Hey Readers! Welcome! I am so glad you are here. For most of us adults, we grew up hearing that caffeine and caffeinated beverages caused increased urine output, and hence, potential dehydration. But, is this really true? In this month's blog post, I go over some recent scientific research that addressed this very question - does caffeine affect fluid balance in the human body?
Before we delve deeper into the research on caffeine and hydration, let's talk a little bit about what caffeine is, how it works, and how it can impact cognitive and physical performance.
What is Caffeine?
Caffeine (1, 3, 7-trimethylxanthine) is naturally occurring in coffee, tea, and chocolate, and it can also be found in pills, powder, and energy drinks. Caffeine is a central nervous system (CNS) stimulant, and it acts primarily by blocking adenosine receptors both centrally and peripherally. When adenosine binds to its receptors, it slows the heart rate and relaxes muscles, and this usually makes people feel sleepy and relaxed. However, when caffeine binds to adenosine receptors, it causes the opposite effect, making you feel alert and energized.
Caffeine has long been touted as having a diuretic (i.e. increasing urine output) effect, thereby contributing to dehydration or a fluid imbalance in the body. Indeed, industrial workers performing prolonged labor in hot climates are generally advised to avoid caffeinated beverages in the workplace, and current military doctrine also recommends that soldiers closely monitor caffeine ingestion when out on missions. However, many researchers and experts have challenged the notion that caffeine contributes to dehydration.
Caffeine and Performance
Caffeine is arguably the most widely used supplement to improve performance among recreational athletes, professional athletes, and busy, working adults, and for good reason. Caffeine-containing supplements are well-known to exert many favorable physical AND mental outcomes, including increasing exercise performance and endurance capacity, reducing perceptions of discomfort and pain, and enhancing alertness and mental focus. Caffeine also works synergistically with exercise to increase energy expenditure (i.e. calories burned) and fat oxidation, if taken prior to exercise.
So, now that we know what caffeine is and how much it can help improve performance, let's dig into the research that addressed caffeine use and hydration.
Caffeine's Effect on Hydration During Exercise
A meta-analysis conducted by Zhang et al (2015) aimed to quantify caffeine-induced diuresis in adults during exercise. 16 studies were included in this meta-analysis, with participants having a median age of 27 years (22 – 36 years). Participants were healthy adults who used caffeine in the form of pills, tea, coffee, and caffeinated beverages, with a median dosage of caffeine at 300 mg (range 114 – 741mg) and most participants consuming about 300 – 500mg of caffeine per day. The authors found that there was little-to-no effect of caffeine on urine output during exercise at any of the dosages used by participants in both the acute phase (i.e. immediately after ingestion) and after the acute phase. Thus, caffeine does not appear to lead to excessive fluid loss during exercise based on this meta-analysis. This is likely due to caffeine’s effect on sympathoadrenal activity of the nervous system, which decreases the flow rate of liquids through the kidneys.
Zhang and friends (2015) found another interesting piece of data in their meta-analysis – females actually had a 6-fold higher amount of urine output with caffeine use compared to males, although the amount of urine output from females was still minimal. This gender difference is likely due to differences in metabolism between males and females, where the effects of caffeine may last longer for females. Zhang et al (2015) concluded that, “caffeine is a safe ergogenic aid that can be used by athletes, fitness enthusiasts, industrial workers, and military personnel without concerns for any negative impact on fluid balance.”
Gonzales et al (2020) found similar results when they investigated the effects of caffeine mouth rinsing on hydration status and sweat rate after a 10km run in recreationally trained runners (i.e. individuals engaging in running exercise 4-7 days per week, completing between 5 and 40 km per week). In this double-blind, placebo-controlled, crossover research study, participants (male and female, aged 30 +/- 7 years) completed two 10km runs, separated by one week, where participants were asked to complete the run “as quickly as possible.” Immediately before each run, participants completed a 10-second mouth rinse protocol with either 300 mg of caffeine or a placebo. The results showed that dehydration, sweat rate, and hydration were not significantly different between the caffeine and placebo trials. Thus, the use of a caffeine mouth rinse protocol did not alter or affect sweat rate or hydration status during physical activity, and this is in agreement with the meta-analysis conducted by Zhang et al. (2015). In the Gonzales et al (2020) study, the participants were already regular caffeine users. Thus, it is possible that habitual caffeine use may lead to a tolerance against any diuretic effect from caffeine.
Caffeine's Effect on Hydration at Rest
Killer et al (2014) compared the effects of coffee intake versus water on hydration status in caffeine-habituated adults at rest. In a counterbalanced, crossover design, 50 non-smoking males, aged 18 – 46 years, who were moderate coffee drinkers (defined as drinking 3 – 6 cups per day, with 300 – 600 mg/day caffeine) were included in the study. In the coffee trial (C), participants consumed four mugs (200mL each) of black coffee per day, equating to a caffeine intake of 4mg/kg body mass. The water (W) control trial involved participants consuming four mugs of water (200mL each) per day. Diet was also controlled and provided to participants during the study. The results showed that there were no significant changes in total body water (TBW) from beginning to end of either trial, coffee or water, suggesting that participants maintained a stable fluid balance throughout the study. There were also no differences between 24-hour urine volume or urine void volume between the coffee and water conditions. Also, there was no significant difference in urine creatinine, serum osmolality, hematocrit, total plasma protein, serum sodium, or serum potassium between the two conditions. In sum, this study showed no significant differences in the hydrating properties of coffee or water across a wide range of hydration status indices. Thus, Killer et al (2014) concluded that moderate consumption of caffeine does not disrupt hydration status in the body and caffeine-containing beverages such as coffee may provide similar hydrating qualities to water.
