Caffeine And Dehydration
Chester J. Zelasko, Ph.D. | February 16, 2007

Health information can be tricky. Sometimes the things we hear and read are based on actual research, and sometimes they’re based on observation and connecting the dots without any real science to support it.

Case in point is the role of caffeine in dehydration. Caffeine is a diuretic; that means it will eliminate water from your body. That has led to the conclusion that if you exercise or work in the heat, you must avoid caffeine or you will become dehydrated. In fact, that’s been reported lately in the press due to the large amount of caffeine in energy drinks. But is there science to support that conclusion? That’s the topic of this Better Life Newsletter.

Caffeine as Diuretic
Caffeine is metabolized in the liver by the cytochrome p450 oxidase system into three different chemicals:
  • Paraxanthine
    This works on increasing the amount of fatty acids in the bloodstream. It’s also the reason to avoid caffeine before getting a blood test for cholesterol and triglycerides--caffeine will increase triglyceride levels.

  • Theophylline
    This chemical relaxes the smooth muscles in the lungs allowing better air flow. However, while the medication theophylline is used to treat asthma, not enough is made from caffeine to benefit asthma sufferers.

  • Theobromine
    This is the chemical responsible for the diuretic effect. It dilates blood vessels and increases urine volume.

Looking at how caffeine functions in the body, it’s easy to see why health experts concluded that caffeine before or during exercise may have negative effects--it seems like a logical deduction. But let’s examine the science to see if it’s been confirmed.

Does Caffeine Cause Dehydration?
In a well-designed study of free-living subjects (people going about their normal lives, not confined to a laboratory), researchers controlled the amount of food and fluids ingested over a two-day span (1); 18 subjects participated with five different regimens of carbonated and noncarbonated, caffeinated and caffeine-free beverages. There was no evidence of dehydration under any regimen. Researchers concluded that the recommendation that caffeinated beverages not be included in the total fluid intake for a given day was not justified.

In another study, researchers examined three different levels of caffeine ingestion in healthy males over an 11-day span (2). No evidence of dehydration was detected in any measurement of hydration status examined.

Turning to exercise, caffeine, and dehydration, several papers have examined this topic. In a literature review article, Dr. Larry Armstrong, one of the premier researchers on the effects of heat on exercise performance, concluded that the research does not support the avoidance of caffeine during strenuous exercise (3). While caffeine will increase urine output, it does not effect electrolyte balance and therefore will not cause dehydration or have a negative effect on performance.

Researchers examined the effect of water, a sports drink without caffeine, and a sports drink with caffeine during three separate 18 km (11.2 miles) runs in 98 well-trained athletes (4). There were no negative effects on performance. Other than an increase in gastric distress and flatulence associated with the sports drink, there were no negative effects attributable to the caffeine.

To Caffeinate or Not to Caffeinate
While caffeine exerts a small diuretic effect, there is no need to avoid caffeine before or during exercise based on the best available research. Further, it seems apparent that coffee and other drinks containing caffeine can count as fluids toward your total fluid intake for the day--and as a four-to-five mug strong coffee drinker, I’m personally glad about that.

If someone hasn’t used caffeine regularly, there’s the possibility that caffeine may have a slight diuretic effect. For those who use caffeine regularly, there appears to be no risk of dehydration during any type of exercise.

However, that isn’t the end of the story. Some people metabolize caffeine at varying rates. While it may not affect hydration, it may affect your nervous system. Caffeine is a stimulant and can increase blood pressure and heart rate. It can also keep you awake if you’re sensitive to it. As always, you must know yourself and how your body responds.

Bottom Line:
The point of this Newsletter is simple: don’t avoid caffeine at rest or during exercise because you’re afraid of getting dehydrated. The research just doesn’t support it.

  1. Grandjean, A et al. The Effect of Caffeinated, Non-Caffeinated, Caloric and Non-Caloric Beverages on Hydration. Journal of the American College of Nutrition. 2000:19 (5) 591–600.

  2. Armstrong LE et al. Fluid, electrolyte, and renal indices of hydration during 11 days of controlled caffeine consumption. Int J Sport Nutr Exerc Metab. 2005;15(3):252-65.

  3. Armstrong LE Caffeine, body fluid-electrolyte balance, and exercise performance. Int J Sport Nutr Exerc Metab. 2002;12(2):189-206.

  4. van Nieuwenhoven MA, et al. The effect of two sports drinks and water on GI complaints and performance during an 18-km run. Int J Sports Med. 2005;26(4):281-5.
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