This question has been asked for many years. Does caffeine cause dehydration? My perception is that the majority of people, athletes and non-athletes, belief is that caffeine does cause dehydration. We all “know” coffee and caffeine are diuretics. And although it may seem logical that caffeine would dehydrate you, the majority of research tells us otherwise.
A study published in Medicine and Science in Sports and Exercise Journal looked at seven endurance-trained (VO2max = 61 +/- 8 mL.kg.min) heat-acclimated cyclists. The subjects pedaled for 120 min at 63% VO2max in a hot-dry environment (36 degrees C/97 degrees F; 29% humidity) on six occasions: 1) without rehydration; 2) rehydrating 97% of sweat losses with water ; 3) rehydrating the same volume with a 6% carbohydrate-electrolytes solution; or combining these treatments with the ingestion of 6 mg caffeine per kg of body weight 45 min before exercise; that is, 4) caffeine, without rehydration; 5) caffeine plus rehydrating 97% of sweat loss with water; and 6) caffeine plus rehydrating with the 6% carbohydrate electrolyte solution.
Caffeine did not alter heat production, forearm skin blood flow, or sweat rate (although it did increase rectal temperature to 39.4 +/- 0.1 degrees C, whereas it remained at 38.7 +/- 0.1 without caffeine). Caffeine ingestion increased sweat losses of sodium, chloride, and potassium (approximately 14%; P < 0.05) and enlarged urine flow (28%; P < 0.05).
The conclusion the study came to was that caffeine ingested alone or in combination with water or a sports drink was not thermogenic or impaired heat dissipation. Caffeine increased urine flow and sweat electrolyte excretion, but these effects are not enough to affect dehydration or blood electrolyte levels when exercising for 120 min in a hot environment.
The reason this topic is important because of the performance-enhancing effects of caffeine. Clearly, with a stimulant like caffeine, too much can be harmful. But it is apparent that 5 mg per kg of body weight (so 420 mg of caffeine for me—about a quad espresso) has performance-enhancing effects without negative effects for a healthy athlete.
A study published in the Indian Journal of Medical Research took nine recreational male runners (aged: 25.4+/-6.9 yr) who were “nonusers” of caffeine (23.7+/-12.6 mg per day), they were either given caffeine (5 mg per kg of body weight) or placebo in the form of a capsule one hour prior to the running exercise trial at 70 per cent of VO2max on a treadmill in a heat-controlled laboratory (31 degrees C/88 degrees F, 70% relative humidity). The runners drank 3 ml of cool water per kg of body weight every 20 min during the running trials to avoid the adverse effects of dehydration. Heart rate, core body temperature and rate of perceived exertion (RPE) were recorded at intervals of 10 min, while oxygen consumption was measured at intervals of 20 min.
Running time to exhaustion was significantly (P<0.05) higher in the caffeine trial compared to the placebo trial. Heart rate, core body temperature, oxygen uptake and RPE did not show any significant variation between the trials but it increased significantly during exercise from their respective resting values in both trials (P<0.001).