Water

Water (hydrogen oxide; molecular weight IS) is the major liquid in humans.

Abbreviation

AQP aquaporin

HlndboDk of Nutrient Metabolism ISBN: II-12-4I7762-X

Copyright ' 2003 Elsevier Ltd All rights of reproduction In any form reserved

Nutritional summary

Function; Water provides the medium lor metabolic reactions and for the flow of blood and other body fluids. The hydroxide and proton ions are used in various reactions for the synthesis and breakdown of body constituents. Evaporation of water from sweat protects against overheating.

Requirements; Intakes must maintain adequate body hydration and he sullicicm for renal excretion of solutes. Intakes of 2()(K)ml or more are usually adequate for many people. Individuals with high sweat output or working at high altitude often need more.

Sources: Most water is consumed pure, w ith very dilute beverages (coffee and leas) or nutrient-rich beverages (milk and other dairy products, juices, sweetened drinks). Deficiency: Low water intake can rapidly lead to dehydration, particularly w jib exertion, exposure to high temperature or dry air. or w hen suffering from diarrhea. Dehydrat ion impairs physical and mental performance, may cause loss of consciousness, coma and death.

Excessive mtohc Intakes of several liters in excess of needs can cause abnormal dilution of body fluids and loss of essential electrolytes.

Endogenous sources full oxidation of all energy fuels generates water. Water yield is about 1,1 mlg for fat and ethanol, 0.6 ml/g for carbohydrates, and about 0.4 ml g for protein. This means thai a daily intake of 260g carbohydrate, 60 g protein, and 6<tg fat generates about 246 ml water upon complete oxidation.

Dietary sources

Water is consumed w ith both liquids and most solid foods. The composition of the drink matters, since high electrolyte and nitrogen content may increase solvent drag and losses in the kidney, while optimal amounts of sodium and glucose (or galactose) can aid w ith water uptake in some situations (e.g.. diarrhea).

Thirst is a complex physiological response to water need (dehydration), but may noi always be strong enough to ensure intake. Aging lends to blunt both the mechanisms causing thirst (e.g. osmosensors) and the strength of the thirst sensation. Drinking rapidly quenches thirst, even before ingested water has been absorbed. Adequate hydration also minimizes thirst sensation. Social sellings and individual habits often cause intake beyond the needs for adequate hydration

Digestion and absorption

The mucosa of the proximal small intestine has a net water secretion of about 30 ml hour (Knutson etui., 1995). The water moves mainly through the gap junctions between the enterocytes. following the osmotic gradient to the more concentrated luminal contents.

The higher than isoosmolar luminal concentration is due to ingested foods, glandular secretions, and sodium secreted by the enteroeytes. Water is largely taken up again into -mall intestinal enteroeytes w ith sodium and glucose via the sodium/glucose cotrans-porter (SGLTI, SLC5A1) and other sodium cotransporters. Since more than 200 water molecules are moved into the enterocyte with each glucose molecule, the absorption of 100 g glucose pulls along about twenty times as much water (about 2000 ml).

Aquaporin 3 (AQP3). whose expression increases from the stomach and is greatest in the ileum and colon, allows water movement across the basolateral membrane (Purdy et at.. 1990). Water transport across the basolateral membrane of colon enteroeytes also uses AQIM (Wang el at.. 2000), In healthy people, less than 200nil of water is lost with feces. This amount can increase to several liters in patients suffering from cholera or other types of severe diarrhea.

figure 11.1 Mcrharitsms of water secretion and absorption in small and large Intestines
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