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Jayne has taught health/nutrition and education at the college level and has a master's degree in education.
Imagine you've entered a contest for the latest technology gizmo. The contest requires you to drink the most amount of water within a short period of time. Or, perhaps you are pledging to a fraternity and one of the hazing rituals is drinking as quickly as possible in a short amount of time. Drinking large quantities of water in a short period of time alters the 'water balance' in the body, resulting in 'water intoxication,' which can be fatal. Both too much water intake and too little water can affect the fluid water balance in our bodies.
What does water balance mean? It means the amount of water consumed through food and beverages is equal to the amount of water excreted. We need to keep the total amount of water in our bodies constant, though. This is referred to as water homeostasis. The amount of water we lose should equal the amount of water we gain. As long as everything stays balanced, the osmotic concentrations, or balancing of sodium with that of water, remain the same in our body fluids. The least tiny change in water - too much or too little, and the 'osmotic concentrations' change.
For example, if we become 'dehydrated,' we lose more water than sodium, so the osmotic concentration becomes higher. In this case, the body would conserve water, rather than sodium, in order to bring everything back into balance again. Or, if we drink more than usual, the body would conserve sodium instead of water to maintain balance. We take in water through the food (roughly 30%) and beverages (roughly 60%) we consume, and our bodies even make a small amount as a result of metabolism (roughly 10%).
We lose 6% of our body fluids from sweating, 6% in feces, and 28% by evaporation from our skin and breath. About 60% of the water we lose is lost through urination. Without any extra activity, illness, or living in a hot environment, we need approximately 2.5 liters or a little over half a gallon of fluid just to replace what we lose on a daily basis.
Our thirst center detects when we are low on water, and triggers the thirst response. This center is located in the hypothalamus. A mere 1% change in total body is all that is needed to activate the thirst center. The thirst center is less responsive in older adults, so they need to be extra careful to consume enough water.
The RDI, or recommended daily intake, for water in the U.S. is 3.7 liters per day for males and 2.7 liters per day for females. The idea that we should drink 'eight glasses of water per day' has no solid scientific backing. Your thirst is a better indicator of water need than a fixed number. If you increase your water intake, your kidneys will take some time to adjust to the new intake level, or vice versa.
There has been some controversy over whether or not caffeine in the amounts of a cup of coffee or tea, or even soft drinks causes a dehydrating effect. Studies have now shown that this amount of caffeine does not cause dehydration, and these beverages can contribute to our water requirements. Drinks that contain 10% alcohol or more, such as most wines, do cause dehydration, so they do not contribute to our water requirements.
Pure water and beverages containing water, such as fruit juice, soft drinks, tea, and coffee, all count towards meeting our water requirements. However, it is a good idea to restrict beverages with excess sugar because they can lead to weight gain.
Our kidneys regulate fluid output. Blood comes to the kidneys to be filtered or cleaned and then returns to circulating around the body. The molecules removed during the filtering process are water, sodium, glucose, and urea , which is a waste product from the breakdown of protein. All the glucose is reabsorbed - our bodies use it for energy. The body will reabsorb as much of the water and sodium as it needs. The body has no use for urea, which along with the excess water and sodium is passed along to the bladder before being eliminated.
If you have been drinking too much water or other fluids too fast, your kidneys can't keep up. The kidneys can only filter about one liter of fluid water per hour. If the intake is more than that, they can't eliminate the excess water and the osmotic concentrate is then altered, which affects brain function by causing the cells in the brain to swell. This can lead to seizures, brain damage, coma, and even death.
The kidneys produce urine of different concentrations in order to maintain water balance. If we are low on water, more water will be reabsorbed back into the blood and the urine will be more concentrated - and a more yellowish color. If we have lots of water, less water will be reabsorbed and the urine will be less concentrated, resulting in a clearer color. Water levels in the body will vary depending on the external temperature of where we are. For example, if we live in Florida, we will sweat more and lose more water than if we live in Maine.
If we exercise, we will lose more water through sweat than if we are couch potatoes. The more fluids we drink, the more dilute our body fluids become, so the kidneys produce more urine to eliminate the excess. Sodium consumption can help to balance the fluids in our body. This is why sports drinks contain sodium, so that the water we consume when exercising won't be lost.
Water balance is the concept that the amount of water lost from the body is equal to the amount of water consumed. Keeping our water and electrolytes, like sodium, in balance is called osmotic concentration. Our thirst center can detect very small changes in the water level within our bodies. The recommended daily intake for water in the U.S. is 3.7 liters per day for males and 2.7 liters per day for females. Pure water and beverages containing water, including fruit juice, soft drinks, tea, and coffee, all count towards meeting our water requirements.
Our kidneys regulate fluid and sodium output. Blood comes to the kidneys to be filtered or cleaned and then returns to circulating around the body. The molecules removed during the filtering process include water, sodium, glucose, and urea. The kidneys can filter about one liter of water per hour. Fortunately, water intoxication is usually done on purpose. We really can't overdose on water accidentally.
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