Individualising a Fluid (Electrolyte) Strategy

It is commonly accepted that many elite football clubs implement hydration testing on their athletes thus giving sports scientists and athletes themselves an idea of their own needs and how to manage and monitor nutrition to peak for competition. But how could we take the next step?

Research in football suggests that elite clubs are ‘sweat testing’ athletes, but is this the case? Maybe as a one off test that gets published in an article, but myself and other researchers believe that electrolyte testing should be used in addition to and together with hydration testing in order to inform a fluid- electrolyte strategy.

Why test for Sodium Electrolytes?

The more an athlete sweats during exercise (without fluid replacement) the greater the decrease in plasma volume (Vrijens & Rehrer, 1999). Therefore, heart rate and core temperature increase, with stroke volume, cardiac output and skin blood flow decreasing (effecting thermoregulation). These physiological responses to dehydration result in premature fatigue (Stolwijk, Saltin & Gagge, 1968). With the loss of fluid through sweat, comes the loss of vital electrolytes, the most essential being sodium. The amount of sodium in sweat averages about 500 mg sodium/lb sweat (ranging from 220 to 1,100 mg) (Buono, Ball & Kolkhorst, 2007). Numerous studies have shown that high salt losses increase likelihood of muscle cramps and hyponatraemia, resulting in decreased performance (Bergeron, 2003, 2008; Eichner, 2007; Neville et al., 2010).

Salty sweaters are shown to have an increased risk of dehydration compared to less salty sweaters (Montain, Sawka & Wenger, 2001). With only one study (investigating 3 premier league clubs) examining how to individualise both water and electrolyte requirements to facilitate performance, sport scientists must consider that sweat rate and composition vary extensively between athletes (Shirreffs et al., 2005). Some athletes lose up to 5 times more sodium than others during the same exercise session (Cort, 2010). Quantification of these losses can individualise a hydration and nutrition strategy, optimising training and competition (Ostojic & Mazic, 2002).

What can we use to test for Sodium Electrolytes?

Many researchers have used the centrifugation method when analysing sweat content. This is neither cost nor time efficient. Clubs want immediate numbers so that a strategy can be implemented. Compact Sodium Ion Meters like the B-722 LAQUAtwin can wirelessly measure the sodium composition of  athletes sweat, immediately after a training session Goulet et al. (2012) tested the Horiba C-122 (previous version of the B-722) and found its measurement of sweat Na+ to be reliable (CV = 3.7%) and accurate within 15 mEq/L (95% limits of agreement) of laboratory-based ion chromatography

How can we monitor hydration and electrolyte losses?

An interesting study by Sherriffs et al. (2005) outlined a methodology that I consider to be first class in obtaining accurate hydration and electrolyte information from athletes during a typical training session. Athletes should be weighed pre and post session. They should also undergo a hydration test using a refractometer to determine their pre training hydration status (both can be repeated post exercise). Players do not use the toilet after the measurements have been taken or throughout training.

During the session they have free access to their own drinks labelled with their name, ensuring they do not drink out of any other bottle. They are instructed not to spit any of their drink out. The bottles are weighed before and after testing to determine the volume of the drinks consumed.

Sweat is collected using adhesive patches (available from any pharmacy) from 1-5 skin sites. Research has suggested that the right forearm site best represents whole body sweat electrolytes (Stofan et al., 2005). Patches are placed after appropriate cleaning of the skin at the start of training and are collected after 60 minutes, allowing adequate time for the athlete to start sweating.

The sweat will be collected and analysed for sodium using a sodium Ion meter that can test sweat immediately after collection. The patch is transferred into the barrel of a syringe. The plunger is then inserted from the back of the syringe to compress the patch and squeeze a sample onto the sampling sheet in the ion meter. The data can then be compared with average sweat sodium concentrations for athletes (20-80 mmol/l) (Shirreffs et al., 2005).

Now we can measure sweat rate (Post Body Weight- Pre body Weight + fluid consumed); total Sodium loss (Volume of water (sweat) that was lost x concentration of sodium mmol); fluid Intake (weight of bottles before – Weight of bottles after); and difference in hydration (osmolality post – osmolality pre)

So what can we do with this information?

Below is data from a study that is currently undergoing publication (Munson et al., 2016). This shows individual sweat sodium concentrations for 12 individual athletes during 4 identical training sessions in an ambient temperature, while consuming a drink containing either water, or 10, 20 or 50 mmol/L of Sodium. I noticed that individual fluctuations were minimal and generally insignificant. So do we need to measure sweat sodium on a weekly basis? No, not at all, but research will tell us that if sweat rate increases with these individuals, then they will lose more sodium through sweat.

Let’s use an example: player 6 has an average sodium concentration of 75 mmol, which will put that player at risk. His average sweat rate was 0.6 L/h putting that at an average total sodium loss of 43 mmol/h, which is high for ambient temperatures. If temperatures were high, I would expect a sweat rate of 1.5L/h which could result in high losses of 113 mmol/h putting him at serious risk of hyponatremia.

Using this individual data, much can be learned about the physiological state of the players. You can provide individual drinks containing high amounts of sodium both pre-game (sodium load) and post-game for recovery. This post game drink can also contain a certain volume of fluid, individualised for the recovery of the athlete. It must be noted that just because a player has a high sweat rate/ sweat sodium loss, it doesn’t mean that they are less fit than others.


Ted Munson

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