What is Hydration and Why is it SO Important

What is Hydration and Why is it SO Important

The Importance of Hydration & Electrolytes

Background: Hydration status as a whole is very important for health. Water is responsible for regulating blood pressure, body temperature, movement of nutrients across membranes, and maintaining proper fluid balance in our cells, plasma, and outside of our cells. Electrolytes certainly have their place when speaking about hydration as they can aid in the overall hydration status of an individual. Electrolytes are responsible for the generation of action potentials that lead to muscular contractions and maintaining fluid balance within and outside of cells. With the many roles of water and electrolytes in our body, it is especially important that athletes maintain optimal hydration status due to the various demands that athletes put on their body during training and competition. In this review, the aim is to explore the many roles various hydration status’ have on electrolyte levels and how it may impact performance for the combat sport athletes. We will also discuss practical recommendations on how one may monitor hydration + electrolyte status, and how to rehydrate optimally to negate any performance decrements that may come with decreased hydration. 



Euhydration is a state where total body water is fluctuating in a normal range. There is no set point of euhydration, rather it is a dynamic process (Greenleaf, 1992).


Hyperhydration is a state where total body water is in excess with expanded intracellular and extracellular fluid volumes.


Dehydration is a state where total body water is in a deficit with constricted intracellular and extracellular fluid volumes.


Rehydration can be defined as bringing total water balance from a deficit back into normal euhydration ranges.


Electrolytes are defined as minerals that are vital for basic life. Minerals such as sodium, chloride, potassium, calcium, magnesium, and phosphorus. 


Hyponatremia is defined as having too little sodium present in the blood.  


The human body is made up of ~60% water and is tightly regulated through two compartments in the body. We have our intracellular fluid (found inside the cell), and extracellular fluid (found outside the cell), which is made up of our plasma volume, and interstitial space. We can see normal fluctuations between 1% hyperhydration and 3% hypohydration throughout the day.  When an athlete moves out of these normal fluctuations is when we can see adverse reactions to health and most importantly for this review, performance. To determine if an athlete has moved too far in one direction, hydration status must be assessed. This comes with its own challenges. Assessing hydration status as a whole can be quite difficult. Sure, there are devices and tests that can measure one’s hydration status, but the result of these tests only gives us a piece of the overall puzzle. With that said, it can be difficult to actually say that an athlete is indeed hydrated. These tests can give us some indication of the athlete’s status and we can provide recommendations to achieve a better hydration status, but to say we have a definitive status is unlikely. Furthermore, in most of the literature, when researchers dehydrate an athlete, they are not blinding the athletes, and the methods used to induce hypohydration are uncomfortable and unfamiliar to the athletes, which makes it hard to draw robust conclusions (Lewis et al., 2019). What is clear, is that many athletes enter into training in a dehydrated state and that BML of ~>2% are consistent with performance decrements (McDermott et al., 2017)


When exercising, the bodies core temperature increases, and leads to sweating, which acts as a way to regulate body temperature. When fluid intake is insufficient to sweat loss, hypohydration occurs. Hypohydration induced by exercise is hypotonic, meaning we are excreting more water than we are solutes. This decreases our plasma/blood volume and increases our extracellular osmolality. To make up for this shift, there is a concentration gradient which moves water from inside the cell to outside the cell, causing intracellular dehydration. Exercise performance is directly affected by this cascade of events by reducing muscle and cerebral blood flow, an increase in body temperature, heart rate, cardiovascular strain, muscle glycogenolysis (breaking down muscle glycogen to glucose) and thirst (Lewis et al., 2019). 

There are many factors that go into an athlete sweat rate, things such as body weight, intensity and duration of exercise, equipment worn and the environment they are training in. When training in a warmer climate, and when intensity of exercise is high and duration is long, we can see an increase in the athletes sweat rate, as well as a greater degree of dehydration. As you can see, sweat rates are highly dependent on the athlete and the conditions they are training in. Average sweat rates are between 1-2L/hr and can be up to 3-4L/hr. This is important to know, so that we can provide recommendation on how to rehydrate during training, if available, or post training, so that the athlete does not enter into a second training session already dehydrated. 

It is also important to talk about electrolytes when we discuss sweating. Sweat is made up of more than just water. Sodium, of all the electrolytes is most notably found in sweat concentration. The composition of sweat is influenced by many factors such as diet, sweat rate, and acclimation status (Maughan et al., 2010). With the link between sodium loss and cramping, it is in the athlete’s best interest to replace sodium loss post exercise. It is difficult to measure sodium losses in athletes as they can either have large sweat rates which translate to large sodium loss, or have a low sweat rate with a higher concentration of sodium. There are several ways to measure sodium concentration, but the most practical measure outside of a facility, is to wear a black t-shirt during training and look for salt stains where sweat has evaporated (Maughan et al., 2010).

