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Does coffee dehydrate you?

Coffee and caffeine have been well documented for its performance enhancing abilities, but does it also make you dehydrated? Andy Blow discusses...

Credit: Lumina Images / Blend Images/ Getty

Athletes in particular are known to consume tons of coffee at least in part because of the performance benefits that the key ingredient 1,3,7-Trimethylpurine – that’s caffeine to most of us – offers.

Whilst the exact mechanics behind caffeine’s performance enhancing effects are still a little bit mysterious at a physiological level, it’s definitely one of only a handful of dietary supplements that pretty much all credible sports scientists agree can boost performance. It appears that the most measurable benefits that come from caffeine ingestion are most relevant during longer bouts of continuous endurance exercise.

But, in reality, there are few sports where it’s not used to try to improve energy levels, focus, concentration and even power or speed. Suffice to say that it’s certainly something worth considering if you’re looking for an additional edge in your endurance racing and training and you’re not using it already.

In this post though I’m more concerned with a possible downside of drinking coffee as an athlete – the theory that it can cause dehydration.

Does coffee make you pee more?

The idea that coffee (or the caffeine in it to be more precise) can promote dehydration is not in any way new. A small but highly influential study conducted in 1928 where participants were shown to excrete up to 50% more urine when they drank caffeinated water and coffee is often credited as being when this idea started to become popular. And, its fair to say that the caffeine in coffee is still widely regarded as being a mild diuretic (a compound that causes the kidneys to produce more urine than they normally would).

Most of us have probably felt this diuretic effect at one time or another, as drinking coffee can definitely make you pee more than most other drinks. This was neatly show in a recent study looking at urine volumes produced in the 2 hours after drinking a range of drinks, where the fluid loss from coffee was greatest.

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

This fact has been relatively widely researched in the last 15 years or so, with many studies seeming to come to the conclusion that although urine output can be increased with coffee/caffeine consumption, the actual result on body fluid balance does not appear to be negative overall.

This has lead to there being a gradual U-turn, in the scientific community at least, on the idea that drinking coffee will actually compromise your hydration status on a day to day basis. That said (and as we’ve pointed out in another recent blog) – accurate measurement of hydration status in the body is no easy task, and this alone may be enough to obscure some of the realities of what goes on with varying levels of caffeine intake.

Does it help if I’m used to it?

It’s well worth pointing out that one of the important confounding factors in all of the research on coffee and caffeine is whether you are ‘habituated’ (used to) drinking coffee on a regular basis or not. Seemingly the diuretic effects on regular coffee drinkers are way less pronounced than on those who have it infrequently, and abstaining from coffee consumption for as little as 4 days can reverse the adaptations that reduce diuresis markedly.

In my own experience (as a regular and enthusiastic coffee drinker) I have to say that whilst the science definitely does seem to point to the fact that there’s no net dehydrating effect of drinking it, for me there seems to be a ‘threshold’ amount (about 2 strong cups a day as it happens) beyond which I, subjectively at least, start to feel like my hydration status is being compromised.

My own theory on this is that it might have something to do with the fact that when you’re training and competing a lot, and therefore losing a lot of sweat on a daily basis, it can actually be quite hard keeping up with total daily fluid needs even if you use drinks that are very well retained in the body.

If you swap some water/sports drinks and juices out in favour of large volumes of coffee, as inevitably happens when choosing what to drink and when, the mild diuretic effects become more significant because overall fluid balance is a little more fragile anyway.

Does everyone have the same reaction to coffee?

The other factor I think is hugely important – and often overlooked – is the potential for differences in the way the body handles the caffeine in coffee from person to person. It’s well understood that there are different genotypes that determine your body’s ability to metabolise caffeine.

It’s therefore not a stretch to think that this might mean the global effects (including the diuretic effects) of caffeine might vary depending on your genetic make up. I certainly know people who seem to be able to drink coffee and tea all day long and show no signs of dehydration, yet there are others who just don’t seem to get on with it at all.

For me the bottom line on the subject is that it’s highly likely that drinking coffee does not cause serious dehydration. That said, I believe that in situations where maintaining good hydration is challenging anyway (e.g. athletes in training, people travelling on long haul flights etc), coffee is far from the optimal choice for staying topped up with fluids, 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.

Andy Blow has a few top 10 Ironman and 70.3 finishes and an Xterra World Age Group title to his name. He founded Precision Hydration to help athletes solve their hydration issues. He has a degree in Sport and Exercise Science and was once the Team Sports Scientist for Benetton and Renault F1 teams.

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