NASM CNC Chapter 11: Hydration

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    Chapter Goals:

    • Discuss the aspects of water in the body after significant losses in sweat.
    • Discuss the methods through which we can decrease hyper and hypo hydration for people.
    • Find the strengths and weaknesses for typical status indicators.
    • Discuss any myths and hot topics for hydration.
    • Be able to talk about the proper hydration strategies we should use during and between the bouts of training.


    Water is a larger percent of the body than any other substance. It helps to fill in spaces outside and inside the cells, and all of the vessels of the body. We see continuous formation and breakdown of water in all of the major metabolic pathways. The opinions on how much water should be taken in vary quite a bit between all of the trainers, but advice can be great for performance improvement and just general heath. 

    In many cases clients will not need any advice regarding hydration since there is sometimes a low volume of sweat loss in typical sessions. 

    The Physiology of Hydration

    Water is equal to around 50 – 70 percent of our entire body mass. And the total water we have will fluctuate, but not by much per day. About two thirds of our water is located within the cells, and this is called intracellular fluid. The other remaining fluid is the extracellular fluid that is found outside of the cells.

    Sodium and potassium are two electrolytes that have the most influence on where water is in the body. 

    Osmolality is a description of the concentration of solute within some liquid. Many places hold fluid, but the main hydration status regulator is the osmolality of ECF, and even more specifically it is the plasma osmolality. Many sites in the body have osmoreceptors that find shifts in osmolality and give off responses in return to reach homeostasis.

    Dipsogenic signal means the physiological factors like low blood pressure and osmolality which prompts water and sodium appetite.

    Baroreceptors are nerves that are specialized and able to sense changes in pressure in the heart or other blood vessels.

    Hypo-osmotic means that a solution has less of solute concentrations when compared to another. 

    The gastric emptying rate is the GER. This is a description of the rate at which the fluids and their other contents pass through the stomach through the sphincter and then to the small intestine. The quicker that the GER is, the faster we see electrolytes and the carbs enter into the mesentery system.

    Hydration and Heat Illness

    The regulation of our body temperature when exercising is going to be influenced somewhat by the total amount of water we have in our body. Hydration status is vital to understand since it can determine if someone may experience a heat related illness or heat stroke even.

    Many people simply assume that hydration alone is the key to the prevention of these heat illnesses. The real best defense is abstention from overexerting yourself in hot environments. 

    Increases in the amount of total water will reduce our generation of heat that happens during strain from cardiovascular exercise. This is because it increases the venous return. 


    There are many elements used to determine the rate of sweat evaporation. Our first factor is heat. The higher the temperature is, the higher the sweat evaporation rate. Humidity is seen as the second factor. Environments that have lower levels of humidity will increase the rate of evaporation of sweat. This is often as much as being unable to feel yourself sweating in the hot and more arid regions when it is windy. Convection is a third factor here. This is the movement of air over an object, hence the windy days playing a part.

    We define a normal hydration status as being euhydrated.

    How Does Hydration Influence Performance?

    There is a major influence from your level of hydration and the exercise you do resulting in heart rate increases or decreases. Th study had 10 guys doing 90 minutes of high intensity walking in a hot environment.

    • Normal hydration and having water during exercise sees a heart rate on average of 129 and a rectal temp of 38 degrees.
    • Normal hydration status along with no water taken in during exercise will see a heart rate of 133 on average and a rectal temperature of 38.1 degrees.
    • Dehydration status along with water taken in during exercise saw a 128 bpm heart rate average and a rectal temperature of 38.2 degrees.
    • Dehydration status and no water taken in at all during exercise saw an average heart rate of 152 and a rectal temperature of a high 39.1 degrees.

    So, the previous information clearly shows there is some significant influence that you hydration status has on both body temperature and heart rate, which when these both increase, they will have a significant impact on the effectiveness of your body in physical activity.

    Defining Hydration Status

    The actual status of dehydration is most of the time caused by excessive amounts of sweat and/or a lack of intake in fluids. It may also be the result of an illness of some kind. 

    We do not define our status of hydration clinically by the total body water that we have, instead we use our plasma osmolality or sodium concentrations to determine these. 

