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Sodium Supplementation During Prolonged Exercise - Effects On Plasma Sodium And Performance

The incidence of exercise-associated hyponatremia in endurance sport has increased notably in recent decades. Sodium supplementation during prolonged exercise can prevent it.

Dr. Cooney Blades
Jan 19, 2024986 Shares18961 Views
This article reviews intervention studies that have assessed the impact of sodium supplementation during prolonged exerciseon plasma sodium concentrations and/or performance.
Exercise-associated hyponatremia (EAH) is a blood electrolyte disorder induced during the 24 hours following exercise.
It is defined as a plasma-sodium concentration (plasma [Na[+]]) less than the normal reference range. For most laboratories, it is found to be 135 millimoles per liter (mmol/L).
Mild hyponatremia (plasma [Na[+]] 130 – 135 mmol/L) is often asymptomatic, although mild symptoms can develop. These include:
  • bloating
  • nausea
  • lethargy
  • vomiting
  • headache
Whilst such symptoms are not life-threatening, they likely have some effect on exercise performance.
They are also non-specific, and can often be confused with the symptoms of dehydration.
As the athlete’s plasma [Na[+]] decreases below 130 mmol/L, the symptoms correspondingly increase in severity.
This occurs as a result of cerebral edema, producing symptoms, such as:
confusionrespiratory distress
comaseizures
disorientationeven possible death
The incidence rates of EAH for endurance races has been reported to be up to 30% amongst Ironman triathletes, and in marathon runners up to 13%.
Such endurance events host large participation numbers. The 2010 London marathon, for example, hosted 36,000 runners.
With this in consideration, such incidence rates correspond to a large absolute number of EAH sufferers in the sporting population.
Whilst excessive fluid consumption has been a clear causative factor of EAH, there is evidence to suggest that excessive sweat sodium loss may also contribute.
It has been speculated that sodium supplementation during endurance exercise could attenuate the reduction in plasma [Na[+]] in these situations, therefore reduce the risk of EAH and potentially improve performance. The research stems from both laboratory and field studies, each with their own advantages and disadvantages.
In this review, intervention trials with a high and low sodium trial, which measure plasma sodium during an endurance exercise protocol will be discussed.

