Homeostasis of the water balance (A) ensures stability of the water content. This stable balance is achieved through various hormonal feedback mechanisms in conjunction with osmorecep-tors. The total average daily water intake results from a combination of drinking, intake of water contained in solid foods, and oxidation water. The latter is an end-product of the oxidative metabolism of energy nutrients. The oxidation of 1 g carbohydrate yields 0.6 ml of water; of 1 g protein, 0.42 ml; and of 1 g fat 1.07 ml. Based on a mixed nutrition, the average daily total amounts to 300 ml of oxidation water. According to the recently established Dietary Reference Intake (DRI), to be properly hydrated, women need to consume 2.7 l, men 3.7 l water/d. This applies to sedentary people in temperate climates. Higher temperatures or activity levels increase these requirements. No Tolerable Upper Intake Level (UL) has been established for water. In the average person, ~80% of water intake comes directly from fluids and ~20% from water contained in foods. Approximately 1.5 l is excreted through the urine. The kidneys can influence water balance by altering the rate of reabsorption. To ensure proper excretion of sodium, potassium, and urea, a minimum fluid excretion of 300500 ml is needed. When no drinking water is available, the water loss through the kidneys can be minimized with appropriate nutrition. This means minimizing those foods that result in the formation of urinary excreted metabolites. For instance, lowering intake of protein and table salt results in a reduction of urea and sodium in the urine and, therefore, lowers the minimum urine volume required. In particu lar situations, e. g., for a prematurely born baby with kidney insufficiency, this mechanism becomes important. Water loss via skin and lungs amounts to 0.9 l/d. Increased respiratory frequency, as occurs in higher elevations, dry and warm surroundings, as well as during physical activity, can greatly increase these losses; 0.5 l/h may be lost via the skin alone in extreme situations. Concurrent loss of sodium takes place, decreasing, however, with regular training. If water loss exceeds 3 l/d, sodium loss needs to be replenished, as does water.
Human fluid requirement is, therefore, dependent on metabolic activity, as well as the environment (B). Small children have a significantly higher rate of energy metabolism compared to adults, causing a higher rate of respiration with greater water loss.
In the digestive tract, actual water intake is of lesser significance (C). Each day, ~8 l of fluids are released into the tract in the form of various secretions. Together with the fluids we drink, this amounts to over 10 l/d, all of which is reabsorbed except for 0.2 l. Diarrhea, vomiting, or increased secretions of saliva or bile acids can greatly increase water loss through feces.
The water content of foods (D) determines their energy content. In general, foods with lower water content have lower energy content. Many vegetables consist of >90% water, whereas isolated components like oil or sugar contain practically no water.
- B. Fluid Requirements -
Fluid requirement per kg body mass
C. Water Exchange in the Intestinal Tract-
Bile 500 ml Pancreatic secretions 700 ml
i- D. Water Content of Foods
S 50 ta
Asparagus Milk Oranges Potatoes
Veal Chicken Beef Cheese Bread Butter Sugar
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