You’ve probably been told that coffee will dehydrate you, but is it true? This is one of the most common beliefs about coffee, that coffee causes a person to urinate more frequently and therefore dehydrates you. But is that true?
When we’re talking about coffee’s dehydrating effect, what we’re really talking about is caffeine. Most people who drink coffee do it at least in part for the caffeine, so the question about the effects of caffeine on dehydration are a valid concern. Some research has shown caffeine to have a mild diuretic effect. Most research, however, shows that for average people, drinking a couple cups of coffee has no effect on dehydration. In fact, since coffee is 90% water anyway, it can be considered part of your daily water intake and helps to hydrate you instead.
What is a diuretic?
It’s always good to start with definitions. A diuretic is anything increases the body’s production of urine. In other words, a diuretic causes a person to urinate more. This has the effect of dehydrating someone, since you’re removing water from the body. A typical adult loses about 1 ½ quarts of water a day from urination. Of course, you can and should counteract this by increasing water intake and thus staying hydrated.
Coffee or Caffeine?
Coffee is a complex plant with over 1,000 volatile compounds present in roasted coffee. The chemical we’re most concerned with, however, is caffeine. Caffeine is a central nervous system stimulant. It is the world’s most widely consumed psychoactive drug. In most of the world it is legal and unregulated. In addition to coffee, caffeine is naturally present in tea, guarana, and dozens of other plant species.
Some research has shown caffeine to have a mild diuretic effect. A lot of this research, however, deals with large intakes of caffeine and high-performance athletes. In other words, unless you’re an aspiring Olympian with a five-cup-a-day habit, you’re probably fine. Even heavy coffee drinks don’t have much to worry about, because studies also show people who’ve built up a tolerance to caffeine experience less of the diuretic effect.
Coffee is mostly water anyway
It’s important to remember that coffee is mostly water. You don’t believe me? Let’s look at the math. The recommended ratio of coffee grounds to water is 14 grams of coffee for 8 ounces of water. That’s a strong cup of coffee, believe me. Fourteen grams is roughly 3 tablespoons of coffee. So 3 tablespoons per cup of coffee.
Eight ounces of water weighs 237 grams. So even if all of the coffee and all of the water you use made it into the cup, your cup of coffee would be 94% water.
Obviously, some of the water and most of the grounds stay in the filter when you brew, so I’m rounding down to 90%, but you get my point. The coffee you drink is mostly water. And that’s assuming you use 14 grams of coffee and only 8 ounces of water.
But most coffee drinkers drink far more than 8 ounces. A Tall Starbucks cup is 12 ounces, for example, while a Grande is 16 ounces. So it’s 90% water or more even if you’re using almost a ½ cup of grounds per Grande, which I’m betting you’re not.
Why does this myth persist?
I think part of the reason this myth persists is that drinking a lot of coffee will make you urinate more. But that’s not because of the coffee. It’s because of the water. Let’s say you drink two Grande-size cups of coffee. That’s 32 ounces of coffee, half of the recommended 8 cups of water per day.
Another reason this myth persists is other caffeinated beverages will dehydrate you. Other drinks with caffeine in them, such as soda and energy drinks, can dehydrate you. Sodas and sports drinks have a lot of salt in them, which can help to dehydrate you. A 12 oz. can of soda, for example has 30-70 milligrams of sodium. Sports drinks might have 100 milligrams of sodium.
So drink your coffee. Count it as part of your daily water intake (drink some plain water too). If you want to know more about the science of coffee, a great book on this is The Little Coffee Know-It-All by Shawn Steiman. (Link to Amazon)