ISSA Nutritionist Chapter 9: Water and Hydration 1

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    Chapter Goals:

    • Know the structure and function of water for life and in the human body.
    • Be able to explain the role of water in physiological processes.
    • Be able to describe how water is absorbed and excreted in the body.
    • Know the importance of water for athletic performance.


    Water is often neglected as far as nutrition goes.

    Macronutrients are the nutrients that are giving energy and calories to the body and are needed in large quantities for daily life, so in this sense water is not included.  

    Truthfully water should be a macronutrient, as the human body is 50 – 60 percent made up of water, and someone can only survive for 3 – 4 days without ingesting water.


    Water is going to be a liquid when at room temperature, and it has a chemical formula of H20, meaning that there are 2 hydrogen atoms, and they are bound to one oxygen atom.

    The chemical structure here means that the water molecules are polar, so one side has a slight positive charge and the other a slightly negative one.

    Water also has a high specific heat, so it is easily able to absorb and transport heat.

    Water is key to keeping homeostasis, which is the equilibrium that the body likes to stay.

    The transfer of fluids is driven through osmosis; this is when the molecules pass through a membrane that is semipermeable, from an area of high concentration to an area that is lower. 

    The total volume of water found in the human body is referred to as total body water.

    We then divide total body water into intracellular and extracellular water.

    Urine production is the means by which someone gets rid of excess fluids, and the sensation of thirst initiates intake of water. 

    Hydration Status

    Total body water describes the amount of water in the body at a point in time. 

    Hypohydration is a state of decreased total body water, euhydration is a state of simply adequate levels of water in the body, and hyperhydration is a state of excessive total body water regulated by the brain.

    Determining hydration status

    Nutrition coaches need to know the current techniques for measuring hydration. 

    One of the easiest and most common techniques is through weighing changes in body mass before and after exercise sessions to get an idea of the rate of fluid loss. The weight of all substances going in must also be tracked during this. Usually there are three consecutive days needed for accurate measures.

    Measuring the plasma and urine osmolality requires some relatively expensive materials used primarily in the lab settings. 

    Urine specific gravity and urine color are some of the common techniques we see used that are less expensive.

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    The color of the urine can range from clear, which means well hydrated, and all the way to severely dehydrated darker colors. 

    The Role of Water in the Body

    Water is involved in many processes of the body, like:

    • Cellular metabolism
    • Regulation of the body’s temperature
    • Solute balances
    • Transport of nutrients in the body
    • The clearance of cell waste
    • Maintenance of homeostasis in the body
    • Digestion
    • Balance of pH
    • Eliminating waste from the body
    • Delivery of oxygen

    The processes are all very crucial to overall survival and establishing water as one of the most important parts of life. 

    Physiology of Fluid Balance

    Two mechanisms are needed for the balance of fluids: one is influenced by the volume of blood, and the other is influenced by the blood plasma concentration.

    Fluid balance can be regulated by the blood concentrations, and this is known as osmolality. 

    In order to understand plasma osmolality, you can compare it to a swimming pool filled with saltwater. When more salt is added, the ratio of salt to water goes up. If freshwater is filtered from the pool, the ratio also increases.

    Plasma osmolality is closely monitored by the nervous system in order to maintain homeostasis. As the plasma osmolality goes up, the nervous system senses the change and triggers the release of the hormone arginine vasopressin.

    Glucose Regulation

    Non-modifiable risk factors like genetics, age, and sex, will all play a role in the development of type 2 diabetes, along with some other factors such as obesity, poor diet, and nonactivity in your lifestyle.

    Kidney Health

    Like with the relationship between water intake and the production of urine, there is a clear link with both hydration and kidney health.

    Chronic kidney disease is shown by gradual decreases in the function of the kidneys over a longer period of time and it is shown by measuring how well the kidneys filter out blood in the body.

    Studies often show that urine protein, kidney blood flow, and hyperfiltration increase in response to elevated AVP.

    Digestion and Absorption of Water

    Water is absorbed through the digestive tract but mainly in the small intestines. 

    The concept of osmosis explains how the water crosses the cell membrane of the microvilli and enters the cells and bloodstream.

