Water intake is often a secondary consideration in dysphagia management. Clinicians, rightfully so, are concerned about how their patients can safely swallow different food consistencies, but may overlook how those decisions may affect water intake and bodily functions at the cellular level. The companion article discusses thickening agents and how alterations to foods often affect their nutrient value and water content and may affect a patient's hydration status. How positively our clinical decisions affect hydration status depends on how well we understand the basic mechanisms of hydration.
For a living cell, this tasteless, colorless, and odorless nutrient called water comprises more than 80% of its composition. Water enters and leaves the body generally in equal amounts, helping to maintain a delicate balance of fluid requirements necessary for optimal cell and organ functioning. Among its many duties are transporting nutrients and waste products; synthesizing and regulating molecules including proteins and glycogen; lubricating joints and the eyes; comprising secretions such as saliva; regulating and economizing body heat; maintaining blood volume; and acting as a solvent for minerals, vitamins, amino acids, and glucose (Whitney &Rolfes, 1999).
Mammals primarily receive water from fluid and food sources and as a byproduct from cell metabolism. This water is then absorbed into the bloodstream through the villi lining the lower gastrointestinal tract and distributed to cells throughout the body. To balance the water intake, water is excreted through urine and feces and evaporated through the skin and lungs as normal bodily functions. Table 1 [PDF] shows the amounts of water gained and lost daily, based on information from James Robinson (1998).
People who exercise vigorously know that water loss may temporarily affect body weight. Water contributes significantly to the total weight of the body. Water comprises about 60% of the body weight of men and about 50% of the body weight of women, a figure that varies depending on age and body fat content. Of the body mass, muscle tissue retains significant water content, while fat is essentially water-free. Thus, in a healthy, lean person, water contributes more to the overall body weight than in an obese person.
Water is stored in cells, body cavities, and blood vessels. It is distributed into two main cellular spaces in the body—the space within a cell (intracellular) and the spaces outside and surrounding the cell (extracellular). Intracellular water accounts for about 55% of total body water, and extracellular water accounts for about 45%. Water in extracellular spaces is less well-defined, as it is subcompartmentalized into special fluids including plasma, mucous secretions, and lymph, spinal, ocular, synovial, and interstitial fluids, among others (Berdanier, 2000).
At a hospital, hanging bags of potassium and sodium chloride solutions infuse the patient with intravenous fluids. Sodium and potassium, in addition to water, are the main components of intracellular or extracellular fluid. Intracellular fluid contains potassium, phosphate, protein, and bicarbonate. Extracellular fluid consists primarily of sodium chloride. Differing concentrations of electrically charged particles within these two cellular fluids are responsible for moving water and minerals back and forth through porous cell walls to maintain homeostatic balance.
Properly hydrated patients maintain the same total body water weight on a day-to-day basis, implying that water levels are kept constant with gains being balanced against losses. In medical settings, nursing staff constantly monitor and chart intake and output of their patients to ensure bodily-fluid processing stability. This balancing process is regulated through the hypothalamus. As the body loses water, cells shrink and the elements and compounds in bodily fluids become more concentrated. Special hypothalamic cells respond to signals of cell shrinkage and trigger the sensation of thirst. Simultaneously, an antidiuretic hormone, vasopressin, is released into the blood stream directing renal functions to reabsorb water from dilute urine and return it to the blood supply. Once water gains exceed losses, the process is switched off and excess water is removed (Robinson, 1998).
Dehydration is defined as a significant loss of total body water content due to pathologic water losses, reduced water intake, or a combination of both. Speech-language pathologists generally do not directly assess for or diagnose dehydration, which is usually identified by physicians, nurses, or nutritionists. However, the SLP should know and recognize symptoms of dehydration, know its potential causes, be aware of the impact of potential clinical recommendations, and be a part of the management team.
Dehydration has three classifications: isotonic, hypertonic, and hypotonic. Isotonic dehydration results from fluid volume loss (hypovolemia) with equal losses of water and sodium volumes of extracellular fluid and little change to intracellular fluid. Because sodium is highly concentrated in these fluids, its loss is combined with water for isotonic loss. The most common cause of isotonic dehydration is significant loss of gastrointestinal fluids associated with fasting, prolonged vomiting, nasogastric suctioning, severe burns, bleeding, and massive diarrhea. Other clinical features include weakness, thirst, decreased blood pressure, dizziness, decreased body temperature, sticky oral mucosa, dry furrowed tongue, and rapid weight loss.
Hypertonic dehydration, or hypernatremia, is seen in young children and older adults as a consequence of fluid deprivation. This condition results from an osmolality imbalance, or a greater loss of water volume than loss of sodium. Hypernatremic dehydration is commonly caused by fever, which results in water evaporation through the lungs and skin combined with the inability to take in more water. Associated causes include the inability to perceive or respond to thirst because of confusion or coma, dysphagia secondary to cerebrovascular accident, insufficient IV maintenance during non-feeding status, or the inability to reach a water pitcher or to receive assistance with drinking.
Hypotonic dehydration, or hyponatremia, results from a greater loss of sodium than water or when sodium concentrations are diluted by excessive increases in total body water. Symptoms include anorexia, impaired taste, muscle cramps, headache, personality changes, nausea and vomiting, seizures, and coma. Hypotonic dehydration may occur as a result of prolonged diuretic therapy with low-salt diet, excessive diarrhea, nasogastric tube suctioning, renal failure, decreased ability to excrete free water, or compulsive water drinking (Wilson, 2003).
Aging and Fluid Balance
Among older adults, fluid and electrolyte imbalance is common. Normal aging is responsible for gradual changes to water and electrolyte physiology. In young persons, organ systems have an enormous capacity for adjusting to variations in environmental conditions, such as regulating water balance during vigorous exercise. With aging, gradual reductions occur in these reserve capacities, including the ability to maintain water balance; total body water decreases to approximately 45%–50% of body weight. At least 15% of this loss can be attributed to lean muscle loss and an increase in body fat.
Women have a smaller decrease in total body water through middle age but experience rapid loss after age 60. Men begin losing total body water during middle age and continue throughout their life span. With the onset of disease, the use of medication, loss of mobility, and other factors, these reserves are further diminished. Aging decreases the sensitivity of the volume osmoreceptors, which are responsible for stimulating thirst and drinking behaviors. Lastly, bladder dysfunction or incontinence may make elderly persons reluctant to drink fluids for fear of experiencing embarrassing situations (Allison & Lobo, 2004).
Water is the essential element for sustaining life. As we experience with our very ill patients, they survive for many days without taking in food, but are unable to survive more than a few days without fluids. In the short term, our clinical decisions regarding swallowing safety, proper food consistencies and varieties, and tastes are important to a patient's quality of life, but adequate and proper hydration are fundamental to the sustainability of life itself.