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Chapter Goals:
- Know water’s structure and function for life and 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.
Introduction
Water is often neglected as far as nutrition goes.
Macronutrients are the nutrients that give 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
Water is going to be 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 can easily 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 when the molecules pass through a semipermeable membrane from an area of high concentration to a lower area.
The total volume of water in the human body is called total body water.
We then divide total body water into intracellular and extracellular water.
Urine production is how someone gets rid of excess fluids, and the sensation of thirst initiates water intake.
Hydration Status
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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 lab settings.
Urine specific gravity and urine color are some of the common techniques we see used that are less expensive.
The urine color 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 to balance fluids: one is influenced by the blood volume, and the blood plasma concentration influences the other.
Blood concentrations can regulate fluid balance, and this is known as osmolality.
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.
The nervous system closely monitors plasma osmolality 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 the relationship between water intake and urine production, there is a clear link between hydration and kidney health.
Gradual decreases show chronic kidney disease 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.
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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.
The gastric emptying rate influences water absorption.
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 may vary between diets as fruits and veggies have different water content than some food items.
Recommended Daily Water Intake
The National Academy of Medicine recommends adult men and women 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 at competition in a state of hypohydration.
During exercise and athletic events, many systems need proper fluid delivery through the circulatory system.
The four systems are:
- The brain needs the delivery of oxygen and metabolic substrates
- The heart and the lungs are for the 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 to meet 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 before exercise or some athletic event is to arrive at 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 calculating the difference, the weight of fluid and food intake and the weight lost from waste should be factored in.
We should avoid weight change during activity, even in hot environments.Â
Replacing fluid lost during exercise, or rehydrating, helps us reduce the recovery times and decrease fatigue after exercise ceases.
We must take in 150 percent of lost water 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 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.
Several risk factors contribute to the development of EAH, and one of these is drinking more fluid than that 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, performance enhancement, and effect on hydration levels. And this is important to consider.
Caffeine is a common supplement that greatly affects athletic performance, but it also is important to weigh out the poor effects on hydration levels.
Creatine is also used to affect hydration status and is a popular supplement among athletes.
Tyler Read
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