We often read that coffee or caffeine dehydrates you. Maintenance of fluid balance is essential to sustain human life and we are recommended to drink regularly and recommendations for daily fluid intake vary from 1.5-3.7 liters per day. It has also been suggested that beverages that contain caffeine (such as coffee) should not be included in these daily fluid requirement guidelines, because of its diuretic effect. It is even recommended to drink a glass of water with each cup of coffee or tea. Does caffeine or coffee really dehydrate you? Or can you drink these beverages and do they contribute to daily fluid requirements.

Caffeine can be a diuretic

Before I discuss a recent study I want to make one thing clear: caffeine CAN be a diuretic! When consumed in large doses (over 500 mg), caffeine elicits a diuretic effect and this is known for more than 80 years! 500mg is a very high dose to ingested as one dose or in one drink. For comparison 400mg per day is considered a moderate daily intake and an average cup of coffee may contain 80 mg (For amounts of caffeine in different beverages read this previous blog).

It is also important to know that regular caffeine consumption may lead to a tolerance developing against its diuretic effect. It has since been suggested that caffeine withdrawal of as little as 4 days is sufficient for tolerance to be lost. So if you are a regular coffee drinker the diuretic effect may be less, even at higher doses of caffeine.

Coffee and hydration study

In a study published in PLOSOne (Dr Sophie Killer was the first author of the study) we addressed the question: Can coffee contribute to daily requirements when consumed in normal daily life, or will it result in dehydration? Where previous studies had studied the short term effects of coffee or caffeine consumption, in this study we were more interested in the longer term effects. We wanted it to be the best and the largest study of its kind so recruited 50 male coffee drinkers and used not one but a wide range of techniques to assess hydration. The effects of coffee consumption were compared against water ingestion during 3 consecutive days. Physical activity was controlled, food and fluid intake was controlled and participants consumed either 4×200 mL of coffee containing 4 mg/kg caffeine or the same amount of water.

Total body water was calculated before and after the trials using Deuterium Oxide, urinary and haematological hydration markers were recorded daily in addition to nude body mass measurement and blood plasma was analysed for caffeine to confirm compliance.

Conclusion

At the end of a very long and labour intensive study it could be concluded that there were no differences between the caffeine and the water trials in any of the markers of hydration. Thus, these data suggest that coffee, when consumed in moderation by caffeine habituated males provides similar hydrating qualities to water.

So now, when we board a British Airways plane we can actually read that coffee and tea also contribute towards daily fluid requirements.

Reference

The study discussed here is available to download for free from the PLOSOne web site:

Killer SC, Blannin AK, Jeukendrup AE. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One. 2014 Jan 9;9(1):e84154. doi: 10.1371/journal.pone.0084154. eCollection 2014.