Water and hydration: Physiological basis in Adults - Recommendations
IntroductionWater in the bodyI. Water in the body: content and distributionI.1. Water content of the human bodyI.1.1.Total body waterI.1.2. Water content of different organsI.1.3. Distribution among body compartmentsI.2. Water absorption and distribution in the bodyBody water balanceII. Body water balanceII.1. Body fluid lossesII.1.1. Insensible water lossesII.1.2. Fecal water lossesII.1.3. Sweat productionII.1.4. Urinary water lossesII.2. Body water inputsII.2.1. Metabolic water productionII.2.2. Dietary intakesII.3. The regulation and maintenance of body water balanceII.3.1.Regulation of fluid intake: physiological thirst, social and environmental factorsII.3.2.Regulation of water excretion by the kidneysII.3.3. Body water balance impairments: dehydration and hyponatremiaRecommendationsIII. Recommendations for daily water intakeConclusionReferences
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As seen in section II above, the human body is able to adapt to a wide range of fluid intake and losses, thanks to a precise homeostatic regulation and to the wide ranges of urine osmolality kidneys are able to achieve. But, in contrast to other nutrients, there is today insufficient research regarding the amount of water required to prevent disease or improve health. As a result, neither upper nor lower consumption thresholds have been clearly linked to a specific benefit or risk.
This can explain why most of the guidelines for total water intake are based today on median population intake. This is true in the USA and Canada, for example, where adequate intake is based on the median water intake from NHANES III data (Third National Health and Nutrition Examination Survey) (IOM 2004). Australia and New-Zealand follow this same methodology (NHMRC 2006).
Recent official guidelines for total water intakes (water + beverages + food moisture) were published by the European Food Safety Authority (EFSA) in 2010. These guidelines are the first ones to use both observed intakes and physiological parameters to set adequate intake. A desirable urine osmolarity of 500 mOsmol/L is proposed, and based on this value and on the osmotic load of a standard European diet, a urinary volume and associated total fluid intake is determined (EFSA 2010). This recommendation does not take into account extra fluid loss due to physical activity, which induces large variation in the adequate water intake.
Table 6 summarizes the water intake recommendations of four international authorities.
Table 6. Reference values for total water intake (food + fluid), L/day.
These recommendations for total water intake include water from food and water from beverages of all kind, including water. For adults, the contribution of food to total water intake is usually considered to represent 20% to 30% (EFSA 2010).
It is interesting to notice that no upper safety limit has been set, due to the ability of the kidneys of healthy individuals to excrete excess water, up to 0.7 -1.0 liter of urine per hour for adults (EFSA 2010).
Take home messages
Today, neither upper nor lower water consumption limits have been clearly linked to a specific benefit or risk, and most of the guidelines for total water intake are based on median population intake.
The European Food Safety Authority has been the first one to introduce a physiological parameter to determine adequate intake and to propose a desirable urine osmolarity of 500 mOsmol/L.
International recommendations for total water intake (food + fluid) of men vary considerably, from 2.5/day in Europe to 3.7L/day in the United States and Canada.