The book is called "Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005)" and is available for free download at: http://nap.edu/10925 . I am interested in how they came up with their recommendations. The quotes are in italics and my comments are in regular text.
the AI [Adequate Intake] for total water (from a combination of drinking water, beverages, and food) is set based on the median total water intake from U.S. survey data. The AI for total water intake for young men and women (ages 19 to 30 years) is 3.7 L and 2.7 L per day, respectively. Fluids (drinking water and beverages) provided 3.0 L (101 fluid oz; ≈ 13 cups) and 2.2 L (74 fluid oz; ≈ 9 cups) per day for 19- to 30-year-old men and women, respectively, representing approximately 81 percent of total water intake in the U.S. survey.
So they just did a study of water intake and took the median value and made that their recommendation, which is that men should drink 3.0 L of fluids and women should drink 2.2 L. What happens if people drink less or more than that?
As with AIs for other nutrients, for a healthy person, daily consumption below the AI may not confer additional risk because a wide range of intakes is compatible with normal hydration. In this setting, the AI should not be interpreted as a specific requirement.
So drinking less than AI is not necessarily a problem.
Higher intakes of total water will be required for those who are physically active or who are exposed to hot environments. Because healthy individuals have considerable ability to excrete excess water and thereby maintain water balance, a Tolerable Upper Intake Level (UL) was not set for water. However, acute water toxicity has been reported due to rapid consumption of large quantities of fluids that greatly exceeded the kidney’s maximal excretion rate of approximately 0.7 to 1.0 L/hour.
Drinking more than AI is not a problem because it can be excreted, so long as a large quantity is not quickly consumed.
So what is the real danger with drinking too little water?
an Adequate Intake (AI) for total water is set to prevent deleterious, primarily acute, effects of dehydration, which include metabolic and functional abnormalities.
Where does the water go? 1) Respiratory. Daily respiratory water loss averages about 250 to 350 mL/day for sedentary persons, but can increase to 500 to 600 mL/day for active persons living in temperate climates at sea level. 2) Urinary and gastrointestinal. the minimum urine output is approximately 500 mL/day. Urine output generally averages 1 to 2 L/day. Exercise and heat strain will reduce urine output by 20 to 60 percent while cold and hypoxia will increase urine output. Gastrointestinal and thus fecal water loss in healthy adults is approximately 100 to 200 mL/day. 3) Sweat. For the average adult, loss of water by insensible diffusion is approximately 450 mL/day. For persons living in hot climates, daily sweat losses often exceed several liters. The average daily sweat loss for 97 men in the desert was 4.9 L.
How much water does the body produce? a reasonable estimate of daily metabolic water production is an average of approximately 250 to 350 L/day for sedentary persons—but which can increase to 500 to 600 mL/ day for physically active persons. Hence, respiratory water losses are roughly equivalent to, or offset by, metabolic water production.
By simple math, we can figure out the minimum requirement. Respiratory (300 mL) + Urinary (500 mL) + Gastroinstestinal (150 mL) + Sweat (450 ml) - Metabolic (300 mL) = 1100 mL. So that is how much fluid must be consumed each day.
Subsequent studies recommended minimum daily water requirements of no less then 0.91 L for survival conditions and 3.0 L for hot weather.
So there you go - the average amount of fluid that an average person must consume each day to survive is about 37 ounces (1.1 L). This level would increase in a hot or cold environment. There is nothing wrong with consuming much more than this, but that is the absolute minimum. Anything more than this (if you are not in a hot environment) is just preference.
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