Primary Reference
James, L. J., Moss, J., Henry, J., Papadopoulou, C., & Mears, S. A. (2017). Hypohydration impairs endurance performance: a blinded study. Physiological Reports, 5(12), e13315. doi:10.14814/phy2.13315
Secondary References
Cheung, S. S., G. W. McGarr, M. M. Mallette, P. J. Wallace, C. L. Watson, I. M. Kim, et al. 2015. Separate and combined effects of dehydration and thirst sensation on exercise performance in the heat. Scand. J. Med. Sci. Sports 25(Suppl 1):104–111.
Wall, B. A., G. Watson, J. J. Peiffer, C. R. Abbiss, R. Siegel, and P. B. Laursen. 2015. Current hydration guidelines are erroneous: dehydration does not impair exercise performance in the heat. Br. J. Sports Med. 49:1077–1083.
Introduction: Endurance exercise causes a rise in metabolic heat production. In response to this rise, the human body increases sweat rate to facilitate heat loss through evaporative cooling. Fluid intake during endurance training rarely matches fluid lost, resulting in hypohydration. Hypohydration decreases blood plasma volume, which increases cardiovascular strain and may lead to a reduction in peak oxygen uptake. These effects are exacerbated when an athlete is participating in a hot, humid environment.
Research examining hypohydration is limited due to methodological flaws. Studies examining the effects of hypohydration on performance are difficult to blind. Subjects are generally aware of the amount of consumption of fluids and whether they are participating in a fluid restricted study. This makes interpretation of the results difficult because it cannot be ruled out whether the subject’s expectations of hypohydration impact the results. One recent method used to blind subjects is the use of intravenous fluid sources. Two recent studies have used this technique (0% vs 2-3% hypohydration) (Cheung, McGarr, Mallette, Wallace, Watson, Kim, et al.,2015, Wall, Watson, Peiffer, Abbiss, Siegel, and Laursen, 2015). The results of these studies found no difference between euhydrated and hypohydrated subjects. These results clearly demonstrate the confounding results generated by a lack of blinding. Limitations to these studies include the use of isotonic saline as a hydration method. This results in serum osmolality remaining irrespective of hydration status. Additionally, by providing individuals fluid intravenously, it prevents the possibility of performance enhancement via psychological relief of thirst by drinking fluids. Therefore, there is a need for additional studies which not only blind subjects but also produce physiological and perceptual responses associated with hypohydration (decreased plasma volume, increased serum osmolality and thirst). The purpose of this study was to use a combined intragastric and oral rehydration to examine the impact of hydration status on endurance performance in the heat. The author’s hypothesized that hydration status would not influence endurance performance.
The author’s did a very thorough job of explaining the theory behind hypohydration and performance. They provided both sides of the evidence (for and against) as well as getting into the current limitations of the research. An excellent observation was made about how the lack of blinding may have influenced previous results. I also think they generated a creative solution to this problem by using a mixed method of providing standard blinded hydration (intragastric) while continuing with oral hydration for the potential perceptual improvements.
Purpose: The purpose of this study was to use a combined intragastric and oral rehydration to examine the impact of hydration status on endurance performance in the heat.
Subject Description: Seven healthy men (25+/- 2 years, height 179 +/-0.07 cm, body mass 78.6 +/- 6.2 kg, VO2peak 48+/-7 mLkg-1 min -1 ) participated in this study. The authors estimated that six subjects would be required to reject the null hypothesis using previous data provided by Kenefick (2010) with a p of 0.05 and statistical power of 0.8.
Materials and Methods: A total of four preliminary trials were performed. Trial one was to determine VO2 peak and peak power output (PPO). This consisted of cycling at 95 W and increased 35 W every 3 minutes until volitional exhaustion. Heart rate, rating of perceived exertion (RPE, Borg) and 1 minute expired gas samples were collected at each increment. During preliminary trials 2-3, a 5 minute warm-up was performed at 50% PPO, followed by 15 minutes of the performance test used in the experimental trials to provide familiarization. The fourth trial was an exact replication of the experimental protocol, to provide greater familiarization.
