For decades, we were told coffee dehydrates you—that every cup of joe actually sucks water from your system. This idea stems from the diuretic effect of caffeine, which makes your kidneys produce more urine. But here’s the twist: modern research shows that for regular coffee drinkers, the diuretic effect is mild and temporary.
A landmark 2014 study asked men to drink four cups of black coffee a day for three days, then measured their hydration levels. The result? No significant difference compared to drinking the same amount of water. Your body adapts, and the net fluid loss is negligible—meaning coffee does contribute to your daily fluid balance.
