When you sweat, you are losing much more than water. You are also losing electrolytes. Sodium and chloride are the most abundant electrolytes in sweat with potassium, magnesium, and calcium present in lower amounts.
The typical understanding amongst athletes, particularly in the endurance space, is that you need to replace electrolytes (specifically sodium) during exercise alongside fluids to maintain performance.
However, the research done in this space is sorely lacking. Currently, there is only one randomized control trial from 2018 that shows an increase in performance with sodium replacement during exercise. But these results have not been replicated since.
Despite this, many athletes will drink sports drinks and use other electrolyte products like Hydralyte.
And there is absolutely a place for these products for some athletes. It is just that the importance of replacing electrolytes is likely not as great as many would think.
Beliefs About Replacing Sodium Losses Amongst Athletes
An interesting study was published in 2018 looking at the beliefs around sodium intake amongst a group of endurance athletes.
Respondents generally believed that sodium intake during exercise prevented hyponatremia (an imbalance of sodium and fluids in the body) and exercise-induced cramps.
79% consciously replaced sodium during exercise and competition, although only 29% had a specific plan. It is likely most just made sure that they were using sports products such as drinks and gels that had some sodium and electrolyte content.
This is interesting because unlike the intake of carbohydrates and fluid during an endurance event or training, there is very little evidence to support sodium replacement.
That isn’t to say it is completely useless but from an evidence perspective, we don’t have a lot to back up this very standard practice amongst athletes and exercisers.
Hydration & Performance
The concern around sodium loss through sweat is mostly related to endurance sports but also stop-and-go sports that involve high-intensity intermittent exercise lasting more than 2 hours such as a game of soccer.
How much sodium an athlete will lose during a session or an event depends on a range of factors including sweat rate and the individual composition of their sweat, environmental conditions, heat acclimatization, and training status.
Athletes can lose as much as 5000mg of sodium through sweating in a single high-intensity workout. So in the case where sodium losses are high, sodium ingestion may play an important role in minimizing dehydration by assisting with fluid and electrolyte balance.
From a theoretical point of view, it makes sense to replace sodium and fluid during a long-distance event or a long period of intermittent high-intensity activity.
But as previously mentioned, the research behind sodium ingestion during exercise and actual impacts on performance doesn’t look very promising.
A 2018 systematic review found just five studies that had been published looking at the association between sodium ingestion and endurance performance.
Only one of the five studies suggested that sodium intake benefited endurance performance. This study was a randomized control trial that provided athletes with either sodium capsules or a placebo during a half ironman triathlon.
They found that athletes that were provided with the sodium capsules had finishing times that were 8% faster than those who were given the placebo.
Eight percent is pretty huge when it comes to nutrition interventions. The performance benefit of caffeine supplementation, adequate fluid intake, and carbohydrates typically show less than half of the performance benefit found in this one study.
Although these findings have not been replicated since and other studies investigating the same thing found next to no performance benefit. Therefore, it’s highly likely that sodium replacement does not have this big of an impact on performance and this one study is the outlier.
It is generally felt that this finding may have been confounded by other variables such as the triathlon transitions and other competitors in the event which cannot be controlled. As is the nature of any research not done in lab conditions.
Effects On Thirst
For a lot of athletes, drinking to thirst is a key way to avoid dehydration during exercise and for most, it can be quite an effective one.
However, thirst sensation is highly sensitive to changes in sodium. If you are losing a lot of sodium through sweat and not replacing it, you’re less likely to feel the sensation of thirst despite also losing fluids.
This is because the body has many ways to maintain a balance between sodium and fluids, including changes to thirst.
Sodium ingestion during or following exercise will actually help to stimulate thirst and drinking as well as stimulate fluid retention by the kidney.
So theoretically, if you were to replace some of your sodium losses during exercise, you would have a greater thirst sensation. This would then help you to drink more fluids. Plus your body will be able to retain more of that fluid.
Therefore, sodium ingestion could assist with efforts to maintain hydration.
We do know that losing more than 2% of your body weight due to fluid losses during exercise is likely to have a significant impact on performance. So if sodium intake assists with avoiding dehydration then it would be well worth utilizing as a tool in training and during events.
How much sodium you should be replacing though is highly debatable.
Sodium & Cramps
Exercise-induced muscle cramps (EAMC) typically occur in single, multijoint muscles (eg, triceps surae, quadriceps, hamstrings) when contracting in a shortened state.
Although some cramps do not affect athletic performance, other times, they can be completely debilitating.
One of the original theories behind the cause of cramps was the dehydration–electrolyte imbalance theory.