Food intake (24h) and physical activity (48h) prior to the first experimental trial were recorded and repeated during the subsequent experimental trial. Subjects were instructed to refrain from strenuous activity or alcohol consumption. A standardized evening meal was consumed between 7 and 10 pm the evening prior to the trail (3.75 gkg body mass of carbohydrate) and breakfast (1 gkg body mass) 2 hours prior to the trial. Subjects were provided 40 mLkg body mass of fluid the day before each trial to ensure fluid intake. Subjects were given 8 mLkg body mass the morning of the trials with breakfast.
The trials were performed in the morning (8-9 am) and were separated by ≥ 7 days. Body mass was immediately recorded and subjects rated their thirst using a visual analog scale. A 20 g plastic cannula was inserted in the antecubital vein prior to an 8 Fr gastric feeding tube to the base of the stomach. A heart monitor was then attached. Subjects were allotted 15 minutes of seated rest and a blood sample was taken, HR and TGI were recorded and thermal sensation rated. Subjects then entered a controlled environment of 34°C and 50% relative humidity and completed a preload of 8 blocks of 15 min cycling at 50% PPO with 5 minutes of rest between blocks. Heart rate, TGI (internal temperature), RPE, and thermal sensation were measured during the last minute of each block. Stomach fullness was measured on a 12 point scale with 12 representing extremely full. Expired gas samples were taken during the final minute of the fourth and eighth exercise blocks. Blood samples were taken after the first, fourth, and eighth blocks. Then the feeding tube was removed, body mass recorded, and thirst ratings recorded. A 15 minute cycling performance test was performed and a final blood draw was done upon completion. A final body mass and thirst rating were recorded.
The performance test consisted of an initial workload set at 90% PPO and subjects could increase or decrease the workload using the console. Subjects were instructed to complete as much work as possible in 15 minutes. No feedback or encouragement on work rate was provided. At each 5 minute time interval, work completed, heart rate, TGI, and environmental conditions were recorded.
Both trials consisted of subject’s ingesting 0.2 mLkg body mass-1 water every 10 minutes of the preload. Water was also infused directly into the stomach through the feeding tube every 5 minutes. The volume of infused water was matched to either replace the amount of sweat (maintain euhydration) or to produce hypohydration of ~2.5% body mass at the end of preload. Sweat loss was determined from the change in body mass during exercise and was determined for both preload and the performance test with the assumption of 1 kg body mass=1 L sweat. A dummy infusion was performed during the hypohydrated trial. Subjects were told the study was investigating drinks of different composition, and the feeding tube was used to conceal drink flavor. Water was provided at ~36°C to eliminate the sensation of cold. Upon completion, subjects were informed of the false intent of the study and asked to guess the real aim. They were also asked if they could identify the hypohydrated and euhydrated trials.
The methods of this study were creative, well-written, and detail oriented. The authors used a creative study design to ensure the subjects were blinded to their hydration state. It was interesting to see the authors gave the subjects a false impression of the studies intent. This approach helped to provide a true placebo group. Additionally, the inclusion of warm water and fake infusions helped to prevent subjects from knowing which study arm they were participating in. Not only did the authors use a creative design, but they also followed up with questions to see if their blinding strategies were effective. This was taking the extra step to ensure their study methods were sound. Many investigations assume their blinding strategies work, without truly knowing. This study eliminates this assumption. Lastly, the authors provided justification for their sample size by using previous research and a power analysis. The use of repeated samples reduces the variability between subjects, as each subject participates in each the experimental and control arms.
Results: Body mass (P = 0.703), serum osmolality (P = 0.878), plasma arginine vasopressin concentration (p= 0.856), and thirst (P = 0.413) were not different between trials. Body mass significantly reduced during the preload in HYP (P < 0.001), but not EU (P = 0.094). Post -performance test, both groups experienced significant body mass loss (HYP P<0.001 and EU P <0.05). The loss in the HYP was greater than EU (P<0.001). Plasma volume decreased ~6% from pre-exercise to 15 min in both trials (P<0.001). Reduction of plasma volume was greater during HYP than EU at 75 min (P<0.05), 155 min (P<0.05) and post-PT (P<0.05). Serum osmolality was greater during HYP than EU at 155 min (p<0.001) and post-PT (p<0.01). Plasma arginine vasopressin concentration was increased at 155 min and post-PT for HYP (p<0.05). Thirst was increased at 155 min and post PT during HYP (P>0.01).
