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Caffeine and Fluid Loss During Exercise

Caffeine and Fluid Loss During Exercise

A large and continuously growing body of evidence demonstrates the performance-enhancing effects of caffeine in endurance exercise (4). Yet athletes are very often hesitant to reach for the coffee mug prior to exercise or competition. The reasons given are diverse, and include jitters, palpitations, a mid-performance “crash”, heat overload, and many other concerns. Probably most commonly cited, though, is the worry of caffeine-induced diuresis (increased urination) leading to dehydration. But how warranted really is this concern?

Eighty percent of adults in the United States consume caffeine daily, with an average consumption of approximately 200 milligrams per day (5). Caffeine is well-recognized as having diuretic effects (5, 8), most likely through its effects on adenosine and the kidneys (1, 6, 9), although this has not been firmly established. But regardless of the mechanisms, the fact that caffeine causes you to urinate more is hardly news to the average person.

The situation changes, however, when one is exercising. Wemple et al demonstrated in 1997 that caffeine given during exercise did not lead to increased urination when compared to placebo (3). In their landmark study, six healthy, highly trained subjects participated in each of four separate trials: rest with caffeine, rest with placebo, exercise with caffeine, and exercise with placebo. They found that at rest, caffeine increased urination compared to placebo, as would be expected from caffeine’s known diuretic properties. However, during the exercise trials, this diuretic effect was lost. The subjects urinated similar amounts during exercise whether or not they consumed caffeine.

These findings were subsequently corroborated in additional studies. A 2015 meta-analysis examined the results of 28 investigations on the subject of caffeine induced diuresis and concluded that the diuretic effect of caffeine did not exist with exercise (1), and that even during resting conditions the diuretic effect was small. The median dosage of caffeine in this meta-analysis was 300 milligrams (4 milligrams per kilogram for a 165 pound person), which is the approximate caffeine content of two cups of brewed coffee (150 mg of caffeine/150 mL) or 6 cans of cola (50 mg of caffeine/360 mL).

Dehydration is absolutely a concern in endurance competition, or for that matter in any prolonged physical activity. This is all the more true when such activities take place in the heat. However, concerns that caffeine can somehow exacerbate this process are unfounded. Caffeine is a well-established ergogenic aid, and will not lead to increased fluid losses during endurance competition.

1. Zhang Y, Coca A, Casa DJ, Antonio J, Green JM, Bishop PA. Caffeine and diuresis during rest and exercise: A meta-analysis. J Sci Med Sport. 2015 Sep;18(5):569-74.
2. Cox GR, Desbrow B, Montgomery PG, et al. “Effect of different protocols of caffeine intake on metabolism and endurance performance.” J Appl Physiol 93: 990–999, 2002.
3. Wemple RD, Lamb DR,and McKeever KH. Caffeine-free sports drinks: effect on urine production at rest and during prolonged exercise. Int. J. Sports Med. 18: 40–46, 1997.
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5. U.S. Food and Drug Administration. [Accessed October 8, 2016] Medicines in my home: caffeine and your body. Available at:
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7. Shirley DG, Walter SJ, Noormohamed FH. Natriuretic effect of caffeine: assessment of segmental sodium reabsorption in humans. Clin Sci (Lond). 2002; 103(5):461–466.
8. Eddy, N.B., and A.W. Downs. Tolerance and cross-tolerance in the human subject to the diuretic effect of caffeine, theobromine, and theophylline. J. Pharmacol. Exp. Ther. 33:167Y174, 1928.
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