In this series of blog posts, we’ve discussed what is meant by the ‘electrolytes’, that are promoted in sports nutrition products. We’ve looked at how much you actually lose during exercise, and what role they play in athlete health and performance during exercise. So far we’ve seen that:
Electrolytes are minerals that dissolve in water into their individual, positively or negatively charged ions: Sodium: Na+ and Potassium: K+ Chloride: Cl- Magnesium: Mg2+ and Calcium: Ca2+ and many others
Sodium and chloride are the two electrolytes lost in large quantities through sweat, but these losses are also regulated in response to the balance of sodium consumed in the diet and recent sweat and urine losses.
Sodium added to drinks before exercise can improve the amount of that fluid retained, rather than lost through urination.
Sodium added to drinks during exercise improves their flavour and tends to encourage consumption, which can be useful in terms of preventing excessive fluid losses during exercise. The effect on fluid and carbohydrate absorption from the gut is likely minimal.
Sodium during exercise can also reduce the fall in blood osmolality and reduce (but not eliminate) the effect of aggressive fluid replacement on the risk of developing hyponatraemia. However, only when athletes exercise for more than 4 hours, and are likely to drink to replace >70% of their sweat losses does the process of sweat sodium testing and targeted replacement appear necessary. Even then, only those athletes with above average sodium concentrations in their sweat (>1g/L) are likely to need to specifically focus on sodium replacement during exercise.
Whilst there are only a few studies in this area, and many have specific methodological concerns, there is currently little evidence that replacing sodium during exercise will improve performance in the same way that adequate carbohydrate or fluid intake will. However, as per the point above, if aggressive fluid replacement is undertaken, then sodium replacement will be useful to balance that fluid intake and maintain a stable blood osmolality.
Other electrolyte losses in sweat are minimal and don’t appear to need specific replacement during exercise.
Only when athletes exercise for more than 4 hours, and are likely to drink to replace >70% of their sweat losses does the process of sweat sodium testing and targeted replacement appear necessary.
So are we replacing sodium losses, or balancing water turnover?
When we think about consuming carbohydrate or fluid during exercise, we’re either thinking about topping up a finite store of a fuel that is preferentially used to fuel the demand of working muscles (carbohydrate), or preventing the negative effects that an overall deficit has on health and performance (fluid). This is the same lens through which the majority of athletes and coaches tend to think about sodium – that when a certain amount of sodium is lost from the body without replacement, that something will go wrong physiologically that leads to either detrimental health or performance outcomes.
Interestingly, research over the last few decades suggests that humans do in fact have stores of sodium in the body that are bound to structures in the skin, muscle and possible other tissues. This sodium does not contribute to the body’s fluid osmolality, but can be added to or released back into the blood as required. Some researchers originally suggested that the release of body sodium stores could help protect athletes from hyponatraemia, and that this justified consuming no or minimal salt during exercise. Recent studies however, suggest that sodium stores are not released for this purpose – instead they are released when body water content is low, as the additional sodium will provide an even stronger signal to the kidneys to conserve water, and make the individual even thirstier.
Humans do in fact have stores of sodium in the body that can be added to or released back into the blood as required. Because off the size of these stores, "running out of sodium" is a highly unlikely scenario.
So do athletes run into problems when a critical amount of sodium is lost from the body?
To date we don’t have any scientific evidence to support this theory. All the roles of sodium discussed in the previous blogs refer to the relationship between water and sodium, rather than a precise amount of sodium (or the development of some type of sodium deficiency) itself. It’s why the need for sodium replacement during exercise appears to be linked to the extent of fluid replacement, and is intended to balance the changes in body water so that the blood sodium (and therefore osmolality) is kept reasonably stable.
What about cramping?
The most common reason that athletes give for replacing sodium during exercise is the prevention or treatment of muscle cramping, with the view that the more sodium you lose, the more you need to replace (regardless of fluid intake and losses). Despite many anecdotal stories of success with this approach, scientific evidence has time and time again failed to demonstrate this link. This evidence includes:
Observational studies of miners and other workers doing hard, repetitive, manual labour in hot conditions in the early 20th century would experience cramping, which seemed more likely to be caused be excessive consumption of plain water rather than dehydration or electrolyte losses. In some cases salt tablets or intravenous saline may have relieved cramping, but this is likely related to the balance of water and sodium rather than a sodium deficit per se (more on this below).