Laboratory Studies

One of the first laboratory studies was undertaken in 1991 by Susan I. Barr, D. L. Costill, and W. J. Fink and published in the journal Medicine and Sciencein Sports and Exercise.
Eight participants completed a crossover intervention of three cycling trials, at 55% VO2max for six hours.
Participants consumed fluid to match their sweat rate, with either water and/or a 25 mmol/L (29 mmol/h) NaCl saline solution, or they ingested no fluid during the three trials.
Barr, Costill, and Fink observed no significant differences in plasma [Na[+]] between the two ‘fluid’ trials (p = 0.27).
These results were similar to those seen in another crossover study by B. Sanders, Timothy D. Noakes, and S. C. Dennis, which was published in 2001 by the European Journal of Applied Physiology.
Six endurance trained male cyclists participated in three trials, again cycling at 55% V̇O2max, but for a shorter time for 4 hours as compared to Barr et al.
This time the participants ingested sodium carbohydrate drink (~0.965 L/h) in the following amount:
  • 4.6 mmol/L (4 mmol/h)
  • 50 mmol/L (48 mmol/h)
  • 100 mmol/L (96 mmol/h)
Similar to the findings from Barr et al., plasma [Na[+]] was maintained regardless of whether participants were supplemented with sodium. However, Sanders et al. built on these findings to suggest why plasma [Na[+]] was maintained.
When salt tablets were consumed, the intracellular fluid (ICF) moved into the extracellular fluid (ECF), expanding plasma volume and preventing any large increase in plasma [Na[+]] levels.
In contrast, when salt tablets were not consumed, the ECF was reduced and the ICF was maintained. Thus, plasma volume decreased and plasma [Na[+]] and osmolality were maintained.
This phenomenon was further explained by a significant decrease in renal-free water clearance during the sodium capsule trials compared to the solution-only trial (p < 0.05), highlighted by differences in urine output, and conservation of ECF when sodium was consumed.
Interestingly neither Barr et al. nor Sanders et al. reported a significant difference in sweat sodium concentration between their respective high-sodium and low-sodium trials.
The results of these laboratory studies suggested that sodium supplementation may have little effect in preventing exercise-associated hyponatremia (EAH). However, other studies challenged this view.
A crossover intervention trial by Dr. Desiree M. J. Vrijens and Associate Professor Nancy J. Rehrer in 1999 and published by the Journal of Applied Physiologydemonstrated that sodium could play an important role in the prevention of EAH.
Again cycling at 55% VO2max, but only for three hours, and as with the Barr et al. study, ingesting fluid equal to sweat rates.
The participants ingested either water, or Gatorade (18 mmol/L Na[+]).
In contrast to Barr et al. and Sanders et al., the Gatorade intervention significantly attenuated plasma [Na[+]] reduction compared to water (water vs. Gatorade = -2.48 mmol/L/h vs. -0.86 mmol/L/h).
Unlike the previous two studies, this study reported that one participant became hyponatremic (plasma sodium 128 mmol/L) during the water trial, interestingly with a fluid intake rate lower than the mean fluid intake rate for the group.
It should be noted, as the intervention in the study by Dr. Vrijens and Assoc. Prof. Rehrer consisted of consuming Gatorade, differences other than sodium were present between the trials, for example:
  • carbohydrate
  • other electrolytes
They also observed that time to exhaustion decreased with lower plasma sodium concentrations. This suggested a role for sodium supplementation in performance, although more likely to be because of the consumption of carbohydrates.
Despite these apparent limitations in the Vrijens and Rehrer study, a later study published in 2009 by the Journal of Athletic Training, with Costas A. Anastasiou as lead author, agreed with their results.
This trial involved 13 untrained men completing a multi-disciplinary intervention of cycling, walking and calf raises for three hours.
Again fluid was ingested equal to sweat loss, and four trials were completed, including:
  • high sodium (Gatorade thirst Quencher 36.2 mmol/L)
  • a low sodium (Gatorade Endurance 19.9 mmol/L)
  • water or an artificially-flavored placebo
Both the low and high sodium solutions attenuated the decline in plasma [Na[+]] during the exercise intervention, compared to the water and placebo. In addition, some of the participants on the water and placebo had plasma sodium concentrations less than 135 mmol/L at the end of the exercise protocol.
Again, discrepancies in carbohydrate ingestion existed between the two sodium and two non-sodium trials, and as differences were only observed between the sodium and non-sodium trials, and not between the low-sodium and high-sodium trials, it suggested that something other than sodium could be responsible.
These four studies provide conflicting results on the ability of sodium ingestion to influence blood-sodium concentrations, which can be attributed to differences in methodology.
Sanders et al. suggested that the steeper declines in plasma [Na[+]] observed in the Vrijens and Rehrer study could also be because of differences in urine output, which were considerably lower in the Vrijens and Rehrer study.
This is likely due to anti-diuretic hormone secretion in response to exercise stress to conserve plasma volume.
When combined with greater sweat sodium losses and large fluid intakes, it is understandable why Vrijens and Rehrer reported a greater decrease in plasma [Na[+]].
These studies also highlight some limitations with laboratory-based data collection, particularly in termsof exercise prescription.
Exercising at a set VO2max may stimulate physiological responses to endurance exercise, but it is not applicable to a racing situation, where the degree of exercise intensity is higher and constantly being modified.
In a study published in 2003 by the British Journal of Sports Medicine, the authors, with Raphael Twerenbold as lead author, attempted to address some of the limitations associated with the laboratory-based studies in a 2003 crossover-intervention study, recruiting 13 well-trained, healthy female runners to participate in three to four-hour running time trials around a 400-meter track.
For each of the respective three trials, the participants consumed:
  • 1 L/h of a high sodium-carbohydrate solution (25 mmol/L)
  • low sodium-carbohydrate solution (15 mmol/L)
  • water
The change in plasma [Na[+]] from pre-run to post-run was significantly smaller in the high-sodium trial compared to the water trial (sodium vs. water, -2.5mmol/L vs. -6.2mmol/L; p = 0.001), which supports the changes observed by Vrijens and Rehrer.
This was further reflected in the proportion of participants that developed mild EAH in each trial (46% when consuming a high-sodium beverage vs. 92% when consuming water).
Despite these large differences in plasma [Na[+]], there were no significant effects on performance.
This suggested that the body is able to cope with sodium imbalances when exercising in cool conditions.
Although the Twerenbold et al. study used a race-like exercise prescription to assess sodium supplementation, it has been criticized, alongside Vrijens and Rehrer and Anastasiou et al. of over-hydrating their participants and inducing dilutional EAH.
The standardized-fluid intakes of 1 L/h were shown to elicit dilutional hyponatremia (below 135 mmol/L) in 69% of their participants, despite being within the recommended fluid intake guidelines for athletes at that time.
Indeed, voluntary fluid consumption tends to be about half an athlete’s sweat rate for most sports, which highlights the importance of investigating the effects of sodium supplementation when athletes consume fluids ad libitum, such as during field studies.

