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By Monica Echeverri

It is commonly believed that coffee causes dehydration. While caffeine consumption has a well-known diuretic effect, is it dehydrating?

Let’s start by breaking down dehydration

Dehydration occurs when water loss exceeds water intake, such as through sweat or urination. The body needs water for eliminating toxins, lubricating and cushioning the joints, and regulating normal body temperature. If left untreated, dehydration can lead to severe complications such as seizures, swelling of the brain, and kidney failure.

Diuretic Effects of Caffeine on Adults

Caffeine has multiple effects on normal water excretion by the kidneys all of which decrease water reabsorption leading to diuresis, e.g. a higher urine output. In the kidneys, caffeine increases water excretion by increasing the rate by which the kidney filters the blood (glomerular filtration rate) by blocking the adenosine type 1 receptor on renal blood vessels. In addition, caffeine inhibits the reabsorption of sodium by the kidneys. Caffeine also affects urine output via an effect on the pituitary gland in the brain reducing the release of a hormone called vasopressin or antidiuretic hormone. This means that when you drink coffee, it causes the body to send signals to your pituitary gland which inhibit the production of the ADH hormone, which in turn causes the kidneys to reduce the reabsorption of water. This will increase the excretion of water via urine.

While drinking coffee may affect hydration status, it is unlikely to cause dehydration. To have a significant diuretic effect, studies reveal that you need to consume more than 500 mg of caffeine per day, which is equivalent to drinking five cups of brewed coffee. On average, an 8-ounce (240-ml) cup of brewed coffee contains 95 mg of caffeine, compared with 30–90 mg for instant coffee, 3 mg for decaf, or 63 mg for a shot (1–1.75 ounces or 30–50 ml) of espresso.

Even regular coffee drinkers appear to experience fewer diuretic effects than non-regular coffee drinkers because they build a caffeine tolerance. The body can build this tolerance in just 1 to 4 days. There is also evidence suggesting that coffee may be just as hydrating as water for regular coffee drinkers. A 2014 study of 50 adult men who regularly drank coffee had no significant changes in their “total water” (water from fluids and food) compared to those who drank water over three days. Therefore, the good news is you can be both hydrated and caffeinated, though it is essential to understand that everyone responds to and tolerates caffeine differently!

Diuretic Effects of Caffeine on Athletes

For athletes, caffeine can be particularly beneficial because it is a known ergogenic aid that enhances physical performance. One of the concerns among athletes is the compounding risk of dehydration from caffeine consumption and excessive sweating. Luckily, research suggests that lower doses of caffeine, and even doses < 3 mg/kg, may still be ergogenic. Thus, consuming lower amounts of caffeine 30-60 minutes before training can still benefit performance and can be achieved either with caffeine anhydrous supplements or regular coffee. Replenishing fluids with an electrolyte drink after training is another helpful way to prevent the risk of dehydration in athletes.

Recommended Intake

With caffeine being the most consumed diuretic in the world, it is important to understand how much is “too much” when it comes to consumption. For most people, it is safe to consume up to 400 mg of caffeine a day, which is equivalent to about two 8-ounce cups of coffee. Another way to assess daily consumption is to evaluate body weight. Typically, it is recommended to consume around 3-6 mg/kg body weight. Consuming more than this may put some people at risk for dehydration. For example, very high doses (9 mg/kg) are associated with unpleasant side effects that do not seem to be ergogenic. Some of the negative side effects of consuming too much caffeine includes increased heart rate, anxiety, sleep disruption, and stomach discomfort. This may make caffeine problematic to those who are prone to anxiety and high blood pressure. At a sensible dose though, caffeine can provide several benefits with few side effects.

How to Prevent Dehydration

While diuretics increase water loss, consuming them may not always lead to dehydration. Drinking up to 5 cups of coffee a day is highly unlikely to lead to dehydration. To prevent dehydration, the best beverage to consume will most likely be a rehydration drink. This is essentially a combination of water, electrolytes, and sugar to help replace lost fluids. Another way to rehydrate is by consuming water-rich foods such as watermelon, oranges, or celery.

Monica Echeverri holds a Master of Science in Human Nutrition and Functional Medicine from the University of Western States and currently works as a food photographer, writer, and recipe developer for natural food brands and wellness publications. She has experience in community nutrition education and enjoys spending her time in the kitchen creating healthy recipes that she shares on her Instagram, @mealsbymonica.