    Developing a Hydration Strategy

    This is going to be a three step process for the development of an individualized hydration plan for clients. This has us doing a sweat loss assessment, followed by finding if the client is training while euhydrated. Once we find those two things, we are able to prescribe the appropriate amount of fluid needed by the client. 

    Sweat Loss Assessment

    This is the first step that happens when developing this hydration strategy for clients. The questions that we are going to consider here, will be:

    • How do you know the amount of fluid deficit that is going to occur?
    • How can you determine the level of fluid replacement that inhibits performance decrements?

    So, the calculation of sweat lost during exercise usually looks like this:

    • Your initial weight in kilograms – your post-exercise weight in kilograms + the fluid consumption between your weigh-ins in kilograms – the amount of urine excreted in kilograms. This all comes together to equal the amount of sweat volume in kilogram/liters. 

    The overall recommendation is for the sweat loss to be less than 2% of your actual body mass. After this number, we see significant effects on performance during activity. This may be the typical view of the sports community, but it is still in high debate if this is an actual valid indicator of performance impairment. It is, however, a simple basis to determine if your activity is impaired.

    Exercise associated hyponatremia, also known as EAH, is a possibly very serios medical condition in which the sodium in plasma levels fall well below 135 mmol/L. this is almost always the result of too much sweat lost during long exercise sessions. 

    Pre-Exercise Hydration Status Assessment

    This is going to have major implications on the hydration recommendations for an athlete. It is a complicated process and it also has weaknesses to look out for. It is not very practical for us to be able to measure the plasma osmolality in a setting that in non-clinical. Thirst sensation on the other hand is something that is measured more easil. Other popular ways for measuring include things like urinalysis and measuring the body mass changes at various times. 

    Thirst and Body Mass as Indicators of Hydration Status

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    Athletes should record their morning body weight after making voids over the course of three days, and then use a 1% decrease from their three day average as a marker for hypohydration. 

    Like with thirst, a time of great losses in sweat followed by restrictive fluid intake when recovering will see huge differences in body mass.

    Urinalysis as indicator of Hydration Status

    The color of your urine is a good gauge to look at hydration status. It is not an exact science of course, but the color is somewhat reliable for seeing exactly how hydrated you are. The more yellow and dark, the less hydrated. The clearer the urine, the more hydrated you are going to be. 

    Urine Specific Gravity is a measurement that represents the ratio of solutes in the urine vs. in distilled water.

    There is a strong relationship between the fluid replacement percent and the USG, and this relationship sees improvements in the runners that lose higher percentages of sweat.

    Fluid Intake During and Between Training

    Before actually giving this advice, it is always important to think of the question of if this advice is even needed. As we stated early in the chapter, sometimes there is no need at all for the average person to change their fluid intake, because they are possibly doing just fine and it would be one more thing to worry about. 

    But, when it is decided that you should bring it up and make a plan, then these are going to be the guidelines to follow and give. 

    Fluid Intake During Exercise

    When working with athletes, the difference in team sports and endurance sports is the fact that the team sports will almost always have a period of rest for the athletes to take in fluids. Here, they can do so without any difficulty. The endurance athletes, there are option for them to plan out their opportunities to drink prior to the event. So, this means you have to have enough knowledge of the event and where everything is located during it. This should be something that is specifically practiced in order to make this process quicker and easier.

    Fluid Intake During Recovery

    Recovery hydration is the term that is used for the fluid and electrolyte intake taking place between two training sessions or competitions. The key for this recovery hydration is for the hydration to replace the sweat losses and then maximize the retention of said fluids prior to entering your next session. 

    If the recovery period is going to be less than 12 total hours, then it is important for the athletes to take in 150% of their sweat, since on average we see one third of the fluid intake lost through urine. 

    We also should watch the actual percentage of body mass lost via sweat and when that loss is over 3% of body mass and the next session is in less than 12 hours, they should just aim for 110 – 120% of their sweat loss taken back in. there should be a focus on their electrolytes also. 

    NASM CNC Chapter 11: Hydration 1
    NASM CNC Chapter 11: Hydration 2
    NASM CNC Chapter 11: Hydration 3

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