Fluid homeostasis is critical for life and maintenance of health in general. Numerous studies have shown that even dehydration of 2% of body weight increases cardiovascular strain (17), decreases exercise performance (18,23), and hinders the thermoregulatory advantages conferred by high aerobic fitness and heat acclimatization (33).
It is also well documented that the ingestion of a CHO solution during intense exercise lasting >1 h delays fatigue (12,32) and improves performance (1,19). This improvement has been attributed mainly to the maintenance of higher glucose oxidation rates during the later stages of exercise and liver glycogen sparing (13,31). However, this is not the case in shorter (<1 h) and intense events (>75% V˙O2max) where the results remain controversial, with some studies showing an improvement in performance (1,5,19) and others having not found such an effect (3,14,24). The physiological mechanism behind these observations is unclear. It has been reported that exogenous CHO under these conditions have minimal contribution to CHO oxidation in the muscle (22). In addition, it is well documented that muscle glycogen concentration is not the limiting factor during intense events lasting up to 1 h (22,29). Therefore, it has been hypothesized that mouth and/or pharyngeal receptors are involved in this process and may play an important role (10). Previous studies have suggested that activation of those receptors influence thirst and thermoregulatory responses in dehydrated subjects (16,30).
Recent scientific data have investigated the role of mouth rinse as a possible mechanism that could influence athletic performance in intense and short events. Despite the mode of exercise used (running or cycling), some studies indicate a positive effect of mouth rinsing with a CHO drink on performance (8,11,26,27), whereas others do not support these findings (4,34). Although water is the drink of choice for many athletes, no study has investigated the potential effect of mouth rinsing or drinking a small amount of water on performance. A novel study using functional magnetic resonance imaging showed activation of brain regions believed to be involved in reward and motor control during mouth rinse with a CHO drink at rest. These findings suggest that the presence of CHO in the mouth might enhance exercise performance by stimulation of the CNS and more precisely in brain regions associated with reward, such as the orbitofrontal cortex, insula/frontal operculum, and striatum (11).
To our knowledge, no study has investigated the influence of the possible activation of oral–pharyngeal receptors, via swallowing a small amount of water, on exercise performance in previously dehydrated subjects. We hypothesized that oral–pharyngeal receptor activation through ingestion of a small amount of water could lead to increased exercise time compared with mouth rinse in dehydrated subjects.
Ten healthy endurance-trained cyclists were recruited to participate in the study. Their physical characteristics are presented in Table 1. Each subject gave informed consent, and the protocol was approved by the university review board. The sample size was considered adequate to achieve 75% power at a 5% significance level. Eligibility criteria for participation in the study included a normal physical examination and absence of any metabolic, cardiovascular, or renal disease. Body composition was assessed by dual-energy x-ray absorptiometry (model DPX-MD; Lunar Corp., Madison, WI).
All trials were performed in the morning after an overnight fast of at least 10 h and at an ambient temperature of 31°C. Trials were separated by at least 1 wk. The day before each trial, participants were instructed to refrain from any consumption of alcohol, caffeine-containing drinks, and any type of vigorous physical activity. To minimize differences in starting muscle glycogen concentrations, subjects were instructed to record their diet 24 h before their first visit. Their diet record was copied and returned to the participants with instructions to follow the same diet before each subsequent visit.
During the first visit, an incremental exercise test to volitional fatigue at a self-selected cadence on a cycle ergometer (Monark 839E; Monark Exercise, Vansbro, Sweden) was performed to determine maximum oxygen consumption (V˙O2max) and maximal workload (W˙ max). Subjects started with a 5-min warm-up at 100 W. After the warm-up, the workload was increased by 50 W every 2.5 min until the HR reached 160 bpm. The workload was then increased by 25 W every 2.5 min until exhaustion. W˙ max was calculated using the following equation:
where W out is the last completed stage and t is the time of the final unfinished stage (21).
On their arrival at the laboratory, subjects emptied their bladders, and urine samples were collected for subsequent analysis. Baseline body weight was then recorded with the subjects wearing only their shorts. Afterward, a venous catheter was placed into a forearm vein and kept patent by flushing of heparinized saline solution (20 U·mL−1) after every blood sample. After 20 min of rest in a seated position in the heat (31°C), the first blood sample was collected. After the sampling, the dehydration protocol and the performance test followed. Finally, body weight changes were calculated, and urine and blood samples were collected at the end of the dehydration phase and at the end of the performance test.
The dehydration protocol included a 2-h phase of alternate walking and cycling (30 min internal) to induce a 2% body water loss. Every 30-min period consisted of 25 min of exercise and 5 min of rest. The intensity of exercise was adjusted so as to elicit an HR of 70% of maximum HR (130 and 140 bpm) during cycling (on average, at 110 ± 10 W) and walking (at 8.0 ± 0.4 km·h−1). Body weight was recorded immediately after every 25-min period of exercise (2). After the completion of each 25-min period, the subjects were towel dried, and their body weight was recorded with the subjects wearing only their shorts. The dehydration protocol induced a −1.9% ± 0.1% change in the total body weight of the subjects.
Immediately after the blood and urine collection, the subjects began their time-to-exhaustion test, consisting of steady-state cycling at 75% of their individual W˙max at a cadence ranging between 80 and 100 rpm, on a Monark cycle ergometer set in constant-power mode. Before the performance test, subjects were instructed to exercise till volitional exhaustion. The following criteria were used to ensure true attainment of their maximal effort: a) inability to maintain cadence above 60 rpm (20) and b) HR at least 85% of the one observed during the W˙ max test. Seat height was the same for each person for all the trials, and cycling shoes with cleats were used. Verbal encouragement was provided only at the later stages of the test. Finally, all clocks, stopwatches, HR monitors, and cycle computers were removed from subjects’ view during the whole duration of the test. For the performance test, the subjects underwent one of the following interventions in a random order:
- Mouth rinse (MR): subjects rinsed their oral cavity with 25 mL of water every 5 min (8). The participants were instructed to rinse their mouths with water for approximately 5 s and then spit the fluid into a bowl held by an investigator. This specific amount of water was weighed to verify that nothing was ingested.
- Drink (DR): the subjects during the exhaustion test ingested 25 mL of water every 5 min. Water temperature was selected at 15°C on the basis of an earlier work describing temperature preferences of dehydrated humans (7).
- No fluid: the cyclists completed the time-to-exhaustion test without ingesting any amount of fluid. This trial served as the control (CON) trial.
For the experiment, Volvic® (Danone, Paris, France) bottled water was used for MR and DR trials ([Na+] = 11.6 mg·L−1).
Blood samples (approximately 15 mL in each draw) were analyzed immediately in triplicate for hematocrit (microhematocrit method) and hemoglobin (cyanomethemoglobin method, Drabkin reagent; Sigma, St. Louis, MO), while changes were used to calculate changes in plasma volume (15). Lactate was also measured in whole blood in duplicate (Accutrend Lactate; Roche Diagnostics, Mannheim, Germany). Sweat rate (SR) was calculated using the following equation: SR = ([pretest body mass − posttest body mass]) + fluids consumed)/time (9). HR was measured continuously using an HR monitor (T3C; Suunto, Vantaa, Finland). During the time-to-exhaustion test, the subjects were also asked to give their RPE using the 6- to 20-point Borg scale (6). Total work was recorded in kilojoules from the summary results provided by the ergometer’s computer.
Data are presented as means ± 1 SE. A repeated-measures ANOVA model was used to evaluate the effect of the treatment in the study variables. A Tukey test was conducted to examine between-trials effects. Data were also analyzed for an order effect. However, repeated ANOVA revealed no statistical significance (P = 0.45). Differences between trials are presented as means ± SE along with their 95% confidence intervals (CI). A type I error of 0.05 or less was the threshold for statistical significance.
Ingestion of a small amount of water induced longer exercise time to exhaustion in mildly dehydrated subjects, when compared with the MR or the CON trial (DR = 21.9 ± 1.2 min, 95% CI = 19.2–24.6; MR = 18.7 ± 1.3 min, 95% CI = 15.8–21.7; CON = 17.7 ± 1.1 min, 95% CI = 15.2–20.2 (P = 0.012)) (Fig. 1). Eight of 10 subjects performed better with DR versus MR and CON, whereas 6 of 10 cycled longer with MR compared with CON (Fig. 2). Ingestion of a small amount of water resulted in a 21.5% ± 6.8% increase in cycling time (DR vs MR) and in a 25.2% ± 7.9% greater time to exhaustion compared with placebo (DR vs CON).
Values for maximum lactate concentration, HR, RPE, and total work at the end of the performance test are presented in Table 2. HR increased gradually throughout the time-to-exhaustion test, reaching values of 87% ± 3%, 89% ± 4%, and 88% ± 3% of HR maximum at the final minute of the time-to-exhaustion trial, for MR, DR, and CON trials, respectively. Maximum lactate concentration 3 min after the end of the performance test was 11.4 ± 1.8, 10.9 ± 1.6, and 11.8 ± 2.1 mmol·L−1, respectively. All the cyclists accomplished their time-to-exhaustion test with values of 15.7 ± 0.4 (MR), 15.5 ± 0.4 (DR), and 15.7 ± 0.5 (CON) according to the Borg scale. Total work was significantly higher in the DR trial where the subjects ingested water; however, no statistical significance was observed in any of the other aforementioned parameters between the three conditions (P > 0.05). According to the recorded changes of total body weight, the subjects began the performance test being dehydrated by −1.9% ± 0.1% (MR), −2.1% ± 0.1% (DR), and −2.0% ± 0.1% (CON) of their total body weight. Moreover, after the performance trial, dehydration reached −2.8% ± 0.1%, −2.9% ± 0.1%, and −2.8% ± 0.1% in the MR, DR, and CON trials, respectively (P < 0.05). Calculated SR values were 1.5 ± 0.2, 1.7 ± 0.4, and 1.5 ± 0.3 L·h−1 for the MR, DR, and CON trials, respectively, with the cyclists consuming 100 mL of water only in the DR trial. Finally, plasma volume decreased over time, reaching −2.2% ± 0.4% (MR), −2.0% ± 0.3% (DR), and 2.1% ± 0.4% (CON) at the end of the dehydration phase and −2.5% ± 0.4% (MR), −2.2% ± 0.3% (DR), and 2.3% ± 0.4% (CON) at the end of the performance test.
