Electrolytes in sport: what exactly are they and how many does the body really need?

When you look at adverts for sports drinks or supplements, it is easy to get the impression that without electrolytes you cannot even manage an hour of easy training. White salt marks appear on endurance athletes' kit after demanding races, and marketing tells us that we have to replace these losses immediately and aggressively with expensive tablets or isotonic drinks [128].

But how much of that is true according to real sports science? In this introductory article we will look at what electrolytes actually are, how the body regulates them, what effect they have on muscle cramps and how much of them we really need during sport.

What actually are electrolytes?

From a chemical point of view, electrolytes are minerals which, once dissolved in water, split into individual ions with a positive or negative electrical charge, which allows them to conduct an electric current in solution [97, 131, 132]. The main electrolytes in the human body are sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), hydrogen carbonate (bicarbonate), phosphate, sulphate, magnesium (Mg²⁺) and calcium (Ca²⁺) [132].

Each of them has an irreplaceable role in the body for the functioning of cells, the transmission of nerve impulses, muscle contraction and maintaining blood pressure [133, 211, 217]. In terms of their distribution in the body's fluids, however, the key thing is how they are divided:

• Sodium and chloride are the dominant ions in the extracellular environment, which means that they are found in the highest concentration in the blood and tissue fluid [132].
• Potassium is by contrast the main ion inside our cells (intracellularly), for example in muscle or brain cells [132, 210].

Because sodium and chloride are found primarily outside cells, it is these two electrolytes that leave the body in significant amounts through sweating [97, 134].

Sweating: cooling first, loss of minerals only second

During sport, especially in the heat and at high intensity, the muscles produce an enormous amount of heat (for every 1 kcal of mechanical work there is roughly 4 kcal of waste heat) [138]. Sweating is primarily a cooling mechanism – by evaporating sweat from the surface of the skin, the body gets rid of excess heat into its surroundings [137, 154].

The amount of fluid and electrolytes lost through sweat is, however, extremely individual:

• Sweat rate: It can range from almost zero up to 5 litres per hour depending on intensity, temperature, humidity, clothing and genetic factors [137, 138].
• Sodium concentration in sweat: The average sodium concentration in sweat is roughly 0.8 g per litre of sweat (about 36 mmol/l) [140]. The range between athletes is, however, enormous – some lose only 0.25 g/l, while others as much as 2.0 g/l [140].

It is important to realise that sweat is always considerably more dilute than blood. The concentration of electrolytes in sweat is consistently lower than in the circulation [142, 155]. That means that when you sweat you lose proportionally far more water than sodium [155]. As a result, if you do not drink during exercise, the sodium concentration in your blood actually rises because of the loss of water, rather than falling [155].

Ingenious regulation: how does the body defend itself against a shortage?

The human body has extremely efficient hormonal mechanisms for maintaining sodium balance [217, 220]. If you have little salt in your food or lose it through sweat, the kidneys react immediately and reduce the excretion of sodium in urine [104, 220].

Similarly, though somewhat more slowly (in the order of 12 to 24+ hours), the sweat glands can also react [160]. Research shows that if an athlete switches to a low-salt diet, the sweat glands adapt and start to reabsorb sodium from sweat back into the body more actively, which reduces the sodium concentration in sweat [155, 157, 160]. A massive drop in the sodium concentration in sweat (by up to 30 %) also happens during heat acclimatisation [161].

Thanks to these adaptive abilities and to the fact that the modern Western diet is very rich in sodium, a real sodium deficit in the body of a healthy athlete under normal conditions is practically impossible [104, 214].

The main trio under the microscope: which minerals to deal with and which are a marketing myth?

Looking at the composition of sports supplements you will find a whole range of electrolytes. What do scientific studies say about them?

1. Sodium (Na⁺) – the king of sports hydration

Sodium is the only electrolyte for which topping up during specific sporting situations demonstrably makes sense [134]. Not because you are at risk of an acute shortage of it, but because it helps to maintain blood osmolality and the volume of extracellular fluid [104, 145]. Sodium in a drink stimulates the feeling of thirst, improves appetite and helps the body retain the water it takes in within the bloodstream instead of quickly excreting it in urine [97, 145, 203].

2. Potassium (K⁺) – essential in the cell, pointless in sweat

Potassium is indeed critically important for muscle contractions and nerve impulses, but we lose an absolute minimum of it in sweat (only 160–320 mg per litre of sweat) [141, 211]. The body watches its level in the blood extremely carefully [211]. Any losses of potassium during training are negligible and you can easily cover them with a normal diet (e.g. one banana) [214]. Targeted potassium supplementation during exercise has no demonstrable benefit for performance [97].

