Body Water Basics
Daily Water Losses
Why is body water important?
We bath in it, swim in it and seek out vacation destinations based on its presence. Water is one of the most important aspects of our everyday life. Also, we thirst for water to maintain good hydration status for optimal health. In fact it is easy to argue that water is our most important nutrient. Each day we must match water intake with losses in order to risk dehydration.
How much of our body is water?
Water makes up about 60 percent of our total body weight, typically a little more for men and a little less for women. For instance, a 175-pound man might attribute more than 100 pounds of his weight to water. Roughly two-thirds of our body water is found within our cells as intracellular fluid, while the remaining one-third is extracellular fluid found bathing our cells. As mentioned earlier, extracellular fluid includes both the fluid between our cells and also the plasma portion of our blood.
When looking at certain body tissue, skeletal muscle is a little more than 70 percent water (by weight), while fat tissue is less than 10 percent water (see Figure 7.1). By and large, it is the ratio of skeletal muscle to fat tissue that has the greatest impact on the amount of water in the body. Because men tend to have a higher percentage of muscle and a lower percentage of fat compared to women, they tend to have a higher percentage of body water. However, regardless of gender, a lean muscular person will have a higher percentage of body water while a nonmuscular, overweight person will have a lower percentage of body water.
Why do we have so much water in our body?
Water is the most abundant substance in the body because it provides the medium or environment for the body. That means that all other substances within the body are either dissolved, suspended, and/or bathed within water. In general, substances such as carbohydrates, protein, and electrolytes dissolved well into body water. Meanwhile, lipids do not and the transport of lipid materials in our blood requires water soluble transporters such as proteins or lipoproteins. For instance, fat-soluble vitamin D hitches a ride upon a vitamin D binding protein (DBP), while sex hormones (i.e., estrogen, testosterone) can latch onto sex hormone binding protein (SHBP). In the meantime, fats and cholesterol are transported in lipoproteins, which are in essence submarines carrying lipid cargo.
How does water help us regulate our body temperature?
Water has the capability to absorb heat to keep us from overheating (hyperthermia) and release it at times overcooling (hypothermia). In comparison to other materials, water can absorb a lot of heat before its own temperature changes. This allows body water to absorb the heat generated during normal metabolism and during times of extra heat production such as exercise. Water then facilitates the removal of extra heat from our body by sweating (discussed below). On the other hand water can give up heat to help keep tissue warm when we are in cooler environments.
What other roles does water play?
Water also provides the basis for the lubricating substances found in our joints. This helps cushion the joint and reduce the physical stress and friction between the bones in the joint. Water is the basis of amniotic fluid that cushions and protects a fetus during pregnancy. In addition, of our urine, bile, saliva, mucus, lacrimal fluid (tears), and digestive secretions all are water based.
DAILY BODY WATER LOSSES
How much water do we lose daily?
Our body loses water constantly and through more than one route. In fact, no other essential nutrient is lost from the body by as many routes and at the same levels as water. Water is lost as urine and through breath as well as from skin surfaces as sweat. For a typical adult it is typical to lose as much as 2 to 3 liters daily. As one milliliter of water is the same as one gram of water, this equals 2 to 3 kilograms or 0.9 to 1.4 lb. This means we need to replace water at the same level as what is lost in order to prevent dehydration. This requirement is higher than all other essential nutrient requirements combined.
How much water is lost daily as urine?
Every day our kidneys process about 180 L (47.5 gals) of blood-derived fluid to regulate blood composition. Of the 180 L, more than 99 percent is returned to our blood, while the remaining 1 percent becomes urine. Dissolved in our urine will be waste products of our metabolism (e.g., urea) and other substances in excess of our needs (e.g., sodium, chloride). About 1 to 2 L (about 4 to 8 cups) of our body water is lost daily as urine. This quantity will change relative to our water consumption. For instance, people who drink a lot of water will tend to produce more urine daily and that urine will seem clearer (more dilute).
How do we know if we are not getting enough water?
People who aren’t getting enough water will void urine that is more concentrated with waste products and excess substances. For these reasons people sometimes look at the color of their urine to gauge their body water or “hydration” status. However, while this can certainly provide insight, food factors, such as the vitamin riboflavin, can darken the color of urine and/or alter its odor, such as with coffee. Researchers use more objective measures such as urine specific gravity to suggest hydration status.
How much water is lost in feces?
Water helps moisten feces for easier transit through and out of the colon. Typically, during normal bowel movements adults lose about 100 to 200 mL of water as part of feces daily. As you might expect, we would lose more water from our body via the feces during bouts of diarrhea. This also means that we need to drink more water as tolerated during, as well as after, these unpleasant episodes.
Do we lose body water when we breathe?
Water is also lost from our body through breathing. When we inhale, air moving through our air passageways (i.e., trachea and bronchi) becomes humidified. This means that we are adding moisture to it. Subsequently, when we exhale, much of the humidified air is lost to the outside environment. This is noticeable on a cold day as humidified exhaled air condenses to form little clouds. The amount of body water lost in this process is about 3 to 500 mL, depending on the humidity level of the air. For instance, in a dry environment, such as a desert climate or at higher altitudes, a little more of our body water is used to humidify the air we inhale. This in turn means that a little more water would be lost during exhalation. Conversely, breathing more humid air decreases the amount of water lost through our lungs.
Do we lose body water in sweat?
We sweat throughout the day to help remove extra body heat produced by normal cell operations, but most of time we do not even notice it because it is so minimal. For an adult this can add up to about ½ L or 2 cups (see Figure 7.2). However, when we exercise or find ourselves in a hot environment, sweating certainly becomes more obvious, especially if it is humid. Increased moisture in the air can hinder the evaporation process, allowing sweat to accumulate on our skin.
