Fact or Fiction: Is Your Cup of Joe Causing Dehydration?

The Coffee Myth

Are you concerned your morning cup of coffee is dehydrating you? You are not alone! It is commonly believed coffee has a diuretic effect which can lead to dehydration. However, new research has found daily consumption may not be as detrimental as we once thought.

What Does the Science Say?

This myth is, indeed, based in some fact. Caffeine is a compound naturally found in tea and coffee and as an additive in soda and energy drinks. It acts as a stimulant and has been shown to have a mild diuretic effect – meaning it increases urination. Theoretically, caffeine augments blood flow to the kidneys which inhibits the absorption of sodium, causing more fluid to be excreted.

However, this effect occurs within a short period after ingestion, typically when large amounts of caffeine are consumed. This exact pathway is debated and recent studies suggest over the course of a day, caffeine does not cause dehydration. In fact, it has been shown habitually drinking coffee can help build tolerance to caffeine’s diuretic effects, meaning over time increased urination becomes minimized.

So what might be the cause of frequent urination when drinking coffee? Increased intake of overall fluid is likely the culprit and is not much different when compared to drinking plain water. Furthermore, the fluid you consume balances the mild diuretic effects of caffeine to help prevent dehydration.

How Much Coffee is Recommended?

Although you do not have to worry about your morning cup of Joe depriving your body of fluid, it is important to be wary of caffeinated beverages. The American Medical Association recommends limiting to 2 to 3 cups of coffee per day (a total of about 200 to 300 mg caffeine). Higher levels can lead to increased heart rate, anxiety, difficulty sleeping, nausea, restlessness, headaches, and tremors – though this will depend on personal sensitivity. Some find that drinking a mild amount of caffeine before exercise can give a quick jolt of energy to make workouts more effective. This has been shown to be an effective strategy for some, but in excessive amounts side effects of caffeine can also hinder performance.

Furthermore, it is important to note how you are drinking caffeine. Sodas, energy drinks, and blended coffee beverages can be high in sugar and fat. Be sure to check serving sizes too. One bottle may contain 2½ servings! Coffeehouse portions can also be deceptive. A medium cup of coffee is typically around 2 cups – a large is pushing 3.

How Caffeine in Common Beverages Stacks Up:

Beverage Size Caffeine Content
Regular Coffee 8 oz 95-200mg
Decaffeinated Coffee 8 oz 2-10mg
Espresso 1 oz 45-75mg
Black Tea 8 oz 15-70mg
Green Tea 8 oz 25-45mg
Iced Tea (bottled) 8 oz 5-40mg
Cola 12 oz 25-40 mg
Mountain Dew 12 oz 40-55mg
5-Hour Energy Shot 2 oz 200-210mg
Red Bull 8oz 75-80mg
Rockstar 8oz 75-80mg

*Adapted from the Mayo Clinic

While mild amounts of caffeine will not cause dehydration, and can give you a boost of alertness and energy, you should always indulge in moderation! Be cognizant of serving size, additional additives in your beverage, and any symptoms of adverse effects from excessive caffeine. Remember, while coffee and tea can contribute to the recommended 6 to 8 glasses of fluid a day, the best source of daily hydration is plain water.

About the author:

Erika Chan just finished her dietetic internship at MGH. Originally from California, she now lives in Boston. She loves exploring the city, eating, and baking in her free time.

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Our Be Fit Nutritionists are comprised of dietetic interns studying at Massachusetts General Hospital. During their internship, they receive training on acute care nutrition, ambulatory and community nutrition, food service systems management, and research. Their comprehensive work is done in collaboration with registered dietitians.

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