In the study, participants drank about 27 ounces of coffee per day for three days in a row. (For reference, a Starbucks Venti is 20 ounces, a Grande is 16, and a Tall is 12.) Then they drank the same amount of water for three consecutive days. Controlling for physical activity and food and fluid intake, the researchers compared a wide range of hydration markers (body weight, total amount of water in the body, kidney function, urine volume, and blood values) and found no significant difference in subjects’ hydration status when they were drinking coffee versus water. Although the study population is small and specific — 50 adult men who were habitual coffee-drinkers and were told not to exercise during the study — its findings echo similar previously-collected data regarding the relationship between caffeine consumption and hydration.
A 2002 review of 10 different studies concluded that even when consumed in larger amounts caffeine caused no detrimental effects on hydration status or even athletic performance Caffeine, body fluid-electrolyte balance, and exercise performance. Armstrong, LE. International Journal of Sport Nutrition and Exercise Metabolism. 2002 Jun;12(2):189-206. . A few years later, in 2005, researchers found that consuming small amounts of caffeine did not impact hydration status Fluid, electrolyte, and renal indices of hydration during 11 days of controlled caffeine consumption. Armstrong LE, Pumerantz AC, Roti MW, et al. International Journal of Sport Nutrition and Exercise Metabolism. 2005 Jun;15(3):252-65. .
Considering coffee’s hydrating qualities, not to mention its ability to increase metabolism, enhance memory, and lower risk of death, I think I’ll make my next cup bottomless Caffeine and coffee: their influence on metabolic rate and substrate utilization in normal weight and obese individuals. Acheson KJ, Zahorska-Markiewicz B, Pittet P, et al. The American Journal of Clinical Nutrition, 1980 May;33(5):989-97 Post-study caffeine administration enhances memory consolidation in humans. Borota D, Murray E, Keceli G, et al. Nature Neuroscience, 2014 Jan 12 The relationship of coffee consumption with mortality. Lopez-Garcia E, van Dam RM, Li TY, et al. Annals of Internal Medicine, 2008 Jun 17;148(12):904-14. .
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