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Original Research

Glycerol-Induced Hyperhydration: A Method for Estimating the Optimal Load of Fluid to Be Ingested Before Exercise to Maximize Endurance Performance

Goulet, Eric DB

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Journal of Strength and Conditioning Research: January 2010 - Volume 24 - Issue 1 - p 74-78
doi: 10.1519/JSC.0b013e3181bd43e2
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In a recent meta-analysis, glycerol-induced hyperhydration (GIH) has been shown to improve endurance performance (EP) by 2.6%, compared with water-induced hyperhydration (WIH) (5). Exercise-induced dehydration >2% body weight (BW) has been suggested to impair EP (13). Consequently, it thus appears that the use of GIH is only required when athletes anticipate that their ad libitum fluid intake during aerobic exercise would not be sufficient to prevent a loss of BW >2%.

In fact, in such a scenario, the use of GIH would act to prevent or delay the arrival of the 2% BW loss threshold, which could lead to an increase in EP compared to if the exercise had been started in a euhydrated state only. For example, Goulet et al. (4) recently showed that by preventing a BW loss >2%, GIH improved endurance capacity by 14% during 2 hours of cycling compared to a trial where the exercise was started in a euhydrated state and BW loss reached 3.3%. Similarly, compared with a walking/running test to exhaustion that was started in a euhydrated state and where dehydration at the end of exercise reached 2% BW, Latzka et al. (8) showed that, as a result of allowing subjects to finish the exercise with a reduced dehydration level, GIH increased endurance capacity by 13%.

The efficacy of GIH in improving EP lies, at least in large part, on the ability of an individual to estimate as precisely as possible the optimal GIH load required before exercise for avoiding or delaying a BW loss >2%. In fact, an underestimated GIH load would prevent an athlete from losing less than 2% BW during exercise (7) or would not maximize the delay of time before reaching this critical BW loss, thereby not maximizing the ergogenic efficacy of GIH. However, the ingestion of an excessively high quantity of GIH before exercise compared with the physiological need may potentially provoke in more sensitive individuals side effects such as stomach bloating, vomiting, nausea, and the like, which, unfortunately, could lead to an impairment in EP (12). Consequently, determining the optimal GIH load to be ingested before exercise is a serious process that should be conducted in a rigorous and systematic manner when the ultimate goal of the individual using it is to maximize EP.

So far, no method or technique exists in the literature that provides the necessary procedures for estimating the most optimal GIH load to be drunk before exercise to maximize EP. There is thus a need for the development of a method of hyperhydration that would permit athletes and coaches to maximize the ergogenic effect of GIH on EP. Such a method has been developed and is reported in the following paragraphs. It provides the necessary steps for estimating the minimal GIH load to be ingested before any exercise to prevent or maximally retard a loss of BW >2% and recommends, based on the result obtained, the use of a GIH protocol that has been shown in the literature to increase body water by at least the amount determined.

Sport science is a scientific process used to guide the practice of sport with the ultimate aim of improving sporting performance. However, Bishop (1) has recently reported in an elegant paper that the translation of sport science research to practice is poor; said differently, there is a lack of transfer from research to practice. Hence, another important purpose of this paper is to bridge the gap between the scientific knowledge of GIH acquired in the laboratory setting and its practical utilization in the field.


Step 1

Using Equation 1, the first step of this method aims at estimating the relative amount of dehydration that will be incurred by exercise. This, in turn, will indicate whether the use of GIH is indicated. An exercise-induced dehydration level <2% BW signifies that the use of GIH is not required. A negative result indicates that overhydration takes place during exercise and that fluid consumption should be reduced until some dehydration is achieved.

Equation 1

where EHSR is the estimated hourly sweat rate, EHRFC is the estimated hourly rate of fluid consumption, and BW is bodyweight.

For Equation 1 to provide valid results, the hourly sweat rate expected in the upcoming race or training session needs to be determined with as much precision as possible. Table 1 reports the sweating rates of runners of different BWs under different exercise intensity and environmental conditions (11). Such data for cycling or other sports are, to my knowledge, not available, or at least not as complete as those reported in Table 1. Hence, an in-field determination of sweat rate would be required for sports other than running. However, even for runners it could be worth determining sweat rate because many factors-such as exercise duration, exercise intensity, environmental conditions, the type of clothing and equipment worn, BW, genetic predisposition, heat acclimatization, training status, and metabolic efficiency-affect it (13). Ultimately, it must be remembered that there is substantial variability in sweating rates among individuals. The method that should be used to determine sweat rate follows:

