Do you need to replace the minor electrolytes you lose in sweat?

Sodium is the main electrolyte lost within sweat and should be your focus when planning your hydration strategy for races and competitions lasting more than 90 minutes. But what about the other ‘minor’ electrolytes - such as potassium, magnesium and calcium - do they need to be replaced during exercise too?

We take a look at what should be in your sports drinks during endurance exercise...

Why sodium matters

The amount of sodium lost in sweat varies from person to person; we’ve seen a huge range across the ~31,000 (and counting!) Sweat Tests we’ve conducted over the years, from one athlete who loses as little as 205 milligrams of sodium per litre of sweat, to a really salty sweater who loses 2,334mg/L. 

This interindividual variability is due to differences in the ability of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) ducts in our sweat glands to reclaim sodium as it moves through the sweat gland before being secreted onto the skin’s surface.

Replacing sodium by consuming drinks at a relative sodium concentration that adequately replaces your individual losses is crucial for maintaining fluid balance and preventing dehydration.

When consumed with fluid, sodium also contributes to the absorption of carbohydrates in the gut through co-transporters, nerve impulse transmission, muscle contraction and cognitive function - so it’s pretty darn important. 

What about the other electrolytes?

The extracellular fluid (the fluid that makes up the space outside the cells and in your blood) contains electrolytes at varying concentrations (see Table 1). 

Table 1. Blood electrolyte concentrations.

As sweat is formed, these electrolytes are drawn from this fluid and then partially reabsorbed back through the sweat gland at differing rates before being secreted onto the skin. The result is a sweat composition that differs from the extracellular fluid it came from, but where sodium remains the most abundant electrolyte.

The concentration of electrolytes found in sweat has been published in a few studies, which reported varying results due to differences in methodologies and between individuals. This table shows the concentrations of electrolytes lost in sweat as outlined by Baker and Wolfe, 2020.

Table 2. Sweat Electrolyte Concentrations.

Note: chloride has a greater mg per litre concentration than sodium because it's a heavier molecule (more milligrams of chloride relative to its molecular weight, e.g., mmol), not because more of it is lost.

Sodium and chloride are the most prominent electrolytes lost in sweat, with potassium, magnesium and calcium being lost at lower concentrations.

Chloride

Chloride is the most abundant anion (negative ion) in the body, accounting for ~0.15% of our total body mass. It’s found primarily in the extracellular fluid and the blood. Chloride plays many roles within the body, including the regulation of cellular pH levels, the maintenance of blood volume, and the prevention of spontaneous or prolonged muscle contractions.

Chloride passively follows sodium out of the sweat glands at a similar concentration, though losses in milligrams per litre of sweat are slightly higher than sodium (see Table 2). So, like sodium, the interindividual variability in losses is large. Sweat chloride losses are used as part of the diagnostic criteria for cystic fibrosis. Beyond a sweat chloride loss of 60mmol/L (2,130mg/L), further tests are conducted to confirm the diagnosis. Most people have sweat chloride concentrations well below this threshold.

As chloride plays a lesser role in the regulation of hydration status than sodium, it is less critical that losses are replaced, in most cases. 

Potassium

Potassium is the most abundant positively charged ion in the intra_cellular fluid (that’s the fluid inside our cells - think within the mitochondria or muscle fibres). It plays a role in several bodily functions, including regulating fluid balance (alongside sodium), facilitating nerve signal transmission, and assisting in muscle contractions.

The movement of potassium between the intracellular and extracellular fluid compartments is dynamic and maintained by the sodium-potassium pump. During exercise, potassium ions shift out of cells into the circulating blood to aid muscle contraction; then, following exercise, insulin and adrenaline drive potassium ions back through the sodium-potassium (Na+/K+) pump into cells, lowering plasma potassium concentration and restoring the intracellular potassium level.

Potassium homeostasis is preserved through the balance of intake from the diet and excretion through urine (and to a lesser extent, sweat). We get dietary potassium from many fruits and vegetables such as potatoes, bananas, oranges and mushrooms. Hew-Butler, 2019 states that potassium-rich foods provide us with enough potassium to prevent or treat hypokalemia (when plasma potassium concentration falls below 3.5 mmol/L) at rest and following exercise in most situations.

Sweat losses of potassium are reported to be between 2 and 8 mmol/L, which equates to 78-312mg per litre (~32oz) of sweat. So, while these losses are significantly smaller than those of sodium (10-90 mmol/L), they aren’t nothing. 

