Caffeine & Exercise part 2: Hydration and withdrawal

This post is a continuation of a 3 part blog post on caffeine. Read the first post here.

Hydration

We’ve all heard that caffeine can cause dehydration, but what does the evidence say? Due to caffeine’s status as a diuretic, (a substance which promotes renal excretion of salt) it’s understandable where this widely held belief stems from. However, as with many oft-repeated nutrition beliefs, there is actually very little evidence supporting this claim.

A 2015 meta-analysis looked at the data surrounding whether caffeine does have a dehydrating effect found that

“Caffeine ingestion did not lead to excessive fluid loss in healthy adults and the diuretic effect does not exist with exercise.”

Interestingly, the study did find that there was a sex difference in the diuretic effect, with a greater effect on urination being found in women than in males. Caffeine-induced dehydration appears to be highly dose-dependent level, with dosages above 360mg significantly increasing its diuretic effect. So, called ‘caffeine naives’, people who don’t regularly have caffeine and as such are highly sensitive to its effects, seem to be far more affected. Another factor on whether caffeine dehydrates you are fluids, usually the liquid you have with your coffee is more than enough to offset its effects, but if you’re having pure caffeine, say in capsule form, without any accompanying liquid, then there may be a noticeable effect.

Overall, as long as you’re not consuming excessively large dosages, aren’t highly sensitive to it, and are having it with fluid, there really isn’t any reason to worry about caffeine dehydrating you.

Tolerance and withdrawal

Talk to any habitual coffee drinker, and you’ll likely hear the phrase ‘I can’t function without my morning coffee’, a classic joke or is there actually something to it? The evidence all points to a categorical yes. Caffeine withdrawal, also known as caffeine-withdrawal syndrome, is marked by effects such as a headache, fatigue, decreased alertness, dysphoria and irritability. There is sufficient empirical evidence that caffeine withdrawal has been considered a disorder in the DSM (Diagnostic and Statistical Manual of Mental Disorders).

Habitual caffeine usage leads to tolerance, and surprisingly quickly. A study found it only takes 250mg taken 1-4 days in a row to develop complete caffeine tolerance; this is an ‘insurmountable’ tolerance, meaning increasing caffeine intake won't overcome it. While it's easy to become immune to the effects of caffeine, for heavy users to fully re-sensitise their adenosine receptors to caffeine’s effects, it can take up to a month to return to baseline.

The physiological effects of caffeine are interesting as not all its effects become desensitised. There are two types of adenosine receptors that caffeine blocks, the A2 receptor and the A2A  receptor. The A1 receptor promotes sleepiness when activated and as such when blocked by caffeine increases alertness.  Blocking the A2A  receptor can increase dopamine levels, which cause the stimulating and mood enhancing effects. Unlike the A2A receptor, the A1 receptor doesn’t seem to become desensitised, which is likely the reason caffeine use doesn’t lose its awakening effect over time.