This theory suggests that this depletion, namely of sodium and potassium, results in the sensitization of select nerve terminals.
Research shows that exercise in hot and humid conditions is more likely to facilitate cramping. Leading experts to believe that it was indeed the amount of fluid and electrolytes lost, that made cramping more likely to occur.
This has been keenly supported by the sports drink and supplement industries, which market fluid and electrolyte products aimed at preventing or reducing the severity of cramping.
Despite this, there is very little research that directly links shifts in fluid and electrolytes with the occurrence of cramps.
We do, however, know that overhydration can increase the risk of cramping.
This is due to the fact that plasma sodium and osmolality is reduced. So if fluids are aggressively replaced during exercise without any sodium replacement this may lead to an increase in cramping. But the aggressive overhydration likely has more to do with it than the lack of sodium ingestion.
Furthermore, sodium, potassium, calcium, and/or magnesium intake before and during exercise do not appear to prevent the onset of cramping.
Sodium & Avoiding Exercise-Associated Hyponatremia (EAH)
Exercise Associated Hyponatremia (EAH) is a serious, potentially fatal condition.
From 1980 to the present day, 14 athletes have died from EAH and many more have been hospitalized.
EAH is characterized by low blood sodium concentration which produces a rapid and dangerous swelling of the brain that can result in seizures, coma, and death.
Whilst EAH is thought to be due to both excessive fluid or an acute sodium deficit, it appears only excessive fluid intake leads to the symptoms of EAH. This is consistent with the view that it is the excessive intracellular fluid, rather than the sodium concentration itself, that is harmful.
Therefore, the best way to avoid EAH is to generally avoid consuming more fluid than what is being lost.
If you are finishing a training session or an event weighing more than what you started, you are overdoing it on the fluids. For the most part, you do not want to exceed more than a 2% reduction in body weight from fluid losses (although many elite athletes actually do) but you definitely don’t want to gain body weight during exercise.
The risk of hyponatremia can also be reduced by ingesting sodium alongside fluids to maintain a good fluid and electrolyte balance. However, avoiding excessive fluid consumption is the most important factor.
Athletes who ingest sodium through sports drinks and other food and beverages are still at risk of EAH if their fluid intake is high enough.
Sodium Replacement During Exercise
Currently, there is no consensus or guidelines on sodium replacement during exercise.
Whether sodium replacement has a role in athletic performance still has a question mark on it so we definitely don’t know the best way to go about it.
Who Should Replace Sodium?
Significant sodium replacement is only required when fluid intake exceeds 80% of sweat losses to reduce the risk of hyponatremia.
However, it is very rare that an athlete would consume 80% or more of their fluid losses. In most endurance sports or stop-and-go sports, the opportunities to drink are limited.
For example, a triathlete relying on aid stations during the final leg of a race or a soccer player only having access to fluids during halftime.
In an ultra-endurance sport or a slower-paced sport where fluids are highly available, athletes may find themselves replacing a good portion of their fluid losses and may need to consider replacing sodium as well.
It is also recommended by the American Association of Sports Medicine that athletes who:
- Lose a lot of sweat (and therefore sodium)
- Have particularly salty sweat, or
- Relace a high % of their sweat losses
Make an effort to replace some of their sodium losses during exercise.
How Much Should Be Replaced?
In the past, it was often recommended that 100% of sodium losses be replaced in exercise. Although, as discussed previously, athletes will rarely replace 100% of fluid losses.
So it does not make sense to replace more sodium than fluids. In fact, this would likely result in an unnecessary increase in plasma osmolality, and an increased fluid shift from the intracellular to extracellular space.
It is incredibly hard to say how much sodium should be replaced. It likely is very individual depending on a range of factors including:
- Type of exercise, intensity and duration
- Fluid availability
- How much fluid the athlete drinks
- How much the athlete sweats
- The individual composition of that athlete’s sweat
If you feel you do need to replace some sodium during exercise, using a commercially available electrolyte-containing sports drink and other such products is probably all you need to do.
Individualizing sodium and electrolyte intake would not only be very difficult but likely unnecessary as well.
All in all, based on our current body of research, for many athletes there may be very little reason to focus on sodium intake during exercise.
Although, it will be interesting to see if this changes in the future when sodium replacement in prolonged activity is more rigorously studied.
Currently, it makes sense for people who lose a lot of sweat, have very salty sweat and/or replace a high percentage of fluid losses, to replace some sodium as well during exercise.
This may assist with maintaining hydration, reducing the risk of cramps, and possibly also reducing the risk of hyponatremia.
Athletes can do this by utilizing sports drinks, salty snacks, gels, and electrolyte powders.