There’s a long-standing belief in the nutrition world that drinking coffee can dehydrate you, and this speculation has come about due to caffeine’s diuretic effects. Just to be clear, a diuretic is a substance that promotes your body to produce urine. As coffee is a prominent source of caffeine, it has been strongly linked with having a diuretic effect and therefore causing dehydration. But is this true?
Well, when consumed in large doses caffeine does in fact have a diuretic effect. However, studies that demonstrated this diuretic effect have used significantly large doses, such as reference one below, which implemented over 600mg of caffeine per day, or the equivalent of 6-7 cups of coffee. Additionally, early studies found that any diuretic effect seems to be most distinct in those who don’t consume coffee regularly (caffeine naive individuals), suggesting that regular caffeine intake leads to a tolerance against its diuretic effects.
A more recent research paper, by Killer and colleagues (reference two below), assessed a more applicable, moderate dosage of coffee on fluid balance and hydration status. During this study, regular coffee drinkers (caffeine-habituated individuals) were prescribed 4 cups of coffee or water per day for a period of three days. Participants were also asked to abstain from exercise and were provided with food throughout the three days. There were no differences in hydration status, urine output or total body water between the water or coffee trials. The researchers concluded that moderate consumption of coffee has no effect on total body water, and in regular coffee drinkers, the fluid contributes to your daily fluid needs.
Conclusion: Drinking coffee in moderate doses (2-4 cups per day) does not have a diuretic effect, and the fluid present in coffee contributes to your fluid needs.