What is Hydration testing?
Hydration testing is a protocol used to determine an athlete’s body fluid balance. An athlete with a normal body fluid balance is said to be euhydrated . This euhydrated status is not a specific point, but rather a state of normal body water. Disturbances to an athlete’s body fluid balance can cause severe performance and health defects [3, 4] hence why hydration testing can be a useful component of athletic programmes. When the athlete is in their euhydrated state, they are more likely to be able to perform at their full capacity than if they are dehydrated .
Many hydration testing methods have been developed and implemented, the most common of which include:
- Body Weight Changes
- Bioelectrical Impedance Analysis
- Urine Specific Gravity
- Urinary indices
- Blood Indices
Whilst there are numerous methods for assessing the hydration status of athletes, each of them have varying levels of validity, reliability and practicality.
Why is Hydration testing important?
It is important to maintain the body’s state of homeostasis as much as possible before, during and after exercise, as this helps to ensure the athlete can perform at their very best and recover adequately. In terms of fluid balance, the goal prior to exercise is to have the body in its euhydrated state . Pre-performance (e.g. training or competition) hydration testing can, therefore, determine if the athlete is in a sufficient state of hydration, and thus ready to participate at their absolute optimum.
Dehydration and Performance
During exercise, a 2% or higher reduction in an athlete’s body weight has been shown to decrease both aerobic and cognitive performance [8, 9].
There is evidence to suggest that an increase in an athlete’s core temperature can decrease their motor-neural output during exercise. This can have a negative effect on their exercise tolerance time and their drive to exercise in the heat due to serotonergic mechanisms .
As mentioned before, dehydration can affect aerobic performance. This is because body water deficits can result in an increase in cardiovascular strain due to an increased heart rate and decreased stroke volume; ultimately resulting in a decrease in cardiac output. Dehydration can also lower cardiac filling due to a reduction in blood volume, this is often accompanied by a rise in skin blood flow and skin compliance .
There have been mixed results when reporting the relationship between dehydration and muscular strength and power. Whilst there have been reports that dehydration does affect muscular function [11, 12), there are also contradicting results [13, 14). This suggests that this is an area for future research in order to determine if there is a relationship between dehydration and muscle functioning.
Dehydration and Injury
The previous section explained how dehydration can affect the central nervous system, effectively causing fatigue. However, there are other health risks that dehydration poses to an athlete, such as cramps, hyponatraemia and heat stroke (this will be covered in the next section).
Despite the extremely high prevalence of exercise-associated muscle cramps (EAMC), the aetiology of this condition is not well understood . There are 3 hypotheses to determine the aetiology of EAMC:
- Dehydration hypothesis
- Electrolyte depletion hypothesis
- Altered neuromuscular hypothesis
Whilst there is not an overwhelming amount of scientific evidence for either of the three  it seems as though the altered neuromuscular hypothesis has the more likely pathophysiological mechanism for EAMC. Basically, exercise will cause muscle fatigue which will, in turn, increase excitatory afferent activity and decrease inhibitory afferent activity. This leads to altered neuromuscular control which then affects alpha motor neuron activity leading to EAMC. Although it is important to recognise the number of factors that will determine the severity (e.g. exercise intensity, duration, conditioning level) .
Prolonged exercise (>5hrs) can cause hyponatraemia, which is defined as having a diluted blood sodium concentration in the body . Hyponatraemia is caused when an individual consumes fluids low in sodium or sodium-free water and does not replace the necessary electrolytes lost . As a result, marathon runners seem to be prone to hyponatraemia when they do not implement effective hydration strategies . There also seems to be a relationship between the severity of the symptoms (e.g. nausea, headache, confusion and fatigue) and the amount by which blood sodium concentration have fallen .
Dehydration and Death
As previously touched upon, there can be serious health risks for individuals who exercise whilst dehydrated. In some extreme cases, there have even been several reports of death due to heat stroke, with dehydration said to be a contributing factor .
Heat stroke is diagnosed when the following criteria are met :
- Body temperature reaches 40.5°C or greater
- Presence of cognitive function impairment
- Anidrosis (absence of sweating)
To prevent this from occurring, there are natural neurophysiological processes in place to maintain homeostasis. Thermoregulation is the process in which heat is dissipated from the body in order to maintain an optimal core temperature (37°C) . A rise in just 1° can activate the body’s thermoregulatory centre, which can then act to reduce the body temperature through several mechanisms ; one of which being perspiration (i.e. sweating).
It is proposed that dehydration reduces cardiovascular function, which, therefore leads to a decrease in skin blood flow and the inability to lose heat via sweat [8, 21, 22).
To summarise, given the impact hydration can have not only upon performance but also on health, this firmly demonstrates the importance of having some form of hydration strategy in place to ensure athletes are well informed and monitored.