Aerobic exercise such as running often involves sweating, but why do we lose sodium when we sweat? And what is the best method of rehydration? George Winter asks the experts.

The Science 

Electrolytes are an important part of the sweating story. They’re substances which, when dissolved, separate into atoms that carry either a positive or negative electrical charge, and they’re vital for cellular function.

Two types of electrolytes, sodium and potassium, are involved in the control of fluid volume. Body fluid is either inside cells, as intracellular fluid (ICF), or outside cells, as extracellular fluid (ECF), which includes blood plasma.

The amount of sodium in the body determines the ECF volume, whereas potassium determines the ICF volume. Water crosses the membranes separating ICF from ECF, and its movement is influenced by the number of ions in each compartment – the osmolality. The less water in the blood, the greater the concentration of electrolytes and the greater the osmolality.

When osmolality rises, the body produces anti-diuretic hormone (ADH), conserving more water inside the body, and making blood and urine more concentrated; when osmolality falls, less ADH is produced and blood and urine are more dilute.

So what does all this mean for runners?

The role of sodium

Humans sweat more than any other animal − rates can exceed 500g/m2 per hour – losing water and electrolytes, particularly sodium, in varying amounts.

The concentration of sodium in sweat is always less than its concentration in the ECF, so sweating without drinking causes the concentration of sodium in the ECF to rise, triggering the thirst mechanism.

But why do we lose sodium in sweat?

Exercise physiologist Professor Tim Noakes suggests in Waterlogged: The Serious Problem of Overhydration in Endurance Sports (2012) that salt loss in sweat occurs “as a mechanism for delaying drinking and so it acutely dissociates the extent of the water loss from the thirst that it produces.”

Generally speaking, we eat around 8.5g of salt a day. So how can we explain the phenomenon of exercise-associated hyponatraemia (EAH), which can occur during prolonged exercise, and the main abnormality of which is a reduced blood sodium concentration?

In 2006 an article in The British Journal of Sports Medicine (2006, 40: 98−105) claimed: “Contributing factors to exercise-associated hyponatraemia include overdrinking of hypotonic fluids and excessive loss of total body sodium [my emphasis]”.

Significantly, the study authors had associations with the sports drink industry. But Prof Tim Noakes in Waterlogged disputed the reference to total body sodium, responding: “There is no scientific basis for this statement”. These differing views are a good example of the scientific conflict that exists between those who promote electrolyte-supplemented sports drinks, and those who don’t.

The low blood sodium concentration in EAH is caused by drinking too much fluid, including sodium-containing sports drinks. This was made clear in February 2015, when the Third International Exercise- Associated Hyponatraemia Consensus Development Conference stated in the clinical Journal of Sports Medicine (2015; 25: 303−320) that “… in most clinical scenarios, the driving force for the development of hyponatraemia is a relative excess of total body water.”

Taking on extra sodium before or during exercise will not prevent EAH developing in those who drink in excess of the dictates of thirst.

Sodium-fortified drinks and gels

For those of us who are used to a highcarbohydrate diet, the evidence is clear: consuming carbohydrate during endurance exercise improves performance. It’s generally accepted that taking around 60g of carbohydrate per hour is the best strategy in events that last for more than two hours, and there’s evidence that specific mixtures of glucose/maltodextrin and fructose can actually boost carbohydrate uptake.

Sports drinks allow two goals to be achieved: first, hydration; second, the delivery of carbohydrate. So for the athlete who’s thirsty and short of carbohydrate, the typical sports drink meets these needs. Sports gels allow the delivery of carbohydrate in a more concentrated form, albeit at the expense of volume, something to remember when hydration strategies are being devised.

Many sports drink and sports gel manufacturers claim that their products supply much-needed electrolytes, the most prominently publicised of which is sodium.

But how crucial is sodium replenishment to the endurance athlete, in fact, and is it even needed, given our prodigious salt intakes?

Exercise-ass ociated muscle cramps (EAMC)

When Dr Michael F Bergeron’s paper entitled “Muscle Cramps during Exercise – Is it Fatigue or Electrolyte Deficit?” appeared in Current Sports Medicine Reports (2008; 7 (4): S50−S55) he suggested two possible causes of EAMC.

The first was as a result of skeletal muscle overload and which the so-called neuromuscular control theory suggests is caused when different factors such as fatigue and muscle damage combine to increase nerve excitability, triggering the condition.

The second possible cause was “extensive sweating and a consequent significant whole-body exchangeable sodium deficit caused by insufficient sodium dietary intake …”

This was endorsed by Dr E Randy Eichner, in Current Sports Medicine Reports (2014; 13: 197−198), who even stated that “[c]ontrarians favour fancy theories like ‘altered neuromuscular control’”.

But the ‘fancy theory’ ridiculed by Eichner received support in a recent paper (Current Sports Medicine Reports 2015; 14: 353−354) which noted that in August 2014 two healthy high school American football players died from EAH encephalopathy after drinking large volumes of fluid to prevent EAMC:

“Underlying these tragedies is the belief held by many medical professionals and the general public that EAMC is caused by dehydration and electrolyte (eg sodium) losses. Yet, new experimental and observational data … suggest that cramping may be due to changes in the nervous system.”

And a recent study of 280 competitors in a 161km ultramarathon concluded that exercise-associated muscle cramping (EAMC) was most common for those with a history of cramping and greater muscle damage during an ultramarathon.

Writing in Sports Medicine – Open (2015), the authors stated: “Impaired fluid and sodium balance did not appear to be an etiology of muscle cramping during an ultramarathon.”

One of the report’s co-authors − Professor Martin D Hoffman – told Running Fitness, “Basically, we’ve found that whether considering supplemental sodium intake or total sodium intake, there is no evidence that sodium protects against muscle cramping, nausea or vomiting, or hyponatraemia in this population of ultramarathon runners during continuous exercise up to 30 hours in hot environments.

Hydration status can also be maintained without sodium supplements, and by drinking to thirst. Obviously, there is a need for caloric intake during such long bouts of exercise.”

Prof Hoffman’s advice is: “As for exercise less than 60 minutes, presuming adequate hydration and nutrition status at the initiation of exercise, there’s no need for any fluid, electrolyte or caloric intake.”

And Prof Noakes states in relation to sodium intake: “Whether the ingested solution contains a great deal of sodium or none at all; by the time the solution passes the site of the small intestine where the bile duct enters, all ingested solutions, whether pure water or an electrolyte containing sports drink will have identical sodium concentrations.”

Sodium in the diet

The prestigious Harvard TH Chan School of Public Health says that American consumers eat far too much sodium and far too little potassium, an eating pattern that puts them at higher risk of heart disease and death. The same can be applied to us in Europe.

The School states: “Making a few changes in food choices can help shift the balance. Potassium levels are naturally high in vegetables and fruits, and sodium levels are naturally low. Large amounts of sodium are often added to foods during processing.

So choosing produce that is fresh or frozen, or choosing foods that have not had salt added during processing, can help curb dietary sodium and boost potassium.” (hsph.harvard.edu/nutritionsource/sodium-potassium-balance/ accessed 8 October 2015).

Sodium is crucial to our regulation of fluid intake, and the evidence is clear that sports drinks and gels are great at delivering carbohydrate and water. But there is no need to consume extra sodium during exercise. Nor is there any need to consume more sodium than that provided by our diets.