Hydration science

Here's some key research papers and studies that have shaped our approach to hydration, along with some of our own contributions to the subject...

The importance of sodium for hydration in endurance sports

Sodium plays a key role in how our bodies function and it's important to replace the sodium we lose to some extent when our sweat losses begin to mount up to some extent. The following studies emphasise the importance of sodium and getting your hydration strategy right.

Effects of oral salt supplementation on physical performance during a half-ironman: A randomized controlled trial: Salt supplementation during triathlon races by Del Coso et al.

Key take-home message: Athletes who added a salt supplement to their usual hydration routines during an IRONMAN 70.3 triathlon took an average of 26 minutes less to complete the course than those who only used sports drinks.

Fluid Loading With Salt Water by Alex Hutchinson

Key take-home message: In a piece for Runner's World, journalist Alex Hutchinson found that adding salt (in the right amount) helps you retain more water, increases plasma volume, and, in theory, improved performance.

Exercise increases the preference for salt in humans by Leshem M, Abutbul A and Eilon R.

Key take-home message: The authors suggest that the immediate and specific increase in NaCl preference after exercise is due to sodium loss (in perspiration) and/or sympathetic arousal that activates the hormones, aldosterone and angiotensin II in humans.

Further reading:

The importance of starting exercise hydrated

Precision Fuel & Hydration recommend 'preloading' with a sodium-based electrolyte drink before prolonged exercise and these studies highlight the value of starting events properly hydrated.

High Prevalence of Dehydration and Inadequate Nutritional Knowledge Among University and Club Level Athletes by Pamela Magee, Alison Gallagher and Jacqueline McCormack

Key take-home message: The study found 31.9% of the University/club-level athlete participants started exercise in a dehydrated state and 43.6% of participants were dehydrated after training or competition.

Salt and Fluid Loading: Effects on Blood Volume and Exercise Performance by Ricardo Mora-Rodriguez and Nassim Hamouti

Key take-home message: The literature suggests that drinking a pre-exercise sodium-based drink (with concentrations not over 164mmol/l of NA) helps enhance performance when taking part in prolonged exercise in a warm environment.

Incidence of pre-game dehydration in athletes by James Finn and Robert Wood

Key take-home message: The study tested the hypothesis that athletes' pre-hydration status competing at an international event in dry, tropical conditions may be inadequate. Indeed, they found athletes were dehydrated and recommended better monitoring of hydration status.

Further reading:

Does dehydration result in performance impairment?

There has been variability in the accepted 'level' of dehydration that an athlete can tolerate before performance suffers, with the recent evidence moving away from the previously accepted 2% level of dehydration resulting in a drop off in performance.

Dehydration and its effects on performance (An excerpt from sport nutrition-3rd edition by Asker Jeukendrup & Michael Gleeson)

Key take-home message: Exercise performance is impaired when an individual is dehydrated by as little as 2% of body weight, although this 'threshold' for dehydration and impaired performance has been challenged and changed in recent years.

Dehydration Impairs Cycling Performance, Independently of Thirst: A Blinded Study by Adams JD, Sekiguchi Y, Suh HG, Seal AD, Sprong CA, Kirkland TW and Kavouras SA.

Key take-home message: A study of 7 cyclists indicated that hypohydration decreased cycling performance and impaired thermoregulation independently of thirst, while the subjects were unaware of their hydration status.

Abstracts for the 4th Annual Congress on Medicine & Science in Ultra-Endurance Sports, May 30, 2017, Denver, Colorado

Key take-home message: Hydration guidelines found in the scientific and popular literature typically advise that sufficient fluid should be ingested to prevent a loss of more than 2% of body mass.

This study challenges the traditional view by showing that during prolonged running (lasting in excess of around 15 hours), much more than 2% body mass can be lost without a reduction in the body water pool. Future hydration guidelines should consider these findings in order to properly educate active people to help them achieve an appropriate fluid intake.

Further reading:

Hyponatremia and the danger of drinking too much plain water

As athletes attempt to avoid dehydration, there can be a danger of drinking too much plain water and this can result in a race-ruining (and potentially fatal) condition known as hyponatremia.

For 'Ironman' Athletes, Study Shows Danger of Too Much Water

Key take-home message: Researchers in Germany tested nearly 1,100 competitors in the annual Ironman European Championships and found more than 10 percent had developed hyponatremia.

Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners by Hoffman MD, Hew-Butler T and Stuempfle KJ.

Key take-home message: The study investigated the prevalence of exercise-associated hyponatremia (EAH) and found a high prevalence in ultra-running, which emphasises the need for a deliberate hydration strategy when taking part in long endurance activities. There was a very high prevalence of EAH in hot temperatures (even in activities lasting less than 3 hours), but less so in moderate ambient temperatures.

NSAID use increases the risk of developing hyponatremia during an Ironman triathlon by Wharam PC, Speedy DB, Noakes TD, Thompson JM, Reid SA , Holtzhausen LM

Key take-home message: NSAIDs (nonsteroidal anti-inflammatory drugs) are commonly used by athletes competing in endurance events and are a risk factor for hyponatremia and altered renal function.

