Does Caffeine Really Dehydrate You?

Caffeine is often associated with dehydration, more specifically, that drinking a cup of coffee is going to result in a net dehydration.

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Anecdotally this may seem to be the case; if you drink a lot of coffee you might feel thirsty, get a headache, feel your blood pressure rise and increase your urine volume aka pee more.  Like many areas of nutrition, it is very easy to confuse correlation with causation. If we “feel” dehydrated, it must be the coffee, right?

Pour a pot of postulation

It has been postulated that caffeine is exerting it’s diuretic effects (if any) through two mechanisms. 

The first is that it inhibits an enzyme called phosphodiesterase, which results in the breakdown of fat in the muscles and fat tissue. Sweet! While the above mechanism is confirmed, it is thought that this inhibition of the phosphodiesterase enzyme in the kidneys results in increased fluid loss.

The main mechanism that caffeine lowers fatigue is by blocking adenosine receptors. As you move through your day expending energy, a by-product of that energy release is adenosine; as it builds up it creates fatigue. By blocking adenosine receptors we can delay the feeling of fatigue. The second postulation is that by when adenosine receptors in the kidney (which contribute to water reabsorption) are inhibited, we lose more fluids (Hocher et al 2010).

Two mechanisms we can confirm are the release of free fatty acids from phosphodiesterase inhibition and fluid loss from kidney adenosine receptor inhibition.  But whether we can say those result in any substantial fluid loss needs to be scrutinized.

Basically, yes we KNOW caffeine has physiological effects which can be both measured and felt but is it actually dehydrating us?

What’s the science buzz?

After recovering from that dad-joke of a header, I referred to a meta-analysis (essentially a study of studies) that compiled research from28 studies with a combined total of 379 participants (Zhang et al 2016).

In this meta-analysis, there were a number of studies showing a wide range of heterogeneity, or individual difference.  Studies use something called Effect Size (ES) to measure the strength of relationship between two variables, in this case: urine loss with or without 300-500mg caffeine. The effect size for caffeine ranged from negative, trivial, small, medium and large.  What that tells us is that there are individual differences at play here, one person may get a greater fluid loss than another simply because of their genetic makeup.

How the results stack up:

• Magnitude of effect size for 300-500mg of caffeine was small
• Dose was not directly predictive of fluid loss i.e if you ingested more caffeine it did not mean you would lose more fluids
• Women lose more fluids than men to caffeine
• Dosing caffeine over longer periods (hours or days) did not have a significant impact on fluid loss
• When paired with exercise, any fluid loss from caffeine was negated (exercise causes fluid retention)

When we look at these results it is apparent that a moderate dose of caffeine is not causing a significant (if any) loss in fluids and when ingested before exercise, results in no loss of fluids.  Whether it is taken in one drink or sipped over longer periods, the effect is none to very small.

Side effects

Anecdotally, many people may simply drink more coffee, diet soda, juice etc than actual water.  For individuals who rely more on “drinks” more fluids, the problem may lie more in their lower overall fluid intake than actual caffeine intake. If someone prefers fluids that have a taste, they may not consume much water anyway and are unlikely to get their recommended intake simply from coffee and soda.

In addition, the tannins in coffee produce an astringent effect by “sticking” to the tongue and making it feel dry and sandy.  Since we associate a dry mouth with dehydration we may assume that this feeling is indicative of such. 

If you have a dry mouth you won’t feel very hydrated and if you don’t consume any water, you probably will be. 

The end recommendation?  Drink your coffee but drink your water too!

Resources:

Zhang, Y., Coca, A., Casa, D., Antonio, J., Green, J., & Bishop, P. (2015, September). Caffeine and diuresis during rest and exercise: A meta-analysis. Retrieved November 10, 2020, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4725310/

Hocher, B. (2015, December 16). Adenosine A1 receptor antagonists in clinical research and development. Retrieved November 10, 2020, from https://www.sciencedirect.com/science/article/pii/S0085253815545730