Post-exercise, potassium’s contribution to rehydration has been studied by Perez-Idarraga and Aragon-Vargas in 2014. 11 athletes ran or cycled until they lost 2% of their body mass via sweating, and were given plain water, coconut water, a sports drink (411 mg/L sodium, 156 mg/L potassium), or a potassium drink mix (402 mg/L sodium, 1,955 mg/L potassium) in the 1-hour post-exercise period. They found that adding potassium to the sports drink did not provide any further benefits for fluid retention and rehydration beyond sodium alone. Both the sports drink and the potassium drink were better for rehydration than water, supporting that it is the sodium that causes the fluid retention effects. 

But what about during exercise? It has been suggested that for very long duration exercise in hot, humid conditions, potassium supplementation may be necessary, especially if real foods are not being consumed and the individual is a heavy sweater. However, the guidelines for when this would apply are unclear in the literature. Low blood potassium (hypokalemia) occurs when serum potassium levels fall below 3.5 mmol/L; this is rarely due to sweat loss alone. And so the current best practice is to replace potassium at a similar rate to the average losses across exercise durations, where sodium replacement is also warranted.

Until more robust guidelines are available, replacing potassium lost in sweat is sensible beyond 2 hours when the small losses have time to accumulate. For the majority of recreational athletes exercising for less than 1 hour per day, specific potassium supplementation is probably not worth worrying too much about, as long as a balanced diet is maintained.

Magnesium

Magnesium is found mostly in the intracellular fluid and serves multiple functions in the body. It is involved in over 300 biochemical reactions as well as its role in muscle contraction and glucose metabolism, meaning it's very important for day-to-day activities and exercise. 

The recommended daily intake of magnesium is 310-420mg. Magnesium in the diet mainly comes from dark leafy vegetables, nuts, fruits and drinking water. 

Sweat losses of magnesium have not been studied widely but Baker and Wolfe, 2020 reported them to be in the range of 0.02-0.4 mmol/L which is equivalent to 0.49 - 9.7 mg/L. 

There is little to no experimental data that indicates magnesium lost through sweat incurs any negative performance or hydration status effects if dietary intakes are met. However, it is worth noting that athletes may experience increased urinary magnesium losses during periods of intense training, so if they have a restricted diet, they may fall short of the recommended daily intake, but this is very unlikely to affect exercise performance or recovery 

Calcium

Calcium is essential to maintain strong bones and teeth but it is also crucial for the proper functioning of nerves and muscle contraction. 

Calcium requirements differ between sexes and change as we age, and dietary intakes of dairy products and green leafy vegetables provide the majority of daily calcium for most folks. 

As little as 0.2-2 mmol of calcium is lost in each litre of sweat (8-80 mg/L), again not very much! Hew-Butler (2019) states that exercise-associated disturbances to calcium homeostasis are exceptionally rare in healthy individuals unless paired with another medical condition. It is unlikely that calcium lost in sweat needs to be replaced with supplements during exercise if the recommended daily intake is achieved. 

However, it is worth noting that for those fitting two sessions into a single day, there is emerging research suggesting that calcium intake may matter beyond just hitting your daily target. A 2023 study by Lundy et al, in elite male rowers found that consuming a calcium-rich meal (~1000mg) before each session stabilised blood calcium during exercise, reducing the hormonal response that drives bone breakdown. This is yet to be researched in a wider population of recreational athletes but could be a consideration, especially for those who have experienced bone-related issues such as stress fractures. 

What electrolytes are in Precision Hydration products?

Precision Fuel & Hydration products have been formulated to contain concentrations of the minor electrolytes similar to those of your sweat losses. We individualise the amount of sodium in each product relative to your losses, since the magnitude and cost of not replacing the right amount of sodium has a larger weight of supporting evidence. Whilst current evidence suggests it may not be essential to replace your minor electrolytes during exercise, there is no harm in your drinks containing them. Day to day, the focus should be on getting them from real foods. Including these electrolytes in our products provides a complete electrolyte profile that mirrors natural sweat composition to ensure your bases are covered. 

Our low-calorie effervescent tablets in the USA contain slightly different concentrations than the rest of the world, but our natural drink mixes are the same everywhere. Our Electrolyte Capsules contain 125mg of potassium per capsule as well as trace amounts of calcium and magnesium to support the 250mg of sodium in each capsule.  

Table 3. Minor Electrolyte Concentrations in PF&H Products. 

Conclusion

Whilst it’s interesting to look at what other electrolytes we lose in our sweat in real detail, let’s end by taking a step back and remembering the bigger picture.

Sodium is the electrolyte that really matters when it comes to staying hydrated. Current guidelines suggest that replacing potassium, magnesium and calcium during exercise is generally not required. Most healthy athletes needn't worry too much at all about looking for magnesium, potassium or calcium in their sports drinks, nor taking specific supplements.

Further Reading

• Why sodium is crucial to athletes performing at their best
• Different types of sports drink and when to use them
• How well do different drinks hydrate you?  
• Should you combine your carbohydrate and electrolyte drinks?