Field Studies

The first field study, published in 2002 by the Clinical Journal of Sport Medicine, with Dale B. Speedy as lead author, investigated the effects of sodium supplementation during the 2000 Cape Town Ironman Triathlon.
Thirty-eight athletes were recruited at race registration, three days prior to the race. These participants were issued with sufficient salt tablets to provide 700 milligrams per hour (mg/h) of sodium, during the race, which lasted for approximately 12.5 hours.
Some of the tablets also contained carbohydrate, but this was not the case for all the salt tablets.
The control group (n = 133) was not given any salt tablets, but completed the same pre-race and post-race measures as the intervention group.
As the participants were not randomized to receive the intervention, the two groups were matched during analysis, according to both body mass change during the race and pre-race plasma sodium.
However, despite the matched analysis, this is still a significant limitation for the study, especially as:
  • neither group was blinded for the intervention and measures of pre-race body mass, and sodium concentration were taken 1 to 3 days prior to the race
  • no dietary standardization or measurements were taken
There were no significant differences in the change in plasma [Na[+]], when matched for pre-race plasma [Na[+]], neither was there any difference in performance between the groups.
Interestingly, the prevalence of EAH during this race was particularly low; only one athlete developed asymptomatic hyponatremia out of the entire field.
Despite similar temperatures and relative humidity, previous prevalence rates have been much higher:
  • up to 29% of race finishers were reported to have developed EAH in the Hawaiian Ironman Triathlon, according to a study by Dr. W. Douglas B. Hiller and published in 1989 by the journal Medicine & Science in Sports & Exercise
  • Speedy et al. reported 18% of race finishers in the New Zealand Ironman Triathlon
The low prevalence rates of EAH observed in this Ironman Triathlon could therefore reflect the results gathered in this study.
Plasma [Na[+]] increased and weight decreased in both the control and supplementation groups, which suggested that participants became hypohydrated, and did not overconsume the fluid.
A similar trial was conducted a year later during the 2001 Cape Town Ironman, and the results were published in 2006 by the British Journal of Sports Medicine, with Tamara D. Hew-Butler as lead author.
In that trial, 145 triathletes, with a finish time around 12.5 hours, were randomized to receive either:
  • salt tablets (10.6 mmol Na[+] per tablet, between 10.6–42.4 mmol/h); or
  • a placebo tablet (596 mg starch per tablet)
The randomization and blinding protocol was advantageous compared to the study by Speedy et al., as it reduced selection-bias in analysis, and allowed a much more comparable control group.
Despite this, the results of the Hew-Butler et al. study supported the findings of Speedy et al. and showed no significant differences in:
  • plasma [Na[+]]
  • body mass change
  • performance
  • prevalence of medical care between the intervention and placebo groups
Hew-Butler et al. observed a number of participants developing mild hyponatremia (post-race [Na[+]] 130–135 mmol/L), even though they lost weight during the Cape Town race.
This should be interpreted with caution as pre-race weight was determined three days prior to the start of the race, so it cannot be confirmed that the loss of body mass occurred during the race and not before the race.
This highlights that although over consuming fluid is an important causative factor, other factors can play a role in the development of EAH.
There was no difference in post-race plasma [Na[+]] between the intervention and control groups in both the field studies, however this may be reflective of the inherent limitations with these study designs, where dietary intakes before and during the race are not controlled.
The Cape Town Ironman provided all athletes with sports drinks (Energade, [Na[+]] = 18 mmol/L), every 20 kilometers (km) in the cycling leg and every 2.5 km in the running leg.
It is, therefore, probable that the control group consumed sodium-containing sports foods, with a similar consistency to those in the study by Vrijens and Rehrer.
Hence, it is difficult to directly compare the results observed in the intervention group to the control group.
Further, the fact that both these races were undertaken in South Africa and the low prevalence of hyponatremia that is reported could indicate that the ad-libitum food and fluid intakes in this country are different to other races, where much higher prevalence of hyponatremia have been reported.
In their study published in 2013 by the Journal of the International Society of Sports Nutrition (JISSN), Samuel David Cosgrove and Katherine Elizabeth Black tried to address some of these control issues by conducting a blinded, randomized-crossover study of sodium supplementation in a 72-kilometer road cycling time-trial.
Nine well-trained cyclists (5 male cyclists, 4 female cyclists) consumed either a 30 mmol/h sodium or a placebo.
In line, with the research at the Cape Town Ironman, Cosgrove and Black found that sodium supplementation had no effect on plasma [Na[+]] change (relative change pre-race to post-race, salt = 0.56%, placebo = 0.47%, p = 0.7) during the time-trial, with neither of the participant becoming hyponatremic on either trial, nor was there any difference in the time taken to complete the time trial between the supplemented and placebo groups.
They suggested the mildly-cold conditions of the time-trials (14°C) that did not elicit large enough sweat sodium losses to warrant sodium replacement in the time-trials.

