Chapter 139 Water
The Most Basic Nutrient and Therapeutic Agent
• Dry mouth is the only sign of water need.
• Water is merely a passive solvent with no chemical properties of its own.
• The human body can never become depleted of water because water is free and readily available.
• The water needs of the body can easily be met by any commercially prepared fluids.1
Water and the Human Body
Water is the predominant compound in the human body. At birth, a baby’s body is 78% water by weight. The average young adult is 70% water, and although the body’s water content declines with age, water content remains at no less than 50% even in the geriatric individual. During youth, most cells with the exception of adipose cells are 90% water. In adulthood, two-thirds of body water resides within cells; the remainder resides in the extracellular spaces.2 Even bone tissue is 22% water.
Water has numerous roles to play in human physiology3:
• Building material. Water molecules facilitate the folding of amino acid chains assembled on the basis of DNA genetic sequences and reinforce the structural integrity of proteins. Water molecules fill every space not occupied by other molecules in every nook and cranny of every cell.
• Solvent. The ionic nature of water molecules makes water an excellent solvent of ionic compounds: salts, glucose, amino acids, etc.4 The tendency of water molecules to surround ions and molecules of solutes allows chemical agents to move freely to wherever they are needed.
• Reaction medium and reactant. The biochemical reactions that sustain life occur within the medium of water. Yet water is not just the medium in the background; it is also an active participant—a reactant in the hydrolysis of macromolecules such as proteins, carbohydrates, and fats. Water is also a by-product of the metabolism of food energy molecules that contain hydrogen: carbohydrates, proteins, and fats.
• Carrier for nutrients and waste products. Water is the transport medium that moves nutrients into cells and waste products out of cells and then out of the body. Water is also the most basic constituent of blood, indispensable to the function of the cardiovascular, respiratory, urinary, and nervous systems.5
• Thermoregulation. The electromagnetic bonds among water molecules give water a great capacity to absorb heat, hold heat, and resist temperature changes. This property allows water to support homeostasis by helping to maintain body temperature. In addition, the evaporation of water from the skin as perspiration releases heat sufficiently to maintain body temperature even when the surrounding atmospheric temperature is higher than body temperature.
• Lubricant. Water combines with molecules forming viscous substances that lubricate and protect tissue functions. Examples include the mucus of the respiratory and gastrointestinal mucous membranes, synovial fluid in joints, saliva, tears, etc.
• Shock absorber. Within the cell, water takes the structural configuration of a gel that maintains cellular shape and form. This property cushions tissues against the shock of movement and mild trauma. Water molecules have an affinity for the electronegative molecules of connective tissue matrix (e.g., chondroitin, keratin, etc.), including the nucleus pulposus of the vertebral discs. It is the water molecules surrounding the compounds of the connective tissue matrix that provide the cushioning effect.
Thirst: The Sense That Cannot Be Trusted
The osmoreceptors in the hypothalamus are sensitive to only certain solutes in the blood (e.g., sodium) but insensitive to others (e.g., urea).6 Thus, the osmoreceptors are not accurate gauges of blood solute concentration and water need at the cell level. The “thirst center” of the hypothalamus, triggered by osmosreceptor reaction to solute concentration, is distinct from the osmoreceptors.
Signs and Symptoms of Dehydration
Water Losses from the Body
Water exits the body via several portals:
• Skin—insensible perspiration (450 mL daily in temperate climates). Of course elevations in body temperature can lead to massive increases in water losses in the form of sweat.
• Kidneys—1 to 2 L of urine daily. Water loss via urine increases during hyperglycemia (diabetes mellitus) as well as in disorders involving antidiuretic hormone (vasopressin).
• Respiratory mucous membranes—breath (250 to 350 mL daily). Additional water loss occurs during respiratory infections, which trigger increased discharge of mucous secretions.
• Digestive system, including feces (200 mL daily). Dramatic water losses arise from disorders whose symptoms include vomiting and/or diarrhea.
Perspiration is hypotonic; that is, it has lower electrolyte content than plasma or extracellular fluid. This phenomenon means that sweating leads to more water loss than electrolyte loss. As the electrolyte concentration in the extracellular fluid rises, it draws water out of cells (intracellular fluid), leading to cellular dehydration. This state of hypertonic dehydration demands replenishment with hypotonic beverages, such as pure water. Water need supersedes salt need during endurance exercise.8
Ideal Water Intake
In units of liters per day, the water intake levels recommended by the U.S. Food and Nutrition Board, based on age and gender, are as follows9:
Hydration for Athletes and Laborers
The National Athletic Trainers’ Association has published guidelines for athletic hydration. Here are some highlights of their recommendations that apply equally to “weekend warriors” and those involved in heavy physical labor10:
• Establish hydration protocols, accommodating sweat rate, rest breaks, fluid access, environmental factors, acclimatization state, exercise duration and intensity, and personal preferences.
• Make fluid-replacement beverages easily accessible in individual containers to permit easier monitoring of fluid intake.
• Ensure that athletic activity begins in a well-hydrated state. Pre-exercise hydration entails consuming 500 to 600 mL (17 to 20 fl oz) of water 2 to 3 hours before exercise and 200 to 300 mL (7 to 10 fl oz) of water 10 to 20 minutes before exercise.
• Replace fluids sufficiently to meet sweat and urine losses and maintain hydration by preventing at less than 2% body weight reduction. This need requires 200 to 300 mL (7 to 10 fl oz) every 10 to 20 minutes.
• Hydrate postexercise to correct any fluid loss, ideally within 2 hours after exercise. When rehydration must be rapid, compensate for urine losses during rehydration by drinking 25% to 50% more than sweat losses to assure optimal rehydration 4 to 6 hours after exercise.
