Does coffee ACTUALLY dehydrate you and harm performance?

The general consensus is that caffeine can boost performance, especially during longer bouts of endurance exercise, so athletes tend to be heavy coffee drinkers. But what effect does coffee have on your hydration status?

Is coffee a diuretic?

The idea that caffeine can promote dehydration isn't new. A small but influential study back in 1928 saw participants pee out up to 50% more urine when they drank caffeinated water and coffee. Most of us have probably felt this effect. 

And caffeine is still widely regarded as being a mild diuretic (a compound that causes your kidneys to produce more urine than they normally would).

A recent study looked at urine volumes produced in the 2 hours after drinking a range of drinks; the fluid loss from coffee was highest.

But, the key thing that’s often not talked about is that the diuresis that coffee consumption can induce doesn't appear to result in a net fluid loss over a longer period, when you take into account the fluid that's absorbed and retained from the coffee itself.

This has been relatively widely researched, with many studies coming to the conclusion that, although urine output can be increased with coffee/caffeine consumption, the actual result on body fluid balance doesn't appear to be negative overall.

This has led to there being a gradual U-turn in the scientific community on the idea that drinking coffee will compromise your hydration status on a day-to-day basis.

Does coffee dehydrate you? 

The diuretic effects on regular coffee drinkers seem to be way less pronounced than on those who have it infrequently and avoiding coffee for as little as 4 days can reverse the adaptations that reduce that diuresis markedly.

Individual responses to caffeine

People handle caffeine differently, there are different genotypes that determine your body’s ability to metabolise caffeine.

So it's not a stretch to think that this might mean the effects of caffeine might vary depending on your genetic make up. We probably all know people who seem to be able to drink coffee and tea all day long and show no signs of dehydration and others who just don’t seem to get on with it at all.

For me, it’s highly likely that drinking coffee does not cause serious dehydration.

Should you drink coffee to maintain hydration status?

With that said, I believe that in situations where maintaining a good hydration status is challenging (e.g. when training or travelling on long haul flight, etc), coffee is not the optimal choice for staying hydrated, especially if you don’t drink it regularly.

I also think that a level of personal experimentation and common sense needs to be employed to figure out what level of coffee consumption works well for you as an individual, as clearly it isn’t the same for everyone.

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Further Reading

• How well do different drinks hydrate you?
• Should you use caffeine as part of your fueling strategy?
• How to use caffeine BEFORE exercise
• How much caffeine should you use DURING training and races?