Exercise-associated hyponatremia in endurance and ultra-endurance performance-aspects of sex, race location, ambient temperature, sports discipline, and length of performance: A narrative review (Knechtle et al., 2019)

Key take-home message: A review of where the literature on Exercise-Associated Hyponatremia currently stands and emphasises the risk factors associated with EAH, including event-specific factors such as: duration of 4+ hours, high availability of fluids, extreme heat and extreme cold.

Further reading:

Should you drink plain water to thirst?

Is drinking plain water to thirst really the correct approach to hydration for athletes all of the time? Quite simply, no.

While drinking water to thirst may be suitable for some athletes in some situations (e.g. during shorter events in cooler climes), it will be necessary for athletes with high sweat losses, salty sweat and those taking part in longer events to take on some form of electrolyte product to help replace the sodium they're losing in their sweat.

Drinking behaviors of elite male runners during marathon competition by Beis LY, Wright-Whyte M, Fudge B, Noakes T and Pitsiladis YP.

Key take-home message: The most successful runners, during major city marathons, drink fluids when they feel necessary and their fluid intake rate fell in line with the American College of Sports Medicine's recommendations of 0.4-0.8 litres per hour.

Waterlogged: The Serious Problem of Overhydration in Endurance Sports by Professor Tim Noakes

Key take-home message: Tim Noakes advocates ‘drinking plain water to thirst’ as the best way to avoid hydration-related issues, such as hyponatremia, and to maximise athletic performance, rather than drinking to a hydration plan and/or consuming any additional electrolytes through sports drinks.

In some situations this may be true, but such an approach fails to take into account the importance of the electrolyte sodium or even considering a more balanced approach to hydration.

Further reading:

Sweat testing and your hydration strategy

Drinking to thirst is a useful approach when combined with a hydration strategy and an understanding of your own sweat losses can help to refine your approach to events.

Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability by Lindsay Baker

Key take-home message: Sweat testing can be a useful tool to estimate athletes’ sweat rate and sweat sodium losses to help guide fluid/electrolyte replacement strategies, provided that data are collected, analysed, and interpreted appropriately.

Normative data on regional sweat-sodium concentrations of professional male team-sport athletes by Mayur K. Ranchordas, Nicholas B. Tiller, Girish Ramchandani, Raj Jutley, Andrew Blow, Jonny Tye and Ben Drury

Key take-home message: The data suggests that self-reported measures of sodium loss might serve as an effective surrogate in the absence of direct measures (i.e. those which are more expensive or non-readily available).

Comparison of conductivity with sodium determination in the same sweat sample (Riedietal.,2000)

Key take-home message: Using 206 participants (Cystic Fibrosis and non-CF), the study determined sweat conductivity using the PH Advanced Sweat Test and sweat sodium concentration using flame photometry (gold-standard). Two measures showed great agreement and correlation.

Further reading:

Sweat rate

As well as your sweat sodium concentration, it can be extremely useful to get a handle on your own sweat rate in order to further understand your sweat losses and refine your strategy for a variety of events in different conditions.

How much do you sweat? by Asker Jeukendrup

Key take-home message: An informative look at how to measure your sweat rate in different conditions and at different intensities.

Variability in the sweating rate and sweat sodium variability in the sweating rate and sweat sodium concentration of ultra-endurance triathletes during exercise by Matthew D Pahnke, Joel D Trinity, Jacob J Baty and Jeffrey J Zachwieja

Key take-home message: The study concluded that there is a large amount of variability in the sweating rate of heat-acclimatised ultra-endurance triathletes during exercise in a warm environment.

H2Q Sweat Prediction Performance Summary Report (Sports Science Synergy)

Key take-home message: This is strong evidence of prediction accuracy and underscores the need for carefully measured sweat losses when evaluating sweat prediction efficacy. Knowledge of individual variation in sweating may always be incomplete, thus the goal is to narrow prediction uncertainty to the point where prediction error is reasonable or acceptable when using the smallest number of parameters possible.

Sweat rates, sweat sodium concentrations, and sodium losses in 3 groups of professional football players (Godek et al., 2010)

Key take-home message: The study argues the need to understand individual losses and evaluates dietary sodium intake guidelines when compared to losses induced with pre-season training. Comes to the conclusion that supplementation is necessary.

Electrolyte abnormalities in cystic fibrosis: Systematic review of the literature (Scurati-Manzoni et al., 2014)

Key take-home message: This analysis of the available literature shows that electrolyte abnormalities in Cystic Fibrosis patients are almost always associated with fluid volume depletion.

Further reading:

Why have a Sweat Test?

A Sweat Test can be a useful tool for athletes who are keen to dial in their own hydration strategy because of significant sweat losses and/or a high sweat sodium concentration.

Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: An update and analysis by sport (Barnes et al., 2019)

Key take-home message: American football and endurance athletes have the greatest need for deliberate hydration strategies due to the significant, sport-specific variation in whole body sweat rate and rate of sodium sweat loss. The reason for higher losses is uncertain, but may be due to equipment worn in American football and/or the absolute workload of endurance athletes, which were accounted for in this study.