Throughout preload, there were main effects of time for heart rate, RPE, TGI, and thermal sensation, which all increased progressively. Heart rate was greater for HYP than EU at 95, 135, and 155 min (P<0.05). There were no time or interaction effects for thermal sensation, stomach fullness or TGI. There was a main effect of trial but no interaction effect for RPE.
Total work was 8.1% +/- 6.4% greater during EU than HYP. This was most evident during the time period between 5 and 10 minutes (p<0.05) and 10 and 15 minutes (p<0.05) but not between 0 and 5 minutes (P=0.211). Heart rate (P=0.942) and TGI (p=0.103) were similar between trials.
Results from the interview showed only one subject correctly guessed manipulation of hydration status was the true aim. Once this information was revealed, all subjects were able to correctly identify the trial order.
The authors did an excellent job presenting the results of this study. They provided more than enough details and examined the data in depth. They provided the results in a clear and presentable fashion. The results were both included in paragraph and table formats. Additionally, details such as standard errors and p levels were included in the reporting. This made it easy for the reader to see the variance and levels of significance. There was more details in the results than was provided for this critique, which goes to show the level of details the authors achieved. Interestingly, it appears from the interview results subjects were able to tell the difference between when they received fluids and when they did not. This leaves speculation whether the authors’ endpoint of blinding the subjects was achieved. It doesn’t appear this study effectively eliminated the possible benefit from a perceptual response of increased fluid intake. The subjects could clearly tell they were either being infused with liquid or not.
Discussion: The goal of this study was to investigate the effects of hypohydration on endurance performance with a blinded intervention. The results of this study rejected the null hypothesis, as performance decreased ~ 8% in a hypohydrated state. These results are consistent with previous literature which has demonstrated exercise performance to be reduced when subjects begin in a hypohydrated state. Hypohydration appears to impair performance through multiple mechanisms. These mechanisms include hypovolemia and hyperosmolality, which appear to cause a cascade of effects which limit performance. This cascade of effects includes a possible reduction in muscle and cerebral blood flow, increased cardiovascular strain, impaired thermoregulation, increased perceived exertion, and increased thirst. This study produced reductions in plasma volume and increases in heart rate, RPE, osmolality, arginine vasopressin, and thirst during hypohydration. Internal temperature tended to be higher in the HYP group, but the study was underpowered to detect significance. The methods used in this study are the likely reason the results differed from the two previously mentioned studies (Cheung, McGarr, Mallette, Wallace, Watson, Kim, et al.,2015, Wall, Watson, Peiffer, Abbiss, Siegel, and Laursen, 2015). The current study ensured the prevention of hyperosmolality by using water rather than an isotonic solution. Serum osmolality plays a role in physiological and behavioral responses to fluid balance, and appears to be an important consideration for hydration studies. Another difference between this study and the previous reports was the experience of the subjects. This study examined individuals whom had experience cycling but were not considered trained cyclists. The two prior studies mentioned used trained cyclists. The majority of research demonstrates training status does not play a major role in performance decrements attributable to hypohydration.
Conclusion/Limitations: In conclusion, the current study successfully achieved hypohydration, manipulated physiological and perceptual responses, while blinding the subjects. This study demonstrated beginning exercise in a hypohydrated state (~2.4% body mass) impaired cycling performance in active men. Future research should focus on applying this concept to real, outdoor cycling. Outdoor cycling has the impact of air and wind, which may exacerbate hypohydration. A limitation to this study was the small sample size and despite blinding the subjects, afterwards the subjects were able to identify when the fluids were administered.
My Thoughts: Overall, this study was performed very well. The authors provided an in-depth analysis of their results while providing ample details of their methods. One topic I wish the authors would have speculated about was the fact that all subjects were able to identify when they received liquids after being told the intent of the study. This appears to show that blinding was not realistically achieved, or at least not as effective as the authors initially believed. If the subjects could tell they were receiving liquids it is possible this could have played a psychological role in performance. Despite this fact, I feel the authors came up with a creative method to try to solve the difficulty of blinding subjects during a fluid intake study. In conclusion, it appears staying hydrated is a key to maintaining performance during long endurance related events. The study used 8 blocks of low-moderate intensity exercise (50% PPO), future research should examine exercise at higher intensities which is more specific to competitive events. The failure to stay hydrated limited the participants’ ability to produce the final “kick” of a race. It can be estimated that the “kick” performance decreases approximately 8% if an athlete fails to stay hydrated during earlier portions of the race.
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