3. Magnesium (Mg²⁺) and calcium (Ca²⁺) – the big myths about cramps and bones

• Magnesium: It is heavily promoted as a cure for muscle cramps [212]. In reality only 4 to 15 mg per litre of it are lost in sweat [141]. Scientific evidence that magnesium supplementation during exercise prevents sport-related cramps practically does not exist [212]. A certain small effect has been shown for night cramps, but not for those brought on by sporting exertion [212].
• Calcium: Although it is essential for muscle work, its losses through sweat are minimal and supplementing with it during sport has no direct effect on performance [213].

So what about those cramps?

If you ask an ordinary runner or cyclist why they take salt tablets, the most common answer will be: "So I do not get cramp." [101]

This hypothesis, however, repeatedly fails in scientific studies [101, 219]. Observational studies from marathons and ultra-endurance races found no difference in hydration, in the use of salt supplements or in the concentration of electrolytes in the blood between athletes who got cramp and those who finished the race without it [101]. The scientific consensus leans towards cramps being caused by complex neuromuscular fatigue – that is, overloading of the nervous system that controls muscle contractions [102, 219].

There is, however, one exception where electrolytes are related to cramps. If an athlete becomes significantly dehydrated and then drinks a huge amount of plain water (without sodium) all at once, there is a sharp drop in blood osmolality [101, 103]. Water starts to move quickly into the tissues, which causes cells to swell and significantly increases the susceptibility of muscles to cramp [103]. In this specific case, adding sodium to the drink prevents the drop in osmolality and neutralises the risk of cramp [103]. The problem here, however, is not a "sodium deficit" but excessive dilution of the internal environment with plain water [101, 103, 104].

So how many electrolytes (sodium) does the body need during sport?

The answer from sports science is surprisingly simple and depends on the length and nature of your exercise [151]:

1. Exercise of up to 2 hours

For training sessions or races lasting less than two hours, topping up electrolytes is from a physiological point of view unnecessary [149, 150]. The loss of sodium is too small to threaten the stability of the internal environment [149].

• Recommendation: It is more than enough to drink plain water or a drink “salted to taste” (about 400–500 mg of sodium per litre) [146, 203]. This amount improves the taste of the drink and supports voluntary fluid intake without burdening the stomach [203].

2. Extreme exercise over 4 hours (ultra-endurance)

Targeted sodium supplementation only starts to be important for extremely long efforts (e.g. Ironman, ultra-marathon, long cycling stages), and then only if two conditions are met [97, 151]:

1. You are replacing fluids aggressively (you drink so intensively that you replace more than 70 % of your sweat losses) [97, 151].
2. You are one of the above-average “salty endurance athletes” (your sodium concentration in sweat is higher than 1 g/l) and you exercise in the heat [97, 151].

In these cases, targeted sodium supplementation is a key defence against the development of dangerous hyponatraemia (a low sodium concentration in the blood) [97, 200, 220].

Watch out for too much salt!

Many athletes make the mistake of buying an expensive isotonic drink (which has the optimal osmolality for fast absorption) and then tipping salt tablets into it as well [204]. This step sends the osmolality of the drink up into hypertonic values, which in the stomach and intestine causes exactly the opposite – water starts to be drawn from the body back into the digestive tract, which leads to dehydration and severe digestive problems [5, 34, 204].

A tip from scientific studies (Rowlands et al., 2021): Extensive analyses show that for optimal hydration during endurance exercise the most effective are hypotonic sports drinks (osmolality < 275 mOsmol/kg) with a moderate content of carbohydrates and sodium (< 50 mmol/l, or < 1.15 g of sodium per litre) [23]. These drinks pass through the stomach and intestine fastest and best maintain blood plasma volume [23, 28, 72].

Summary for practice

• Electrolytes are not a miracle elixir: For ordinary fitness training, a run of up to 90 minutes or an hour in the gym you do not need any special isotonic supplements [134, 150, 214].
• Muscle cramps are about fatigue: A shortage of magnesium or the loss of salt itself does not cause cramps during exercise [101, 212, 219]. Cramps are more likely when you drink too much plain water after becoming dehydrated [103].
• Deal with sodium according to taste: A small amount of sodium in an isotonic drink (400–500 mg/l) is great for palatability (taste) and hydration [146, 203].
• Ultra-endurance athletes, take note: Only for efforts over 4 hours in the heat does it make sense to consider individual sweat testing and more precise sodium replacement [97, 151].