Does Coffee Really Dehydrate You?

Does Coffee Really Dehydrate You?

What’s the background?
Caffeine is known to have a diuretic effect when consumed in high doses. Since coffee contains caffeine, it is therefore widely assumed that drinking coffee leads to dehydration. However, some studies have reported that regular consumption of caffeine leads to resistance to its diuretic effects, which may mean that a regular caffeine habit does not lead to a reduced hydration status.
Moreover, since coffee contains other compounds in addition to caffeine, these may interact with one another and lead to different effects. And while many studies have explored the effects of caffeine on hydration status and other variables, few have actually investigated the effects of coffee. This is surprising because coffee is one of the most popular beverages consumed worldwide.

What did the researchers do?
The researchers recruited 52 healthy and non-smoking males aged 18 – 46 years who were moderate coffee drinkers, as defined by the consumption of 3 – 6 cups per day (300- 600mg/day caffeine). Coffee consumption was assessed by a 3-day food diary.
The researchers tested the hydration status of the subjects following two treatments (coffee and non-coffee).
The coffee treatment involved the consumption of 4 mugs (200 mL) of coffee per day (4mg of caffeine per kg of bodyweight) while the non-coffee treatment involved the consumption of 4 mugs of water (200 mL) per day. During each of the treatments, diet, other fluid intake and physical activity were all controlled. Tests were performed in relation to body weight, urine and blood markers.

What happened?
The researchers reported that there were no significant differences between the treatments in respect of: bodyweight changes, 24-hour urine volume, urine specific gravity, urine osmolality, urine creatinine, serum osmolality, hematocrit, total plasma protein, serum sodium, serum potassium did not differ between conditions did not differ significantly between the two treatment conditions. Thus, the findings demonstrated no significant effect of coffee across a wide range of hydration assessment indices in comparison with water.

What did the researchers conclude?
The researchers concluded that moderate coffee consumption (4mg of caffeine per kg of bodyweight over 4 mugs per day) did not result in dehydration. They suggest that advice should not be given to the effect that moderate coffee intake is dehydrating.

What are the limitations?
The study was limited in that it is unclear whether it was the moderate amount of caffeine over the course of the day that led to the lack of any effect on hydration status or an interaction between the compounds in coffee itself. It is also unclear whether a greater amount of caffeine in a larger quantity of coffee (e.g. 6 – 8 mugs per day) would have a significantly different effect.

What are the practical implications?
Strength athletes who habitually consume caffeine pre-workout in the form of coffee only do not need to take care to drink additional water to compensate for the dehydrating effect of coffee.
Individuals who enjoy drinking coffee habitually in moderate amounts do not need to consume extra fluids to compensate for the dehydrating effect of coffee prior to engaging in exercise.

The study: No Evidence of Dehydration with Moderate Daily Coffee Intake- A Counterbalanced Cross-Over Study in a Free-Living Population, by Killer, Blannin and Jeukendrup, in PLOS one, 2014

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