Rethinking the hydration paradigm

With War Season upon us, many thoughts turn to how to survive - and enjoy - the next large camping event. Friar Galen of Ockham, OP, a physician in the modern world, offers some information on hydration and how it really works.

Friar Galen writes:

How many times have you heard "don't drink caffeine when you're dehydrated - you'll pee out more than you take in and make it worse" I know I can't count the number of times I've given that advice. But through the years of repeating this advice and observing what people actually do, I began to have some nagging doubts. After searching the medical research literature, it appears the issue is even simpler than I had thought.

If you think of the body as a tank of fluid, it gets filled by drinking and loses volume through urine and sweat (for simplicity, I'll ignore other mechanisms of loss). The rate of urine production is automatically adjusted to try to keep the tank full - when more fluid comes in, more is let out; if not much is coming in, then urine output is slowed. Certain substances like caffeine interfere with this control mechanism by increasing the rate of urine production. When we exercise or spend time in the heat, fluid loss increases through sweat (the tank also gets a bit bigger due to extra blood flow to muscles and skin, but again I'll ignore this for the sake of simplicity). The body responds by decreasing urine production and letting you know you're thirsty. You (hopefully) respond by drinking.

Now the debate begins: what is the best fluid for rehydration?

The classic argument has been between water and electrolyte fluids (sports drinks). I've always maintained that water is better, at least in the short term. You lose more water than salts when you sweat, and you replace the salts you need in your normal diet. The sports drinks aren't absorbed as fast, but include the essential salts and energy (carbohydrates). Either one has been long accepted as appropriate for rehydration. What has been verboten has been anything containing caffeine (or alcohol, but that's a story for another day). Caffeine is indeed a diuretic - a substance that overrides the body's reflex to decrease urine production. So at first look, it makes sense to tell people who are trying to fill their tank to avoid something that pulls out the stopper.

But let's think about this some more. If your goal is to fill the tank, you can still accomplish that goal even if you have made the drain a little bigger. You just need to put more in the input side. So, it follows logically that if you would choose to rehydrate with a caffeine-containing beverage, you should be able to, but you will have to drink more than if you avoided caffeine. Obviously this can't go on indefinitely, as you'll end up with a net loss, but for the short term there shouldn't be any negative effects. As it turns out, caffeine may not open the drain as much as is commonly thought.

A review article in the Journal of Human Nutrition and Dietetics from 2003 looked at published studies from 1966 through 2002 that deal with caffeine and fluid balance in humans. By aggregating the studies from this 36 year period they came to the following conclusions:

  1. Large doses of caffeine (above 250 mg) have an acute diuretic action;

  2. Single caffeine doses at the levels found in commonly consumed beverages have little or no diuretic action;

  3. Regular caffeine users become habituated to the effects of caffeine, diminishing its actions.

To put this into perspective, typical soft drinks contain 20 - 70 mg caffeine, tea 40 - 75 mg, and coffee 60 - 200 (though if you get the premium grande from your favorite emporium, it could easily be 600 mg or more). Therefore, most soft drink and tea drinkers are usually ingesting less than the 250 mg, the point at which most studies showed the diuretic effect starts. Moderate intake of caffeinated drinks is not likely to trigger an increase in urine output, and should not interfere with rehydration. Intake over 250 mg may not have much effect either as people who drink that much are usually doing it every day and therefore not going to have as strong of an effect as on someone who has been abstaining from caffeine.

So what is a reasonable recommendation?
We should continue the mantra of "hydrate, hydrate, hydrate" to ensure people are reminded to keep up their fluid intake. What they actually choose to drink can largely be left up to their choice based on what tastes good to them at the time. From a standpoint of physiology, water is the best rehydration solution and is the fastest absorbed into the body (but be aware of the very rare complication of hyponatremia ). Sports drinks do a good job of rehydration and are flavorful and typically taste better than water. But if nothing tastes better to the fighter coming off the field than a couple of ice cold Mt Dews, by all means, enjoy!

Galen of Ockham, OP
MKA Keith E Brandt, MD, MPH, FAAFP
Maughan, R.J., J. Griffin. Caffeine ingestion and fluid balance: a review. J Hum Nutr Dietet, 16, pp 411 - 420. 2003.

© 2012 Galen of Ockham (MKA Keith E. Brandt, MD, MPH, FAAFP). May be used in SCA publications as long as content is not modified and proper credit given. For all other uses, please contact the author at Republished on with kind permission of the author.