Table 1
Table 1:
Predicted sweating rates (L/h−1) for runners of different body weights (50, 60, and 70 kg) and running at different speeds (8.5, 10, 12.5, and 15 km/h−1) and under different ambient temperatures (18 vs. 28°C)*.
  1. When well hydrated, urinate and then weight yourself in the nude (pre-training BW in kg).
  2. Exercise for 30 minutes under the environmental condition and at the exercise intensity you expect in your upcoming race or training session.
  3. Do not urinate or drink while doing the exercise or before taking your post-training BW.
  4. Dry yourself thoroughly.
  5. Weigh yourself (post-training BW in kg).
  6. Subtract the post-training BW from the pre-training BW.
  7. Divide the number you calculated by 0.5. The result is your hourly sweat rate in liters. If the exercise is longer, divide the difference in BW by the exercise time in hours.

Step 2

Once it has been established that the use of GIH may be beneficial, the second step consists of identifying the minimal amount of extra body water required before exercise that would be sufficient to prevent a loss of BW >2%, which is given by Equation 2. The result of Equation 2 should never be negative, in which case it indicates that the determined relative dehydration level is below 2% BW, suggesting that the use of GIH is not needed.

Equation 2

Step 3

Once the quantity of fluid to be ingested before exercise has been determined, the third and final step consists of identifying and selecting a GIH protocol (Table 2) increasing body water by an amount equal to, or slightly greater than, the result of Equation 2. Obviously, rarely will a GIH protocol exactly compensate for the result given by Equation 2. When an exact match is not possible, then the ideal GIH protocol in this situation is the one providing an increase in body water that is above and closest to the result of Equation 2. In the case where the estimated loss of fluid induced by exercise is such that no GIH protocol can reduce dehydration <2% BW, then the optimal level of hyperhydration in this particular situation would correspond to the maximal capacity of GIH in increasing body water.

Table 2
Table 2:
Glycerol-induced hyperhydration (GIH) protocols allowing the increase of body water by 7, 12, and 22 mL of fluid/kg BW−1.


A method for determining the optimal GIH load to be ingested before exercise to maximize the benefits of this hydration strategy on EP was presented. No such method existed before in the literature. A key feature of this method is that it also provides 3 GIH protocols from which users can choose to optimally compensate the estimated GIH load that should be ingested before exercise.

The optimal load of fluid to be ingested during GIH is suggested to be the one that is just sufficient to prevent a loss of BW >2% during exercise in the case where such a loss can be prevented. In fact, the American College of Sports Medicine (ACSM) in its 2007 Position Stand on Exercise and Fluid Replacement indicates that EP is maximized when dehydration during exercise is kept <2% BW (13). Also, there is no evidence in the literature suggesting that maintaining the hydration level during exercise closest to euhydration level may offer an EP advantage. However, when it is expected that a loss of BW >2% cannot be prevented during exercise even though GIH is used, then the optimal level of GIH in this situation represents the one that would delay for as long as possible the attainment of the 2% BW loss threshold. Maximally delaying the time required for reaching the 2% BW threshold should assure that the decrease in EP from exercise-induced dehydration is minimal.

Noakes (14) recently argued that it is the development of the thirst sensation, and not a particular absolute loss of BW, that may be the causal factor for the decrease in EP observed during exercise when body water levels are not optimal. It has been shown that GIH decreases perceived thirst during prolonged exercise compared with when the latter is started in the euhydrated state (4). Hence, GIH may also help increasing EP by decreasing thirst sensation. Independently of the mechanism involved in the dehydration-mediated decrease in EP, the use of GIH is assured to have an impact on EP in a positive manner because it has a positive effect on both the loss of BW and the perception of thirst during exercise.

As nearly as important as the determination of the optimal GIH load to be ingested before exercise is the capacity to choose the right GIH protocol optimally compensating the GIH load computed. In fact, choosing a GIH protocol that would confer a too high or too low increase in body water in comparison to the determined optimal GIH load could affect EP in a way that is similar to that of being unable to estimate the right excess amount of fluid to be ingested before exercise. The use of 3 GIH protocols was recommended to compensate the amount of fluid needed before exercise. These protocols are ideal to be used in conjunction with the proposed method because they cover the minimal (7), intermediate (6), and maximal (9) capacity of GIH to increase body water.