Discussion

In this review, the authors have referenced some of their own studies.
These referenced studies have been conducted in accordance with the Declaration of Helsinki (1964) and the protocols of these studies have been approved by the relevant ethics committees associated with the institution in which they were performed.
All human subjects, in these referenced studies, gave informed consent to participate in the studies.
It is interesting that all of the studies, which have shown a beneficial effect, have been between the trials containing carbohydrate-electrolyte beverages and water trials.
This raises the question as to whether it is the sodium per-se or another substance within the drink, which is eliciting the effect.
Indeed, the ingestion of carbohydrates is likely to attenuate the stress of exercise.
However, as these carbohydrate electrolyte beverages contain more than just sodium and carbohydrate, any of the drink components could have influenced the results.
Also, as they were not separately assessed, the exact reasoning for these results cannot be confirmed.
The obvious criticism that the studies reporting beneficial effects are because of the overconsumption of fluids may well be true, but both Barr et al. and Sanders et al. replaced sweat losses, but yet did not see any differences in plasma sodium between the water and sodium containing trials.
Nearly all of the studies have failed to result in high rates of hyponatremia, even when large amounts of sodium-free fluid have been ingested, this conflicts the observational data from endurance races, where up to 30% of athletes have been reported to have EAH.
Future research is required to concentrate on those with a historyof hyponatremia to determine why some athletes are more susceptible to hyponatremia.
This sub-population may benefit from sodium supplements, but before conclusions can be made further investigations are necessary.

Conclusion

Whilst there are some suggestions from laboratory studies that sodium supplementation could reduce EAH incidence and improve performance, particularly during exercise in the heat, recent field trials have demonstrated that sodium supplementation has no effect on plasma sodium concentrations during a racing situation.
Nonetheless, a well-controlled crossover field trial about sodium supplementation during prolonged exercisein a hot environment is still required to develop practical recommendations.
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