Weight changes, sodium levels, and performance in the South African Ironman Triathlon by Sharwood K, Collins M, Goedecke J, Wilson G and Noakes T.

Key take-home message: Percentage change in body weight of participants at the 2000 South African Ironman Triathlon was linearly related to post-race serum sodium concentrations.

Sweat rate and fluid turnover in American football players compared with runners in a hot and humid environment by S Godek, A Bartolozzi, J Godek, and W Roberts

Key take-home message: The American footballers had a high sweat rate with large total daily sweat losses. Consuming large volumes of hypotonic fluid may promote sodium dilution.

Recommendations for fluid and electrolyte replacement must be carefully considered and monitored in footballers to promote safe hydration and avoid hyponatraemia.

Further reading:

The importance of rehydration

In many cases, what you eat and drink in your daily diet after racing or a particularly intense/long training session should be sufficient to replace the sodium lost in your sweat.

This won't be the case for all athletes though, so a more aggressive rehydration strategy with a strong electrolyte drink might be required if an athlete has lost significant amounts of sodium and fluid, or if they're looking to perform at their best soon after their most recent exertion (e.g. a multi-stage racer).

2007 American College of Sports Medicine position stand. Exercise and fluid replacement by Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ and Stachenfeld NS.

Key take-home message: Individual sweat rates can be estimated by measuring body weight before and after exercise. During exercise, consuming beverages containing electrolytes and carbohydrates can provide benefits over water alone under certain circumstances.

After exercise, the goal is to replace any fluid electrolyte deficit. The speed with which rehydration is needed and the magnitude of fluid electrolyte deficits will determine if an aggressive replacement program is merited.

Vascular Uptake of Rehydration Fluids in Hypohydrated Men at Rest and Exercise by J. E. Greenleaf, G. Geelen, C. G. R. Jackson, J.-L. Saumet, L. T. Juhos, L. C. Keil, D. Fegan-Meyer, A. Dearborn, H. Hinghofer-Szalkay, and J. H. Whittam

Key take-home message: Fluid formulations containing sodium compounds near isotonic concentrations and the more dilute sodium formulations were effective for maintaining plasma volume in exercising, hypohydrated men.

Further reading:

How to rehydrate quickly and improve your recovery

Different types of sports drinks

There are a variety of sports drinks on the market and they serve different purposes, with Precision Fuel & Hydration's primary focus on sodium replacement through a hypotonic product. Here's why...

Optimal composition of fluid-replacement beverages by Baker LB and Jeukendrup AE.

Key take-home message: The optimal fluid-replacement beverage is one that is customised according to specific physiological needs, environmental conditions, desired benefits, and individual characteristics and taste preferences.

Osmolality and pH of sport and other drinks available in Switzerland by Samuel Mettler, Carmen Rusch and Paolo Colombani

Key take-home message: The osmolality of some commercial sports drinks, which are designed to be consumed during exercise, tended to be in the hypertonic range, although such drinks should rather be slightly hypotonic.

Hypervolemia induced by fluid ingestion at rest: effect of sodium concentration by Sugihara A, Fujii N, Tsuji B, Watanabe K, Niwa T and Nishiyasu T.

Key take-home message: Beverages containing 120 mmol l(-1) Na(+) induce hypervolemia (i.e. fluid overload) with a minimum incidence of gastrointestinal problems.

Further reading:

Hydration and nutrition

Precision Fuel & Hydration generally advocates keeping nutrition and hydration separate by using solid fuels or gels for carbs and using your fluids for hydration. Different methods will work for different athletes though, so the best approach is to find a strategy that works for you.

Competition Nutrition Practices of Elite Ultramarathon Runners (Trent Stellingwerff)

Key take-home message: The major finding was that all three athletes practice nutrition strategies that provide high carbohydrate availability throughout their races.

The addition of electrolytes to a carbohydrate-based sport drink: Effect on continuous incremental exercise done against progressively greater workloads (Roberts, 2017)

Key take-home message: The design is not applicable for investigating electrolyte replacement as the exercise is minimal (just a 12-minute incremental bike test!) and the conditions moderate. It's more interesting how the addition of electrolytes may influence heart rate.

Nutritional intake and gastrointestinal problems during competitive endurance events (Pfeiffer et al., 2012)

Key take-home message: There is little information about the actual nutrition and fluid intake habits and gastrointestinal (GI) symptoms of athletes during endurance events. So, this study highlights factors which may play a part in carbohydrate and fluid intake – namely temperature (triathlon events = 24-29°C, marathon = 12°C) and individual differences between athletes.

Further reading:

Sodium depletion and cramping

There are many potential causes of cramps in individual athletes, from the neuromuscular theory of cramp and the sodium-depletion theory. A recent study has suggested that electrolyte intake can help reduce or prevent cramping.

Electrolyte beverage consumption alters electrically induced cramping threshold (Earp et al., 2019)

Key take home message: Electrolyte consumption decreased cramp susceptibility and pain in test subjects, but did not prevent cramping in any participants. Results suggest that electrolyte consumption independent of hydration can influence cramp susceptibility in young people.

Further reading: