Myth #1: Coffee Dehydrates You
Everyone knows that coffee contains caffeine (unless it’s decaffeinated, of course), and it’s also common knowledge that caffeine is a diuretic—a substance that encourages urination and thus lowers the amount of fluid (mostly water) in your body. So, coffee is dehydrating, right?
Actually, no—not according to scientific research. A 2003 study concluded that consuming moderate amounts of coffee or tea (as part of a “normal lifestyle”) does not lead to any significant fluid imbalance, because most regular drinkers of caffeinated beverages develop “a profound tolerance” to the diuretic effects of the drug. Moreover, coffee and tea are mostly water, so drinkers are likely replacing most of the fluids they may lose.
Myth #2: Espresso Is More Caffeinated Than Drip Coffee
Espresso is concentrated coffee (or coffee brewed with less water), and its strong flavor may lead many to assume a higher potency of caffeine. The key is volume: According to the USDA, 2 ounces of espresso (about the volume of a double shot) contain about 120 milligrams of caffeine, which is the rough equivalent of a 10-ounce cup of drip or steeped coffee. So the two preparations are roughly comparable. In fact, if you’re in the habit of ordering a single shot of espresso, either as a straight shot or as an Americano (espresso plus boiling water), your caffeine consumption is actually less than if you ordered a standard cup of drip coffee.
You can make the same comparison with alcohol—a glass of wine, a pint of beer and a shot of whiskey all contain about the same amount of alcohol, despite differences in volume.
Myth #3: Coffee Stunts Your Growth
This myth actually arose as a result of a smear campaign launched by Postum, an alternative coffee beverage, back in the early 1900s. The ads terrified parents by telling them that, thanks to coffee, their children would suffer learning disabilities and their physical growth would be adversely affected. The misinformation took hold, and, to this day, the myth is still going strong.
In fact, no substantial conclusions have been drawn, and the evidence has never led back to coffee—the studies were actually examining the effects of soda on bone health.
Myth #4: Dark Roasts Have More Caffeine Than Light Roasts
Dark roasts certainly have a bolder taste, and it’s for this reason that they’re believed to contain more caffeine. Interestingly, the real effect of roasting is to evaporate the water in the beans, changing their mass. If you measure by whole beans, you'll discover you can fit more light beans (and thus more caffeine) in your cup than dark.
Myth #5: Coffee Sobers You Up
According to the American Psychological Association, coffee’s head-clearing properties (at least related to excessive alcohol consumption) are wishful thinking. In fact, it’s possible the caffeine in coffee makes it more difficult to realize you are intoxicated, concludes the 2009 study published in Behavioral Neuroscience. Head researcher Thomas J. Gould, Ph.D., stated that “despite the appeal of being able to stay up all night and drink, all evidence points to serious risks associated with caffeine-alcohol combinations.”