Hopkins, W. G., Rowlands, D. S., & Bonetti, D. L. (2010). Effects of water, hypotonic, isotonic and hypertonic sports drinks on fluid absorption and endurance performance. Presentation 860 at the 2010 Annual Meeting of the American College of Sports Medicine, Baltimore, Maryland; June 2-5.

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This study compared absorption and effects on performance of a hypotonic sports drink (3.9% carbohydrate, 10 mM NaCl, 220 mOsm) with an isotonic sports drink (7.6% carbohydrate, 12 mM NaCl, 280 mOsm), a hypertonic sports drink (6% carbohydrate, 21 mM NaCl, 330 mOsm), and a water placebo. Well-trained cyclists(N = 11) consumed each of the four drinks on separate days at a rate of 250 ml every 15 minutes during a two-hour steady ride at constant power (55-60% of baseline peak power). The preload was followed by a continuous incremental test to peak power. Tests were performed at room temperature (18-22C) and separated by three to seven days. The physiological measures were: uptake of D20-labeled water; plasma volume; osmolar, electrolyte, glucose, and lactate concentrations in blood samples taken before, during, and after exercise; volume and osmolarity of urine collected before and after exercise; sweat volume inferred from change in body mass; and heart rate and tympanic temperature recorded throughout exercise.

The uptake of D20 label during the first half and second half of the two-hour preload with the hypotonic drink was substantially faster than with water (by 3% and 18% respectively) and the other sports drinks (by ~9-33%). Serum osmolarity was substantially lower and accompanied by smaller reductions in plasma volume during the first half of the two-hour preload with the hypotonic drink compared with the other sports drinks. Tympanic temperature, heart rate and gut discomfort were higher during the first half of the two-hour preload during the trials with the hypertonic drink compared with the hypotonic drink and water. There were no clear differences in peak heart rate and peak lactate, but peak power with the hypotonic drink was substantially higher than with water (3.0%) and slightly higher than with the other drinks (1.2% and 1.3%).

Implication. The physiological outcomes are consistent with earlier and faster absorption of the hypotonic drink compared with the other drinks, resulting in a possibly small beneficial effect on endurance performance.

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