1. Pre-exercise hyperhydration

Pre-exercise hyperhydration is a state of elevated body water induced acutely prior to exercise by means of fluid ingestion with or without waterbinding agents, such as Glycerol⁴.

Glycerol-induced hyperhydration may be used to increase the athlete’s capacity to tolerate fluid loss and offset (i.e., delay, prevent or attenuate) the deleterious effects of dehydration (e.g., fluid loss >2% BM) that can occur during exercise⁵ . This strategy may be beneficial in a range of challenging situations that commonly arise in sport, such as:

• When preparing for competition in which high fluid losses are anticipated through prolonged exposure to hot environments or when it is not possible for fluid intake to match sweat losses.
• For athletes competing in sports where fluid consumption is impractical, such as during the swim-leg of an Ironman (World Triathlon Corporation) Triathlon race or during tournament-style of play in team sports, where matches are played in close succession over a day and there is limited time between matches to replace fluid loss.
• When competition regulations limit an athletes access to fluids, for example tennis or football (soccer) match play.
• Avoiding the need to drink during competition for example, maintaining a streamline position in a cycling time trial or avoiding an adjustment in the race line during a marathon such that the onset of the fluid intake is postponed.
• When an athlete’s voluntary fluid intake is reduced because of gastrointestinal distress or a reduced drive to drink.

The benefits associated with pre-exercise hyperhydration can be put into perspective when the consequences of dehydration (i.e., increase in body core temperature and thermal strain) during exercise can negate the physiological advantages resulting from increased fitness and heat acclimatization.⁶

How is pre-exercise hyperhydration achieved?

Compared to hydrating with water, adding effective osmotic agents such as glycerol (and sodium; for more information, refer to ‘Electrolytes’) to a hydration solution will lower urine production and thus increase fluid retention² . Accrued fluid retention with glycerol is possible through Glycerol’s direct effect on reabsorbing fluid through the kidneys.⁷ Specifically, glycerol is reabsorbed by the kidney tubules, increasing the concentration gradient of the renal medulla, thereby water reabsorption in the nephron is enhanced.⁸

Since glycerol and sodium enhance fluid retention through different physiological mechanisms, sodium can also be added to glycerol hyperhydration solution because their combination can be more effective than either osmolyte alone. The following diagram illustrates the fluid retention achieved through three common hyperhydration strategies adapted from previous work conducted.^2,3

Figure 1. Comparative effects of different hyperhydration solutions on fluid retention. WIH – Water-induced hyperhydration, GIH – Glycerolinduced hyperhydration, SIH – Sodium-induced hyperhydration, G+SIH – Glycerol + Sodium-induced hyperhydration.

How and when do I hyperhydrate with glycerol?

Effective protocols of Glycerol-induced hyperhydration include the addition of 1.2 - 1.4 g/kg body mass Glycerol in ~25 ml/kg body mass fluid in the 90 - 180 min prior to exercise.¹ For example, a 75 kg athlete would weigh out 90 – 105 g glycerol on a set of calibrated kitchen scales and add their fluid of choice (i.e., 1875g water, cordial or sports drink).

As an alternative hyperhydration strategy, the addition of 3.0 g/L sodium, with or without glycerol, can be added to a hydration solution.³ To maintain palatability, an electrolyte supplement may be appropriate

What to expect?

• When ingested orally, glycerol is rapidly absorbed and distributed throughout the body’s fluid compartments, until it is gradually excreted over the next 24 - 48 h.
• In terms of the timing with other pre-event activities (e.g., warm up, marshalling duties, final preparations), peak urine production is likely to occur 60 - 80 min after ingestion.
• Greater fluid retention for up to 4 h after ingestion.
• Reduced urinary volume that is more concentrated.
• Compared to other hyperhydration strategies, the additional of glycerol to a solution is well tolerated with low abdominal discomfort.
• When combining glycerol and sodium-induced hydration strategies, you should expect to see lower urine production and greater fluid retention compared with glycerol-induced hyperhydration on its own.
• When compared to pre-exercise euhydration, pre-exercise hyperhydration has been shown to reduce dehydration-induced increases in heart rate and heat storage.

2. Post-exercise hydration

Post-exercise rehydration strategies should aim to correct fluid and electrolyte losses accumulated during an event to enhance recovery and subsequent performance in training and/or competitions held over consecutive days. Under circumstances that limit time or prevent the consumption of meals or snacks that facilitates complete fluid balance restoration, glycerol may be used and offers the following benefits:

• reduced diuresis associated with rehydration. In the case of rehydrating after exercise performed late in the day, glycerol-induced rehydration can reduce overnight diuresis to avoid interruptions to the athlete’s sleep patterns.
• aggressive rehydration after weigh-in where weight-making practices (e.g., dehydration) have been implemented to achieve a target weight in weight-division sports.

How and when do I hydrate with glycerol after exercise

The volume required to restore fluid balance will depend on the net deficit from sweat loss during the previous exercise bout. As a general guide, it may be necessary to drink up to 150% of weight loss.⁴

Add 1.0 g/kg body mass of glycerol to each 1.5 L fluid consumed.⁹