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Arnaoutis, G, SA Kavouras, I Christaki, LS Sidossis . Water ingestion improves performance compared with mouth rinse in dehydrated subjects. Medicine and Science in Sports and Exercise 2012, 44(1), 175-179

It is well recognised that the ingestion of carbohydrate-electrolyte sports drinks can enhance performance of endurance exercise and can also have some other benefits such as a reduction in core temperature, in heart rate and in the perception of effort in exercise that is not continued to exhaustion. These effects were generally attributed to an effect of the increased availability of the ingested carbohydrate as a fuel for the working muscles and action of the water and sodium to balance the losses in sweat and maintain the blood volume. That picture changed a few years ago with the publication of a study that showed that simply rinsing the mouth with a carbohydrate drink that was then expectorated without swallowing was also effective in improving exercise performance. The interpretation of this observation was that receptors in the mouth responded to the presence of carbohydrate and signalled to the brain that carbohydrate was on its way.

We know that there are also receptors in the mouth and pharynx that respond to water ingestion and can even meter the amount consumed. This raises the obvious question as to whether rinsing the mouth with plain water – rather than with a carbohydrate-containing solution – has any effect on exercise performance or on the physiological and psychological responses to exercise. This paper addressed that question by requiring ten healthy trained male cyclists to complete three time-to-exhaustion cycling tests at 75% of their maximum power output after being dehydrated by 2% of their total body weight. Dehydration was induced by a 2-h moderate-intensity exercise (70% of maximum HR), which included 30-min intervals alternating between jogging and cycling in the heat (31°C). All subjects repeated the protocol in random order on three separate occasions: a) mouth rinse with 25 mL of plain water before and every 5 min of the trial (MR), b) ingestion of 25 mL of plain water before and every 5 min (DR), and c) control (CON), where no fluids were provided. Time to exhaustion was longer in the DR trial than on MR and CON trials (Mean ± SEM: 21.9 ± 1.2 vs 18.7 ± 1.3 and 17.7 ± 1.1 min, respectively; P < 0.05). There were no differences in heart rate responses, maximum blood lactate concentration, or the subjective rating of effort between the three trials. Based on these results, the authors concluded that ingestion of even a small amount of water increased exercise time in dehydrated subjects possibly through activation of pharyngeal receptors, but that simply rinsing the mouth with water had no significant effects. It remains to be seen, though, whether a longer exercise period would show similar responses.