Assessing pre-exercise hydration status

There are several ways to assess hydration status. Some common ones you will see in research are Urine Specific Gravity and plasma osmolality. They both have their benefits and their drawbacks. The obvious drawbacks are the instruments needed to test which can be quite difficult if you don’t have access to such equipment. Urine specific gravity measures the number of particles in an individual’s urine compared to the amount of water. A normal, or euhydrated measure is between 1.010 to 1.020. Plasma osmolality measures the number of particles in blood (sodium, chloride, potassium, bicarbonate, urea, glucose, and various proteins) compared to total volume. Both of these tests’ outcomes can be influenced by diet, posture, fluid and food intake as well as a number of other factors, and therefore neither are a good indicator of hydration status (Armstrong et al., 1994). Evidence of this comes from a study where 6 out of 39 individuals’ plasma osmolarity decreased after they had lost 3-8% body mass (Sawka et al., 1996). Additionally, in a different study, men and women drank 500-ml bolus of fluid acutely exhibited an increased plasma osmolarity, which is the opposite of the expected outcome (Sollanek et al., 2004). Lastly, as previously stated, exercise can have an impact on hydration status. More specifically, exercise induced dehydration compared to passive dehydration seems to have a larger impact on plasma osmolarity. This was seen in Munoz et al., where plasma osmolarity changes were twice as large during mild cycling compared to passive exposure. The authors hypothesized it was likely attributed to changes in concentration gradients which pulled water into the working muscles. It is for these reasons that assessing hydration by way of plasma osmolarity and Urine specific gravity are not the most practical in certain settings and not the most reliable to predict hydration status in athletes. 

A more practical way of monitoring an athlete’s hydration status is to track body mass changes and urine concentration/color or thirst. To practically track hydration status, it is best to measure body mass in the morning combined with some measure of urine concentration during the first void in the morning. The use of this technique implies that 1g of lost mass is equal to one ml of water lost (Cheuvront et al., 2004). As long as post exercise fluid loss is accounted for and replaced, body mass tends to remain stable daily over a time from of 1-2 weeks with regular exercise (Cheuvront & Sawka, 2006). If the athlete is in a weight reduction period, where body mass reduction is intended, it is best to track another hydration method such as urine color to dissociate between tissue loss from water loss. 

When examining urine, fluctuations in urine output can vary based on sweat losses and water intake. When sweat losses are high and water intake is normal or high, reduced urine production can be observed, with that comes a darker more concentrated urine. Consistent low daily urine output and darkening of urine color in a sample taken in the morning me be an indication of dehydration (Cheuvront & Sawka, 2006).

Lastly thirst can be a tool to assess hydration. Saliva can also be widely variable and therefore cannot be used as a single assessment to determine hydration status. What thirst can provide, is that the athlete needs fluid, and a more structures drinking schedule is needed pre, intra, and post training. 

Outside of laboratory techniques to assess hydration, it is best to use two of the three techniques above to attain a better idea of the athlete’s hydration status. 

Dehydration and Performance

From the available research it clearly shows that when hypohydration met or exceeded 2%, aerobic exercise performance reduced (Mcdermott et al., 2017). This is likely due to the decreased availability of fluid to deliver nutrients and oxygen to working muscles, central nervous and cardiovascular systems. These physiological alterations can be heightened in warmer and temperate conditions. Core body temperature increases a long with reductions in stroke volume, and peripheral sweat gland perfusion all contribute to performance decrements. 

While studies show that a body mass loss of 2% impairs performance, there is a large inter-individual variability that needs to be considered. James et al., 2019, reported that hypohydration ranging from 1.5%-19.2% saw deleterious effects to performance. For this reason, it is best to conduct a more individualized approach to the athlete’s hydration, specifically to how much fluid loss is practical and physiologically possible to replace given the athletes athletic setting. 

The position statement from the National Athletic Association concluded that strength, power and anaerobic performace decrements were also seen at hypohydration levels of atleast 2% (Mcdermott et al., 2019). 

Previous research when looking at exercise performance and dehydration have implemented atypical methods of subjects’ normal behavior and produce uncomfortable symptoms/side effects. Some of these techniques include prolonged passive fluid restriction, exercise-induced dehydration combines with fluid restriction during or after exercise, or diuretic drug administration. These techniques can lead to headaches, increased sensations of pain, and increased thirst, all of which can contribute to the deleterious effects of dehydration on performance (James et. al., 2019). Therefore, it can be likely that some of the impairments in performance were related to the above factors and not the level of dehydration that was achieved. 

Lastly, there may be a benefit from training in conditions of slight dehydration related to competition setting to attenuate some of the negative adaptations to exercise performance related to dehydration. 


Most athletes finish exercising with a fluid deficit and may need to restore euhydration during the recovery period ( Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance 2016). Rehydrating after exercise is very much a tailored process as sweat rates and exercise duration has an impact on how much fluid has been loss. The Position Statement also suggested that in addition to water, electrolytes, mainly sodium, should not be avoided in the post exercise hydration period. They also suggested that 125%-150% of body mass loss during training should be made up in the form of fluids. This translates to 1.25-1.50L per kg lost during training. The athlete or coach needs to account for any water consumed during training to represent accurate fluid loss. 

Athletes are cautioned to not over consume fluids to the point where body mass increases. The reason for this is that we don’t want the athlete to further