An important question is whether the proposed method could produce any adverse effects on health or EP. None of the 3 GIH protocols whose use is recommended for this method has been shown to produce side effects. Lyons et al. (9), however, reported that their protocol could produce abdominal bloating but that this feeling subsides within 15 minutes after the end of water ingestion. Because GIH overloads the fluid compartments, one may wonder whether it could precipitate the development of dilutional hyponatremia, which is defined as a serum sodium concentration of 135 mmol/L−1 or less (15). Goulet et al. (5) showed in their meta-analysis that although GIH decreases serum sodium concentrations, the magnitude of decline is not sufficient to cause hyponatremia. Furthermore, no case of hyponatremia has ever been observed in any of the studies conducted on GIH and exercise (5). Concerns may be raised as to whether the extra body water having to be carried as a result of GIH could decrease the power to weight ratio and, as a result, impair EP. To the best of my knowledge, this question has never been scientifically investigated. However, Ebert et al. (3) evaluated the concept of “functional dehydration” on endurance capacity during hill climbing in cyclists and demonstrated that dehydration was associated with a decrease in EP compared with the maintenance of euhydration, implying that the hydration benefit was superior to the enhanced power-to-weight ratio provided by dehydration. As it is recommended that GIH be used for those exercise situations where it is evaluated a priori that dehydration may impair EP, then it is reasonable to believe that the hydration advantage provided by GIH would outweigh the “disadvantage” of the GIH-induced increase in BW, at least during cycling exercise.

The proposed method has some limitations that merit being briefly discussed. First, it does not take into account many of the factors that influence fluid balance during exercise-namely, the release of water molecules bound to muscle glycogen, metabolic water production, and urine loss (10). The use of muscle glycogen and the oxidation of carbohydrate and fat will add effective water to the body fluid pools. This phenomenon will contribute to reducing dehydration level without modifying BW. Hence, the assessment of hydration status during exercise using BW changes lacks precision and has lately been the object of criticism (10). However, relying on the changes in BW during exercise is the only realistic measure and representation of dehydration for the athlete and the field-based coaches and practitioners. Moreover, the proposed method assumes that values for sweat rate and the capacity of the selected GIH protocol in increasing body water will exactly correspond to those encountered in the upcoming race or training session, which may not always be correct. To estimate the level of dehydration incurred by exercise, sweat rate needs to be determined, which necessitates time and efforts. A key characteristic of this method is that it is based on the assumption that a loss of BW >2% negatively affects EP, which may not hold true for all individuals. In fact, some athletes may tolerate more BW loss before EP becomes impaired, whereas in others a loss of BW <2% may already be sufficient to compromise EP. Finally, the validity of this method is unknown because it has not been scientifically evaluated. Nevertheless, the proposed method has the merit of being the first to attempt bridging the gap between the science of GIH developed and acquired in laboratories and its concrete utilization in the field by endurance athletes. It also represents the best tool that athletes have hitherto for maximizing the effectiveness of GIH on EP.

Of note, all but 1 (reliability of the selected GIH protocols in increasing fluid retention) limitation of the proposed method also apply to the method used by scientists and coaches for the development of customized rehydration protocols during exercise that highly respected sporting organizations like the ACSM and National Athletic Trainers' Association (NATA) recommend for maximizing the performance of athletes during exercise (2,13). Hence, it is my belief that the limitations associated with the procedure presented here should not detract users from utilizing it because its benefits on EP are likely to far outweigh the uncertainty brought about by some of its limitations.

Practical Applications

During aerobic exercise, dehydration >2% BW has been shown to decrease EP. For practical, logistical, or physiological reasons, it is not always possible for athletes to prevent dehydration >2% BW through ad libitum fluid intake during prolonged exercise. In such a scenario, the use of GIH before exercise may help support EP by increasing the body fluid reservoir up to a level sufficient to prevent a dehydration level >2% BW. Therefore, the efficacy of GIH in maximizing EP lies, at least in large part, on the ability of an individual to estimate as precisely as possible the optimal GIH load required before exercise for avoiding a loss of BW >2%. This important step in the utilization of GIH can be adequately performed only if the appropriate tools are provided to the athlete. Unfortunately, up until now no method existed in the literature that provided the necessary procedures for estimating the most optimal GIH load to be drunk before exercise to maximize EP. In this paper, such a method was developed and provided the necessary steps for estimating the minimal GIH load to be ingested before any exercise to prevent a loss of BW >2% and then recommended, based on the result obtained, the use of a GIH protocol that had been shown in the literature to increase body water by at least the amount determined. The development of this procedure should help maximize the ergogenic effect of GIH on EP and will help bridge the gap between the scientific knowledge of GIH acquired in the laboratory setting and its practical utilization in the field.


The author declares no conflict of interest and at the time of the conduct of this study was financially supported by the Fonds de la recherche en santé du Québec (FRSQ).


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fluid overloading; hydration strategy; glycerin; dehydration; exercise performance

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