How many times have you been told, “That cup of coffee will dehydrate you”? There’s a long-standing belief in the nutrition world that drinking coffee can dehydrate you, and this speculation has come around 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, with the average cup of coffee containing 80mg 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 6-8 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 in 2014, 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 to control for fluid intake in food and fluid lost during exercise. 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.
Despite the long-stranding belief, drinking coffee in moderate doses (2-4 cups per day) does not have a diuretic effect and will not dehydrate you. In fact, the fluid present in coffee actually contributes to your daily fluid needs. So don’t worry about that cup of coffee, it won’t dehydrate you, it may even help your mental or exercise performance for the next 4-6 hours after you’ve had it.
About the Author
Mark Funnell holds a BSc (Honours) degree in Sport and Exercise Science and an MSc in Sport and Exercise Nutrition from Loughborough University. He is currently working as a Performance Nutritionist for the England and Wales Cricket Board with both the England Women’s Academy and Midlands Regional Disability Cricket squads. Additionally, Mark is a graduate member of the Sport and Exercise Nutrition Register (SENr), an ISAK Level 1 Anthropometrist and a UK Anti-Doping advisor.