Electrolytes - what are they and how do they contribute to proper body hydration?

Hydration is essential for maintaining bodily functions and athletic performance. Dehydration, a decrease in body water content, impairs physical performance, increases body temperature, and can lead to heat stress. Body water makes up approximately 60% of the weight of an adult male and 50% of the weight of an adult female. Muscle tissue contains about 75% water, while fat tissue contains only 5%.

Fluid loss occurs primarily through sweat, urine, and breathing. During physical activity, 0.5–2.0 l of fluid can be lost per hour, depending on intensity and ambient temperature. Along with water, the body loses electrolytes, primarily sodium and chloride, and to a lesser extent potassium, calcium, and magnesium.

What are electrolytes?

Electrolytes are minerals that dissociate in water into ions carrying an electrical charge. They enable the transmission of electrical signals in the body and are involved in maintaining water and acid-base balance.

Sodium and chloride are lost in the largest amounts through sweat. Electrolyte losses vary according to the intensity of sweating and individual variability. The average concentration of sodium in sweat is 35 – 60 mmol/l.

The role of electrolytes in hydration

Electrolytes help maintain fluid volume, acid-base balance, nerve impulse transmission, and muscle contraction. Sodium is the major cation of extracellular fluid and is essential for maintaining plasma volume and water reabsorption in the kidneys.

Sodium-containing beverages promote drinking (increase palatability), improve water retention, and reduce the risk of excessive fluid loss.

If only sodium-free water is consumed during exercise, a decrease in plasma sodium concentration – hyponatremia (< 135 mmol/l) may occur. This condition can occur especially in endurance athletes with prolonged intake of water without minerals.

Electrolyte losses through sweat

Sweating is the main mechanism of heat dissipation. Each liter of sweat removes about 2.4 MJ (573 kcal) of heat from the body.

Sweat loss among FC Barcelona players:

• in cold conditions (20 °C, 52% humidity): 0.55 – 0.82 l/h,

• in the heat (29 °C, 46% humidity): 1.43 – 1.81 l/h.

Sweat sodium ranged between 22 – 61 mmol/l, total sodium loss 0.9 – 3.4 g/h.

Electrolyte replenishment during exercise

During prolonged physical activity, the goal is to limit body weight loss to less than 2%.
According to recommendations in Sport Nutrition and ACSM, it is advisable to consume 200-300 ml of fluids every 10-20 minutes during exercise, the amount depending on the intensity and environmental temperature.

Composition of drinks consumed during performance

Sports drinks should contain:

• An ideal drink during exercise contains 20–60 mmol/l sodium (460–1380 mg/l) and 20–80 g carbohydrates/l (4–8% solution).

• Osmolality should be 200–350 mOsm/kg – an isotonic or slightly hypotonic solution ensures rapid absorption.

• The mixture of glucose and maltodextrin increases the rate of carbohydrate utilization without overloading the gastrointestinal tract.

This composition ensures the maintenance of plasma volume, slows down the decrease in body temperature and improves endurance performance.

Other electrolytes: potassium, magnesium, and calcium

In addition to sodium and chloride , potassium, magnesium, and calcium , which are found predominantly inside cells , also contribute to fluid balance.
Potassium (K⁺) is the main intracellular cation (≈150 mmol/l), ensuring the transmission of nerve impulses and muscle contraction. Losses through sweat are small (4–8 mmol/l) and are easily covered by the normal diet.
Magnesium (Mg²⁺) acts as a cofactor for many enzymes and is involved in energy production and neuromuscular activity; it is found in sweat only in small amounts (≈1 mmol/l).
Calcium (Ca²⁺) is essential for muscle contraction, nerve transmission and blood clotting; its losses through sweat are negligible (≈0.3 mmol/l).

Overall, losses of these minerals during exercise are low and their replenishment is not as critical as in the case of sodium.

Optimal electrolyte intake

According to the chapter, the main goal during exercise is to maintain plasma osmolarity and sodium stability .
Sodium helps retain water in the extracellular space and prevents excessive urine excretion.
Sodium-containing beverages also promote the desire to drink and voluntary fluid intake.

Sodium supplementation during exercise is not necessary for short activities (< 1 hour), but is necessary for endurance events lasting more than 2 hours and in hot environments.
With sweat losses above 1.8 l/h, the drink should be richer in sodium (> 40 mmol/l) to prevent a decrease in plasma concentration.

Factors affecting sodium needs

• Exercise duration > 4 h → need to replace more than 70% of sweat losses.

• Sweating rate > 1.8 l/h → higher sodium intake recommended.

• Sweat sodium concentration > 60 mmol/l → replace 60–90% of sodium losses with a drink.

The goal is to keep plasma sodium concentration stable and prevent hyponatremia, not to replace all losses during the exercise itself.

Practical principles of drinking during exercise

• In case of high sweat loss > fluid intake → support fluid intake .

• If sweat loss is lower than intake → limit excessive drinking to avoid hyponatremia.

• Fluids should be cool (15–22°C), palatable, and served in small volumes at short intervals.

• Each athlete should have an individual strategy based on their own sweat rate and fluid tolerance.
  

Electrolyte replenishment after exercise

For full rehydration after exercise, it is necessary to replace 1.5 liters of fluids for every kg of body weight lost.
The drink should contain 20–30 mmol/l sodium (500–700 mg/l) , which promotes fluid retention and rapid restoration of balance.
Drinking only water after exercise leads to lower fluid retention and slower rehydration.

Resources:

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