DAILY BODY WATER LOSSES
How much water do we lose daily?
Our body loses water constantly and through more than one route (Figure 7.2). 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 pounds. 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.
Figure 7.2. Our sweat glands begin deep in our skin and they ooze sweat when they are stimulated by our brain when our body temperature rises and by circulating epinephrine during exercise.
How much water is lost daily as urine?
Every day our kidneys process about 180 liters (47.5 gals) of blood-derived fluid to regulate blood composition. Of the 180 liters, 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 liters (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 milliters 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 300 to 500 milliliters, 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 liter or two 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.