Caffeine, dehydration, and health: what you should know

Although coffee has been linked to several promising health benefits such as improved physical performance, lowered risk of type 2 diabetes, and even protection from Alzheimer’s disease and dementia, caffeine has been linked to negative and potentially detrimental side effects.

Handwritten chalk chemical formula of Caffeine on school blackboard.

Coffee drinking is the, dare we say, religious experience that binds people from countless cultures all around the world. The particular social significance of coffee and its consumption varies widely from place to place, but one thing always remains true: people everywhere can’t (or refuse to)  imagine their lives without coffee. In the United States, it’s perhaps less of a social phenomenon than it is a private ritual for workers who’ve come to see it as their daily lifeline. Factor in people with predilections for other caffeinated beverages like tea, energy drinks, and soda, and it comes as no surprise that the effects of caffeine on the body have been the source of great public interest for many years. That’s why we’re digging into the what’s and the how’s of caffeine consumption and dispelling some common beliefs about the relationship between caffeine and dehydration.

The Chemistry

But first, let’s talk a little bit about what caffeine is and what it does to your body. Put simply, it’s a stimulant of the methylxanthine class, making it the world’s most widely consumed psychoactive drug. It works by stimulating the brain and nervous system, increasing alertness and making you feel less tired (the very reason it holds a special place in so many of our hearts). More specifically, it’s an “adenosine antagonist,” which means it blocks your brain’s absorption of a chemical (adenosine) that would normally make you feel drowsy throughout the day. People who regularly consume caffeine in large amounts, however, can lose sensitivity to caffeine. This is why people who drink lots of coffee, for example, often need to drink more of it (hello double espresso shots) to feel the same stimulating effects. It is totally possible, however, to undo your high caffeine tolerance by slowly cutting back on your consumption or by staying away from it for an extended period of time (yeah, we know).

The Effects

Caffeine can also act as a mild diuretic, meaning it causes your kidneys to flush extra sodium and water from the body via the process of urination. This brings us to the one myth about caffeine that refuses to go away: caffeine causes dehydration. The belief can be traced back to a small but influential study carried out in 1928 where increased urination, and therefore dehydration, was linked to caffeine consumption. While it is an intuitive, seemingly logical conclusion, the actual science behind caffeine and hydration is (thankfully) a bit more complicated than that.

Drinking caffeinated beverages  may increase urine production, but it certainly doesn’t result in a net loss of fluids from the body. You pee more because you’re intaking more fluids. And as with water, iced tea, and even water-dense fruits like watermelon, the body processes coffee at a more rapid rate than, for example, milk, which contains fats and proteins that take longer for the body to process. When you drink coffee on an empty stomach, the exit rate can be even quicker, but the same goes for regular old water. The truth is that coffee is as much of a hydrating drink as it is a diuretic. What that means is that you can factor in your daily cup(s) of joe into your total water intake. What that doesn’t mean is that you can or should replace water with coffee or any other caffeinated beverage.

Although coffee has been linked to several promising health benefits such as improved physical performance, lowered risk of type 2 diabetes, and even protection from Alzheimer’s disease and dementia, caffeine has been linked to negative and potentially detrimental side effects. It’s been known to trigger anxiety and nervousness, insomnia, digestive issues, and high blood pressure, for example. Beyond these effects, consuming excessive amounts of caffeine, though not dehydrating as defined by net water loss, can also impair the function of water in your body. In large enough amounts, it can be detrimental to the balance of sodium and other electrolytes.

By The Numbers

Despite the potential dangers of caffeine, however, its consumption isn’t a pressing public health issue by any means. The lack of correlation between the world’s biggest coffee-drinking countries, of which the Netherlands, Finland, Sweden, and Denmark top the list, and the healthiest countries according to the Bloomberg Global Health Index, led by Italy, Iceland, Switzerland, and Singapore, suggest as much.

But, how much caffeine consumption is too much? The answer varies from person to person for several reasons. People, for example, have differing levels of sensitivity to caffeine, and bodies metabolize it at different rates. For healthy adults, however, the FDA advises that 400mg a day (about four or five cups) is generally safe for healthy adults. In large enough amounts, as with most things including water, consuming enough caffeine can indeed be fatal. Unfortunately, there is empirical data on what the lethal dose of caffeine is, and it’s in the range of five to ten grams. That’s the equivalent of about 50 – 100 cups of coffee, so you probably don’t have to worry.

Bodies, as always, are complicated, and caffeine doesn’t have the same effect on everyone. This is the very same reason why we don’t recommend any specific amount of daily water intake as a general rule. The decision to stay, break up with, or distance yourself from your caffeinated beverage of choice is ultimately yours and should be based on careful observation of your bodily responses to it.

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