Studies of athletes at endurance and ultra-endurance races, where blood samples and questionnaires were undertaken in those who did or did not cramp. These studies found no relationship between hydration status, use of salt replacement products or blood electrolyte concentrations between those who did and did not cramp. It has been acknowledged however that these blood tests were often not taken at the time of cramping, but often several hours later.
Laboratory studies where sodium has been given with or without fatiguing exercise, and the level of electrical stimulation of muscles to cause them to cramp has been measured. Previous work suggests that those who tend to cramp during competition are also those who cramp more easily with electrical stimulation. These studies suggest that dehydration or a large sodium deficit per se does not change cramping risk using this method.
The most common reason that athletes give for replacing sodium during exercise is the prevention or treatment of muscle cramping. Despite anecdotal stories, scientific evidence has time and time again failed to demonstrate this link.
The most recent scientific view of cramping during exercise is that it is most likely a complex syndrome, with multiple different factors that can lead to changes in the nerves that control muscle contraction. These factors are broad and include muscle fatigue (for various reasons), and changes in the function of the nervous system itself (this can include pain, certain health conditions and medications, physical and psychological stress). So as discussed previously with exercise induced gastrointestinal syndrome, it is almost too simplistic to blame cramping on one single factor.
The most recent scientific view of cramping during exercise is that it is most likely a complex syndrome, with multiple different factors that can lead to changes in the nerves that control muscle contraction.
In the past few years however, a small group of studies have emerged that suggests that sodium may still play a minor role in cramping risk for some people. In people who were already dehydrated from exercise, consuming a large amount of plain water (as opposed to a sodium containing drink) appeared to increase the risk of cramping when induced by electrical stimulation. It is still unclear exactly how or why, but one likely explanation is that when large amounts of plain water are consumed, the drop in blood osmolality causes much of the water to rapidly enter the tissues of the body, and the sudden expansion in the size of cells may play some role. Clearly more research needs to be done to confirm this effect and make practical recommendations, but regardless it seems clear from the science so far that any effect of sodium on cramping risk is once again to do with the relationship between sodium and water, rather than a ‘sodium deficit’ per se.
Summary
In summary, sodium has an important role in the body of maintaining the osmolality of the blood, which in turn keeps the balance of water between the inside and the outside of our body’s cells. It also plays an important role in regulating the overall amount of water in the body, by influencing both how much how is lost or retained by the kidneys, and our thirst and desire to drink. A true sodium deficit, however, appears to not be an important factor for athletes during the timeframe in which exercise is performed. Furthermore, our modern diet is so abundant in sodium that a true deficiency over days or weeks is considered virtually impossible, especially when you consider that both the sweat glands and the kidneys can (and will) adapt to minimise sodium losses if required. Whilst sweat sodium losses during exercise vary significantly from person-to-person and day-to-day due to a range of factors, the ultimate need for replacing sodium during exercise is to balance out fluid intake and losses and maintain an appropriate osmolality, rather than preventing a actual sodium deficit. In most cases this does not require any sodium due to the way the sweat glands remove proportionally more water than sodium, and so sodium during exercise is more about taste than physical need. However, in very long duration exercise when water is aggressively replaced, there can be an important role for purposeful, targeted sodium replacement.
Since many messages about electrolytes are targeted towards athletes participating or competing in events less than 4 hours, it is fair to say that the message as it is portrayed in the media and by many companies is more hype than it is backed by scientific evidence.
Whilst sweat sodium losses during exercise vary significantly from person-to-person and day-to-day due to a range of factors, the ultimate need for replacing sodium during exercise is to balance out fluid intake and losses and maintain an appropriate osmolality, rather than preventing a actual sodium deficit.
References
Miller KC. et al. An Evidence-Based Review of the Pathophysiology, Treatment, and Prevention of Exercise-Associated Muscle Cramps. J Athl Train. 2022; 57(1):5-15.
Lau WY. et al. Water intake after dehydration makes muscles more susceptible to cramp but electrolytes reverse that effect. BMJ Open Sport Exerc Med. 2019; 5(1):e000478.
Maughan RJ. & Shirreffs SM. Muscle Cramping During Exercise: Causes, Solutions, and Questions Remaining. Sports Med.2019; 49(Suppl2):115-124.
