Water is the essential solvent for all biochemical reactions in the human body — it constitutes approximately 60% of the total body weight and approximately 75% of the brain weight, and it is the medium in which all of the metabolic reactions, the transport processes, and the signalling pathways occur. The water is the universal solvent — it dissolves the ions (sodium, potassium, calcium, chloride, bicarbonate), the metabolites (glucose, amino acids, fatty acids, nucleotides), the proteins, the enzymes, and the other molecules that are involved in the cellular metabolism, and it enables them to interact with each other and to participate in the biochemical reactions. The water is also the reactant or the product of many of the most important biochemical reactions in the body — including the hydrolysis reactions (in which water is used to break the bonds between the molecules), the dehydration synthesis reactions (in which water is removed to form the bonds between the molecules), and the oxidative phosphorylation (in which water is produced as the byproduct of the electron transport chain). Without adequate water, the cellular metabolism is impaired, the toxins accumulate, and the cellular dehydration, the organ failure, and the death develop — the hallmark of the water deficiency. The typical water intake is 2-3L daily (from the beverages, the fruits, the vegetables, and the food), and the adequate intake is 3.7L daily for men and 2.7L daily for women — but the water deficiency is common (particularly in the elderly, in the athletes, in the hot climates, and in people with the febrile illnesses), and it is one of the most common and most dangerous causes of the hospital admission in the elderly.
Water and the Cellular Osmolarity
The cellular osmolarity is the total concentration of the solutes inside the cell, and it is determined by the concentration of the ions, the metabolites, and the proteins that are present in the intracellular fluid. The water moves across the cell membranes by the osmosis (from the low osmolarity compartment to the high osmolarity compartment) in response to the osmotic pressure gradient, and this water movement is the primary determinant of the cell volume and of the cell shape. When the extracellular fluid becomes hypertonic (as occurs in the dehydration, in the hypernatraemia, and in the diabetic ketoacidosis), the water moves out of the cells, the cells shrink, and the cellular function is impaired. When the extracellular fluid becomes hypotonic (as occurs in the hyponatraemia and in the water intoxication), the water moves into the cells, the cells swell, and the cerebral oedema (brain swelling) and the neurological dysfunction develop. The cellular water balance is therefore as important for the cellular function as the ionic balance, and the disruption of the water balance is one of the most common and most dangerous pathological states.
The clinical importance of the water for the cellular function is underscored by the observation that the dehydration (defined as the 1-2% loss of the body weight due to the fluid loss) produces the measurable impairment of the cognitive function, the mood, and the motor performance — even before the symptoms of the thirst are perceived. A study in 30 healthy adults found that the dehydration (induced by the exercise in the heat and by the fluid restriction) at only 1.6% of the body weight produced the significant impairment of the attention, of the working memory, and of the reaction time — demonstrating that even mild dehydration has a significant impact on the cognitive function and that the thirst mechanism is not a reliable indicator of the degree of the dehydration.
Practical Application
For general water intake, the evidence-based approach is to drink 2-3L of water daily (or approximately 1mL of water per calorie of food intake, which corresponds to approximately 8-10 glasses of water daily for most adults). The water intake should be increased in the hot climates, during the exercise, during the febrile illnesses, and during the pregnancy and the lactation (where the water requirement is increased by 300-500mL daily). The water is the best hydrating fluid (because it is absorbed rapidly and because it does not contain the calories, the caffeine, or the other additives that are found in the other beverages), but the other fluids (the herbal teas, the milk, the juice, and the water-rich fruits and vegetables) also contribute to the daily water intake. The thirst mechanism is a reliable indicator of the water need in most healthy adults, but it can be impaired in the elderly, in the very young, and in people with the neurological conditions that affect the hypothalamic osmoreceptors. For comprehensive hydration and cellular health support, water pairs well with the electrolytes (particularly the sodium, the potassium, and the magnesium, which are lost in the sweat and which must be replaced during the prolonged exercise or the excessive sweating), with the fibre (which absorbs the water in the gut and which forms the gel that is essential for the bowel regularity), and with the antioxidants (which are found in the water-rich fruits and vegetables and which protect the cells from the oxidative stress that is associated with the dehydration).
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