjean AC , Reimers KJ, Bannick KE, Haven MC, (2000). The effect of caffeinated, non-caffeinated, caloric and non-caloric beverages on hydration. J Am Coll Nutr. 2000 Oct;19(5):591-600
(13) Sophie C. Killer, Andrew K. Blannin, and Asker E. Jeukendrup (2014). No Evidence of Dehydration with Moderate Daily Coffee Intake: A Counterbalanced Cross-Over Study in a Free-Living Population. PLoS One. 2014; 9(1): e84154. Published online 2014 Jan 9. doi: 10.1371/journal.pone.0084154
(14) Tucker MA, et al. (2015). Hydration Status over 24-H Is Not Affected by Ingested Beverage Composition. J Am Coll Nutr. 2015;34(4):318-27. doi: 10.1080/07315724.2014.933684. Epub 2015 Mar 1
(15) Jane Higdon, (2005) Tea. Linus Pauling Institute, Oregon State University