Does Caffeine Really Dehydrate You?

This article is an excerpt from Sports Nutrition for Endurance Athletes by Monique Ryan MS, RD, LDN, CSSD. In her comprehensive guide to sports nutrition, Ryan uses her 30 years of experience coaching professional and age-group athletes to simplify this complex subject into proven, real world guidelines. Sports Nutrition for Endurance Athletes shows runners, cyclists, and triathletes how to address specific nutritional needs for short- and long-course racing and busts dozens of myths and misconceptions along the way.

In North America, about 90 percent of adults regularly consume caffeine, mainly in liquid form, whether from cola, tea, coffee, or other caffeine-laced beverages. Caffeine is also present in cocoa and chocolate. Given this statistic, it is safe to assume that most endurance athletes ingest caffeine on a daily basis and perhaps even during training and competition. While the proper use of caffeine can enhance physical performance, it has long been labeled as a diuretic, or a substance that actually increases fluid losses by increasing urine production. In her book Sports Nutrition for Endurance Athletes, author Monique Ryan gets to the heart of this debate around caffeine and the thought that it dehydrates athletes.

Athletes have often been cautioned to limit their daily caffeine and not to count caffeine-containing beverages toward total daily fluid intake, but is caffeine truly dehydrating? For almost a decade scientists have challenged this assumption about caffeine. Reassessments of existing data and new studies of the effects of caffeine on the hydration status of athletes show that, while caffeine often does act as a mild diuretic, stimulating urine production from the kidneys, it does not produce a greater increase in urine volume when compared with the same volume of water or caffeine-free fluid consumption. One study compared the effects of caffeinated and non-caffeinated beverages (both caloric and calorie-free) on the daily hydration status of healthy males. Over a twenty-four-hour period, there were no significant hydration differences among the various beverages. Another study had athletes rehydrate with a caffeinated beverage between exercise periods. The researchers found little evidence to support that caffeine can slow down an athlete’s efforts to rehydrate. Bottom line: Caffeine is not the powerful diuretic it was once thought to be.

Endurance athletes can rest assured that a moderate daily intake of caffeine of about 1.4 to 2.7 milligrams per pound (3–6 mg/ kg) of body weight should not compromise their daily hydration status when consumed within the context of a well-balanced diet. Caffeine intakes safely falling into this range would be 230 to 460 milligrams of caffeine for a 170-pound (77 kg) man, or 190 to 380 milligrams for a 140-pound (64 kg) woman. A can of soda contains about 40 to 45 milligrams of caffeine, but the amount of caffeine in a cup of coffee varies widely depending on how the coffee is brewed (for the caffeine content of some beverages, see Table 1.1). Certainly, caffeine-containing drinks should not compromise the majority of your fluid intake.

Avoid excessive doses of caffeine, however, as high amounts are associated with side effects such as nervousness, gastrointestinal upset, irritability, and insomnia. Higher doses can also negatively affect your hydration status and do not result in performance improvements much beyond the moderate doses. Heavy coffee drinkers may find that the acidic nature of the drink can cause reflux.

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See what to eat and when with Sports Nutrition for Endurance Athletes. Ryan demystifies optimum daily nutrition and shows simple steps to make the best decisions about what you eat and drink.

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