The human body consists of approximately 60% water, which is not evenly distributed between all tissue (Jeukendrup & Gleeson, 2018). For example, blood contains most fluid while fat is hardly hydrated at all. Body water can roughly be divided into two compartments: intracellular and extracellular fluid. About 2/3 of the total body water is present in the intracellular space. Most micro- and macronutrients are water soluble, and an important water-soluble group is minerals, also called electrolytes when dissolved in water. In this article, the interplay between water and electrolytes will be discussed as well as the function of water and electrolytes.
Minerals are naturally occurring inorganic solids present in small amounts in the diet. When dissolved in water, they either have a positive or negative charge, depending on the type of mineral, and are called electrolytes. Electrolytes can be acids, bases, or salts and occur in the body in extracellular or intracellular fluids. Electrolytes have multiple functions in the body due to the electrical charge and imbalance in electrolyte concentration disturbs homeostasis (Apostu, 2014). One of the main functions of electrolytes is regulating fluid levels. In the body, most membranes are semipermeable, meaning water can diffuse across the membrane but the molecules solved in water cannot without the use of transporters (Seifter, 2019). When a solution is divided into two compartments with a semipermeable membrane, like the intracellular and extracellular space, only water can move over the membrane to get an equilibrium between the two compartments. Not only concentration gradients influence the water balance but also hydrostatic pressure. Hydrostatic pressure is created by the difference in water volume percentages between cells and blood plasma (Darwish & Lui, 2019).
Besides regulating fluid levels, electrolytes are important in transmitting nerve signals, muscle contraction, regulating blood pH, and macronutrient absorption in the small intestine.
Sodium is a positively charged ion that is mainly present in the extracellular fluid, making sodium very important in regulating extracellular fluid volume. Extracellular fluid volume and blood pressure needs to be maintained for transport of substrates and excitability of nerve and muscle cells (Strazzullo & Leclercq, 2014). Sodium is together with mainly potassium and chloride to some extent, responsible for nerve conduction. Electrolyte flow from inside to outside the cell and reverse is needed transfer the signal over a nerve. Nerve conduction is not only important in the brain but is also needed for muscle cells to contract and relax (Castelfranco & Hartline, 2016). Furthermore, sodium plays a role in maintaining cellular homeostasis and water balance. A high sodium concentration increases water absorption in the gut and reabsorption in the kidney (Evans et al., 2017). There is a potential link between sodium loss and muscle cramp and thus it is important to for athletes suffering from cramps to consume enough salts. For physical activity shorter than three hours it is recommended to consume an isotonic drink (0.5-0.7 g/L Na+) (Orrù at al., 2018). Athletes exercising longer than three hours are recommended to include sodium, 0.7-1 g/L of water, in their drink to increase palatability, promote fluid retention and prevent hyponatremia (Jeukendrup & Gleeson, 2018). For ultra-endurance exercise it is recommended that the sodium concentration of beverages is around 1.7 to 2.9 g/L (Rehrer & Van Vliet, 2007).