In the present study, we examined the effect of the potential activation of oral and pharyngeal receptors, through mouth rinse or ingestion of a small amount of water, on performance in dehydrated subjects. To the best of our knowledge, this is the first study that investigated the role of the oral–pharyngeal receptors with water on cycling performance. The main finding is that the cyclists who ingested the small amount of water cycled significantly longer when compared with either the MR or the CON trial.
In all published studies, mouth rinsing was performed with a CHO-containing solution (8,11,25–27,34). When mouth rinse enhanced performance, the researchers attributed this improvement to a central effect due to the presence of CHO in the oral cavity, which, in turn, stimulates the reward and/or pleasure centers in the brain (8,11,25,26). Moreover, this central drive was activated despite the type of CHO used and regardless of flavor. To avoid any motivation via the effect of the use of CHO, we provided plain water. We demonstrated that a small amount of water can also enhance performance in exercise intensities typically observed in endurance events, better than mouth rinse. In addition, the efficacy of water suggests that probably, the sensation of swallowing along with the cool sense in the digestive track can motivate moderately dehydrated subjects and lead to an increase in performance. Finally, the absence of any type of CHO within the oral cavity entirely rules out any risk of possible contribution to total CHO oxidation rates via unintentional swallowing.
There is evidence that drinking itself activates the oropharyngeal receptors, which, in turn, influence significantly fluid balance, thermoregulation, and possibly exercise performance. According to the results by Takamata et al. (30), the act of drinking has a reflex effect on the osmotic regulation of sweating and on arginine vasopressin (AVP) in dehydrated subjects. Moreover, in a classic study published by Figaro and Mack (16), subjects performed three identical dehydration protocols followed by 75 min of rehydration consisting of 1) ad libitum drinking (control), 2) infusion of a similar volume of water directly into the stomach via a nasogastric tube (infusion), and 3) ad libitum drinking with simultaneous extraction of ingested fluid via a nasogastric tube (extraction). The researchers found reflex inhibition of AVP and thirst in control and extraction but not during infusion, suggesting that oropharyngeal reflexes modulate thirst and the secretion of AVP.
Lastly, Casa et al. (10) demonstrated that oral, in contrast to intravenous, rehydration in dehydrated subjects resulted in better maintenance of many physiological parameters. Although no statistically significant differences were observed between the two methods of rehydration, there was a clear trend toward performance enhancement when the fluids were passing through the typical oral–pharyngeal–stomach–intestine pathway.
In a recent published study by Rollo et al. (28), the researchers investigated the influence of ingesting versus mouth rinsing a CHO–electrolyte solution during a 1-h run. In contrast to the results by Pottier et al. (25), their main finding was that ingestion of a 6.5% CHO–electrolyte solution was associated with a greater distance covered during a 1-h performance run in comparison with mouth rinsing and the ingestion of the same volume of a placebo solution. However, it is important to note the possible effect of total fluid volume ingested on performance. Contrary to our methodology where participants were dehydrated and ingested 100 ± 16.6 mL of water, the volunteers in the aforementioned studies ingested 5–10 times greater volume of fluids (∼500 and ∼1000 mL, respectively).
We acknowledge that a possible limitation of our study is the performance test that it used. Although very common in exercise testing, the time-to-exhaustion test can be subjective. However, the parameters measured in the final stages of the test (blood lactate, HR, and cadence) revealed no significant differences between the subjects, implying that all subjects were approximately at the same fatigue level when the performance test was terminated. Furthermore, dehydration at the end of the exercise test, as expressed by reduction of body mass, did not differ among the trials (CON = −2.79% ± 0.13%, MR = −2.77% ± 0.18%, and DR = −2.94% ± 0.17%; P = 0.48), indicating that the aforementioned performance differences in cycling time to exhaustion were independent of the dehydration level.
In conclusion, the present study demonstrated that ingestion of a small amount of water could lead to increased exercise time in dehydrated subjects, even in short and intense events, possibly through activation of the oral–pharyngeal receptors. The underlying physiological mechanism and its implication in sports performance need further examination.
No conflict of interest or financial disclosure exists.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:©2012The American College of Sports Medicine
SPORTS DRINK; DEHYDRATION; HYDRATION STATUS; EXERCISE; CYCLING; ORAL/PHARYNGEAL RECEPTORS