“Coffee dehydrates you.”

The notion that coffee leads to dehydration is a widespread misconception, likely stemming from early scientific observations of caffeine's diuretic effect. Studies conducted in the early 1900s identified caffeine as a substance that increases urine production, leading to the assumption that any caffeinated beverage would cause a net fluid loss. However, these initial studies often overlooked the substantial water content of coffee itself and did not adequately account for the physiological adaptations in regular coffee drinkers.

Modern scientific understanding provides a more nuanced picture. While caffeine does have a mild diuretic effect, the large volume of water in a typical cup of coffee largely offsets this. Research has consistently shown that moderate coffee consumption contributes to an individual's daily fluid intake, rather than causing dehydration. Furthermore, regular coffee drinkers develop a tolerance to caffeine's diuretic properties, meaning the effect is even less pronounced for those who consume coffee habitually. In fact, for regular consumers, coffee is considered as hydrating as water.

The persistence of this myth can be attributed to several factors. The initial, simpler understanding of caffeine as a diuretic became ingrained in public consciousness. People might also confuse the increased frequency of urination after drinking coffee with actual dehydration, failing to recognize that the body is simply processing the fluid it has taken in. Additionally, symptoms of excessive caffeine intake, such as jitters, might sometimes be mistaken for signs of dehydration. However, numerous studies, including a 2014 review, have concluded that caffeine does not cause dehydration in healthy adults, even during exercise.