Armstrong (2002) conducted a review of caffeine intake and hydration status. 12 out of 15 of the studies included in this review found no statistical difference in urine volume between caffeine (100 – 680mg) versus water or placebo. Also, caffeine versus placebo trials revealed no differences in core body temperature, sweat rate, plasma volume shifts, or performance times. Thus, the Armstrong (2002) review suggests that recommendations to avoid caffeinated beverages for hydration purposes is simply unsupported by the research evidence. It is unlikely that people will incur detrimental fluid-electrolyte imbalances if they consume caffeine in moderate doses (i.e. less than 680mg). However, Armstrong (2002) stated that further research is required to clarify whether larger doses of caffeine (i.e. greater than 680mg) result in fluid imbalances.
Seal et al (2017) found similar results to Armstrong (2002) in their research study, but they also included higher doses of caffeine intake, as recommended by Armstrong (2002). In the Seal et al (2017) study, ten healthy adults (8 males and 2 females), aged 27 +/- 5 years, who regularly consumed 1 – 3 cups of coffee per day, were included in this study. In this counterbalanced, crossover design, participants engaged in an overnight fast, and then they provided a urine sample the next morning in the lab. After providing the first urine sample, participants consumed a standardized breakfast (i.e. one slice of wheat toast with butter) and either water, 200mL of instant coffee with low caffeine at 3mg/kg body weight (i.e. 269 +/- 45mg caffeine), or 200mL instant coffee with high caffeine at 6mg/kg body weight (i.e. 537 +/- 89 caffeine). Urine was again collected from participants at 60, 120, and 180 min after the test drink ingestion. The results showed that there were no differences in urine output between the low-caffeine trial and water trial at any time point. However, the urinary output was significantly greater in the high-caffeine trial compared to the low-caffeine and water trials at the 2- and 3-hour time points. Urine sodium and potassium excretion was also higher in the high-caffeine trial compared to the low-caffeine and water trials. Thus, Seal et al (2017) concluded that higher doses of caffeine (i.e. 537mg and higher) result in an acute diuretic effect as observed via the higher cumulative urine volume 2- and 3-hours after ingestion, compared to more moderate doses (i.e. ~269mg). Thus, people who ingest about 2 – 3 cups of coffee per day will likely not experience disruptions in fluid balance. But, people who drink 4 or more cups per day could experience caffeine-induced diuresis.
Caffeine is one of the most widely used supplements for improving physical and mental performance. Caffeine has long been touted as a diuretic (i.e. increasing urine output), contributing to dehydration. However, this notion has been challenged by several recent research studies. Zhang et al (2015) and Gonzales et al (2020) both found that caffeine does not affect fluid balance during exercise, concluding that caffeine supplementation is safe for improving performance. Killer et al (2014), Armstrong (2002), and Seal et al (2017) examined caffeine's effect on hydration at rest in habitual caffeine users. All three research studies found that caffeine in moderate doses (below ~500mg per day) does not appear to impact fluid balance or hydration in the body. Thus, adult caffeine intake up to about 400 – 500 mg per day appears safe in healthy individuals, and it may even provide similar hydrating properties to water. At higher doses (>500mg), adverse side effects are possible, including: increased heart rate, increased blood pressure, jitteriness, nervousness, and gastrointestinal distress. Even though we have all been taught for years that caffeinated beverages can dehydrate us, it seems that this is an incorrect claim. The scientific research seems to overwhelmingly suggest that caffeine (in moderate doses at least) will not negatively affect fluid balance in the body. So, if you are the type of person who truly loathes drinking plain water, you might be okay to drink that soda or tea. However, you do need to be careful for the additional calories and added sugars that can often be found in flavored beverages.
As always, the information presented in this blog post is derived from my own study of nutrition, human movement, anatomy, and yoga. If you have specific questions about nutrition and hydration for your body, please consult with your physician, dietician, personal trainer, or private yoga teacher. If you are interested in private yoga and/or personal training sessions with me, Jackie, email me at firstname.lastname@example.org for more information about my services. Also, please subscribe to my website so you can receive my monthly newsletters (scroll to the bottom of the page where you can submit your email address). This will help keep you "in-the-know" about my latest blog releases and other helpful yoga and wellness information. Thanks for reading!
~Namaste, Jackie Allen, M.S., M.Ed., CCC-SLP, RYT-200, RCYT, NASM-CPT, NASM-CES, NASM-CNC
Armstrong, L.E. (2002). Caffeine, Body-Fluid Electrolyte Balance, and Exercise Performance. International Journal of Sport Nutrition and Exercise Metabolism. 12: 189 – 206.
Goldstein, E.R., et al. (2010). International society of sports nutrition position stand: caffeine and performance. Journal of the International Society of Sports Nutrition. 7(5): 1 – 15
Gonzalez, A.M., et al., (2020). Acute Caffeine Mouth Rinse Does Not Change the Hydration Status Following a 10km Run in Recreationally Trained Runners. Biomedical Research International. 2020: 1 – 5.
Killer, S.C., Blannin, A.K,, & Jeukendrup, A.E. (2014). No Evidence of Dehydration with Moderate Daily Coffee Intake: A Counterbalanced Cross-Over Study in a Free-Living Population. PLOS One. 9(1): 1 – 9.
Seal, A.D., et al. (2017). Coffee with High but Not Low Caffeine Content Augments Fluid and Electrolyte Excretion at Rest. Frontiers in Nutrition. 4(40): 1 – 6.
Zhang, Y., et al. (2015). Caffeine and diuresis during rest and exercise: A meta-analysis. Journal of Science and Medicine in Sport. 18(5): 569 – 574.