    The last absorption of water takes place in the large intestines and can be compromised if the junctions between the microvilli are loose and allow water to pass back into the lumen of the intestines. 

    Water absorption is influenced by the gastric emptying rate.

    When the fluid is emptied from the stomach into the small intestines, it still needs to be taken in and absorbed.

    Dietary Sources of Water

    Most of the total water intake comes from ingesting drinking water, but other beverages count toward this, too, since they also can have significant amounts of water. 

    About 20 percent of total water intake comes from food products. This is possibly going to vary between different diets as fruits and veggies have different water content compared to some food items.

    Recommended Daily Water Intake

    The National Academy of Medicine recommends adult men and women to take in 3.7 and 2.7 liters per day, respectively. 

    This is less for the European food safety authority, which recommends 2.5 liters for men, and 2 liters for women. 

    Hydration and Athletes

    The consequences of low hydration levels can be seen in all systems of the body. many athletes routinely arrive to competition in a state of hypohydration. 

    During exercise and athletic events, there are many systems needing proper fluid delivery through the circulatory system. 

    The four systems are:

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    • The brain needs delivery of oxygen and metabolic substrates
    • The heart and the lugs for distribution of blood and exchange of gas
    • The skin for the removal of heat from the body
    • The working muscle for the delivery of fuel and oxygen

    When sweat levels are high, the result is decreased volume of blood, and this can work to compromise the physiological systems. The body will initially provide blood to the brain, heart, and lungs and reduce the amount sent to the surface of the skin and the working muscles. 

    Enhance Performance

    Research shows that total body water loss of 2 percent or more, will reduce overall performance. 

    In cardiovascular exercise, the system maintains cardiac output for the meeting of metabolic demands for the exercise. 

    For us to determine the cardiac output, we multiply the heart rate by the stroke volume.

    Hydration Strategies Before, During, and After Exercise

    The goal of hydration prior to exercise or some athletic event is to arrive to a euhydrated state, but many of the athletes arrive in a hypohydrated state. 

    Some steps for measuring the amount of fluid needed for the exercise or athletic performance are:

    • Measure the body nude and in a state of euhydration
    • Carefully tracking the weight of all fluid and food intake during activity
    • Weighing any urine and fecal matter produced during the activity if needed
    • Removing any remaining sweat from the skin surface after exercise
    • Using the same scale, obtaining a second nude body weight

    We should first obtain the baseline body weight and post activity body weight should be obtained. After we calculate the difference, the weight of fluid and food intake and the weight lost from waste should be factored in.

    We should aim to have no change in weight at all during activity, even in the hot environments. 

    Replacing fluid lost during exercise, or rehydrating, helps us to reduce the recovery times as well as decrease the fatigue present after exercise ceases.

    It is important that we take in 150 percent of lost water in order to return to our pre-exercise levels. This is especially true when we look at the need for exercising in the next 24 hours after a session.

    Practicing Safe Hydration During Exercise

    Thermoregulation is how organisms control their core temperature.

    The cardiovascular system is the primary system responsible for this regulation during exercise. 

    Another consideration to take into account for exercise is the exercise-associated hyponatremia. This happens when the plasma sodium levels fall below a normal range and start to affect performance in various ways.

    In severe EAH, the movement of fluid into the cells substantially increases, and this causes expansion.

    There are several risk factors that contribute to the development of EAH, and one of these is drinking large fluid amounts than what is lost from your sweat and urine when exercising.

    To avoid EAH, there are several strategies to implement, like:

    • Develop a plan of hydration during prolonged endurance events.
    • Heat acclimatization is shown to decrease sweat levels.
    • Increasing the amount of dietary sodium before and during events can reduce or delay.

    Performance Supplementation and Hydration

    Some supplements are taken for their activity and performance enhancement and their effect on hydration levels. And this is important to consider.

    Caffein is a common supplement that has great effects on athletic performance, but it also is important to weigh out the poor effects on hydration levels. 

    Creatine is used to affect hydration status, too, and it is a popular supplement among athletes.

    ISSA Nutritionist Chapter 9: Water and Hydration 2
    ISSA Nutritionist Chapter 9: Water and Hydration 3
    ISSA Nutritionist Chapter 9: Water and Hydration 4

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