Water is necessary for biochemical reactions, transport of nutrients, regulating body temperature, and for protection and provision of cellular structure (Jeukendrup & Gleeson, 2018; McArdle et al., 2016). Especially during exercise, regulation of body temperature is important. Intense exercise increases heat gain and to stay in heat balance, the body must increase heat loss. There are four mechanisms to lose heat: evaporation, conduction, convection and radiation, of which evaporation is the most adaptable. Evaporation is facilitated by sweat, thus an increase in sweat production during exercise helps to cool down the body. Environmental factors, like humidity, wind, solar radiation, and temperature affect the efficiency of the different heat loss mechanisms. For example, high humidity lowers the efficiency of evaporation, while no wind affects mainly conduction and convection. When heat loss mechanisms are insufficient to keep the core temperature low, hyperthermia and perhaps heat strokes may arise. Not only environmental factors affect heat loss mechanisms, also exercise intensity and duration, training status and insulation influence heat loss like sweat rates. For example, intense exercise for several days in the heat like a tournament, increases sodium concentration in sweat. Beside losing heat, sweating also causes loss of fluids and electrolytes. Replenishing fluids and electrolytes during exercise is important to prevent dehydration and its negative effect on exercise performance. Dehydration results in a decreased skin blood flow, sweating rate and thus heat loss. Research shows that dehydration symptoms like impaired alertness and exercise performance already occur when bodyweight has dropped 2% due to fluid losses (Baker et al., 2008; McArdle et al., 2016; O’Neal et al., 2011; S. M. Shirreffs, 2009). Loss of electrolytes can impair cell and organ functioning, reduce exercise performance, and cause muscle cramps (Academy of Nutrition and Dietetics et al., 2016; Coyle, 2004). Since the human body is not able to spontaneously compensate for the loss of electrolytes and fluid through sweat, this has to be ingested pre-, during and post-exercise. However, formulating a beverage that compensates all fluid and electrolytes is difficult due to several factors including interindividual differences in sweat rate, the effect of environmental factors, and exercise duration and intensity (O’Neal et al., 2011). Athletes can estimate their sweat rates by measuring body weight pre- and post-exercise. The difference in weight, corrected for water intake and output through urine, is due to loss of fluids during exercise.
Strategies to prevent dehydration can be applied pre-, during- and post-exercise. In all cases, a cold drink is preferred as it will also lower body core temperature (McArdle et al., 2016). Drinks can be divided into three categories based on their sugar and salt concentration: hypertonic, isotonic and hypotonic. Hypertonic drinks have a higher sugar and salt concentration than blood, making them more difficult to absorb. Isotonic drinks have similar concentrations as blood and are faster absorbed. Hypotonic drinks have a lower concentration than blood and are best absorbed. A downside is the overconsumption of drinks. Overconsumption of especially hypotonic drinks, increases the risk of hyponatraemia (Hew-Butler et al., 2017; Noakes & Speedy, 2006). Hyponatraemia is a state where the blood becomes too diluted (sodium < 135 mmol/L), causing neurological complications with symptoms similar to dehydration like headaches and dizziness (Dong & Wiltshire, 2019). Athletes might think they are dehydrated and start drinking more, which only increases the dilution of the blood. Therefore, isotonic and perhaps hypertonic drinks are preferred on the long run as they will restore electrolyte balance and prevent hyponatraemia. Especially, sodium and chloride ingestion are important because these are the main electrolytes lost through sweat.
Before, during and after
Baker and Jeukendrup (2014) published an extensive review on the optimal composition of fluid-replacement beverages pre-, during, and post-exercise. Pre-exercise drinks are mainly to prevent dehydration before the start of the exercise. Sodium addition to a pre-exercise drink shows increased plasma volume and exercise performance, especially in moderate-trained individuals but less in well-trained individuals (Coles & Luetkemeier, 2005; Greenleaf, Jackson, et al., 1998; Greenleaf, Looft-Wilson, et al., 1998; Greenleaf Robin Looft-Wilson & Wisherd M A McKenzie C D Jensen, 1997; Rehrer & Van Vliet, 2007; Sims et al., 2007). During exercise, Baker and Jeukendrup (2014) adopt the recommendations of The American College of Sports Medicine and the Institute of Medicine regarding sodium and potassium. Beverages should contain ±20-30 mmol/L sodium and ±2-5 mmol/L potassium to help replace losses and still be tasteful. However, they make a side note that higher sodium concentrations are better in compensating the losses. Other research showed no rehydration advantages with the addition of potassium to a during exercise drink containing sodium (Perez-Idarraga & Aragon-Vargas, 2014). Post-exercise drinks are used to replenish fluid loss, since only 50% of the fluid loss during exercise is replenished (McArdle et al., 2016). Fluid intake of 150% total fluid loss is needed to correct for urinary losses during the rehydration period (Baker & Jeukendrup, 2014). Sodium recommendations are similar to the during exercise drink to promote rapid fluid balance and restore electrolyte balance.
The perfect fluid and electrolyte replacement beverage depend on taste and physiological outcome.