Thirst (Encyclopedia of Food & Culture)
THIRST. Thirst is a conscious sensation that results in a desire to drink. Although all normal humans experience thirst, science can offer no precise definition of this phenomenon because it involves numerous physiological responses to a change in internal fluid status, complex patterns of central nervous system function, and psychological motivation. Three factors are typically recognized as components of thirst: a body water deficit, brain integration of central and peripheral nerve messages relating to the need for water, and an urge to drink. In laboratory experiments, thirst is measured empirically with subjective perceptual scales (for example, ranging from "not thirsty at all" to "very, very thirsty") and drinking behavior is quantified by observing the timing and volume of fluid consumed.
Psychologists classify thirst as a drive, a basic compelling urge that motivates action. Other human drives involve a lack of nutrients (for example, glucose, sodium), oxygen, or sleep; these are satiated by eating, breathing, and sleeping. Clark Hull published a major, relevant theory describing the nature of human drives in 1943. He observed that learned habits, in addition to the thirst drive, influence drinking strongly. If a behavior reduces thirst, that behavior is reinforced and learned as a habit. Irrelevant behaviors (for example, sneezing, grooming) provide no reinforcement, have no effect on drinking, and do not become habits. Further, Hull realized that external incentives, such as the qualities or quantity of a fluid, also influence fluid consumption. On a hot summer day, for example, a cold beverage is more attractive than a cup of hot tea. Yet when chilled to a very low temperature, a cold beverage becomes an aversive stimulus to drinking behavior. Physiologists have popularized the term alliesthesia (from Greek root words referring to altered sensation) to describe the fact that the sensation of thirst may have either pleasant or unpleasant qualities, depending on the intensity of the stimulus and the state of the person.
Numerous investigations have verified that thirst and drinking behavior are complex entities. For example, drinking behavior (that is, the timing and the amount of fluid consumed) is not linearly related to the intensity of perceived thirst. Nor should we infer that individuals experience thirst simply because they drink. These facts indicate that thirst and drinking behavior are distinct entities that influence each other and are influenced by numerous internal and external factors.
Physiological Components of Thirst
Thirst is often viewed by physiologists and physicians as a central nervous system mechanism that regulates the body's water and minerals. The significance of the thirst drive is emphasized by three facts: 50 to 70 percent of adult body weight is water, the average adult ingests and loses 2.5 liters of water each day, and body weight is regulated within 0.2 percent from one day to the next. Clearly, water is essential to life and the body responds in a manner that ensures survival.
In 1954, Edward Adolph and colleagues proposed a multiple-factor theory of thirst that has not been refuted to date. This theory states that no single mechanism can account for all drinking behavior and that multiple mechanisms, sometimes with identical functions, act concurrently. Because water is essential to life, the existence of redundant mechanisms has great survival value. Among these, thirst appears to be regulated primarily by evaluation of changes in the concentration of extracellular fluid, measured as the osmolality of blood plasma. (Osmolality is a measurement that describes the concentration of all dissolved solids in a solution, that is, dissolved substances per unit of solvent. In research and clinical laboratories, the unit for osmolality of blood is mOsm/kg or milliosmoles per kilogram of water.)
Below a certain threshold level of plasma osmolality, thirst is absent. Above this threshold, a strong desire to drink appears in response to an increase of 2 to 3 percent in the level of dissolved substances in blood. The brain's thirst center lies deep within the brain, in an area known as the hypothalamus. This anatomical site contains cells that respond to changes in the concentration of body fluids. When the thirst center is stimulated by an increased concentration of blood (that is, dehydration), thirst and fluid consumption increase.
As the brain senses the concentration of blood, it allows a minor loss of body water before stimulating the drive to drink. This phenomenon has been named voluntary dehydration. Specifically, several research studies since the 1930s have observed that adults and children replace only 34 to 87 percent of the water lost as sweat, by drinking during exercise or labor in hot environments. The resulting dehydration is due to the fact that thirst is not perceived until a 1 to 2 percent body weight loss occurs. Interindividual differences, resulting in great voluntary dehydration in some individuals, have caused them to be named reluctant drinkers.
Reduced extracellular fluid volume, including blood volume, also increases thirst. Experiments (for example, reducing blood volume without altering blood concentration) have demonstrated that volume-sensitive receptors in the heart and blood vessels likely regulate drinking behavior by increasing the secretion of hormones. This effect is relatively minor, however. Animal research suggests that a change in extracellular fluid concentration accounts for most (for example, 70 percent) of the increased fluid consumption that follows moderate whole-body dehydration, whereas a decrease of fluid volume per se plays a secondary role.
Thus, thirst is extinguished when body fluid concentration decreases and fluid volume increases. Osmolality-sensitive nerves in the mouth, throat, and stomach also play a role in abating thirst. As fluid passes through the mouth and upper gastrointestinal tract, the sense of dryness decreases. When this fluid fills the stomach, stretch receptors sense an increase in gastric fullness and the thirst drive diminishes.As dehydration causes the body's extracellular fluid to become more concentrated, the fluid inside cells moves outward, resulting in intracellular dehydration and cell shrinkage, and the hormone arginine vasopressin (AVP,
Repeated training sessions in cool or hot environments alter fluid consumption in four ways. First, physical training increases the secretion of the hormone AVP, which stimulates drinking and body water retention. Second, exercise-heat acclimation (that is, adaptations due to exercise in a hot environment over eight days) increases the volume of fluid consumed and the number of times that adults drink during exercise. Third, frequent rest periods, in the midst of labor or exercise, will increase fluid replacement time and enhance fluid consumption. Humans tend to drink less when they are preoccupied or are performing physical or mental tasks. Fourth, learned behaviors can enhance fluid consumption when thirst is absent. This phenomenon is widely appreciated among military personnel and athletes who are trained to consume water at regular intervals, whether they are thirsty or not.
Several research groups have reported that chronological age influences thirst and drinking behavior. Elderly men experience a blunted thirst drive and reduced fluid intake, perhaps due to their brains' reduced ability to sense changes in plasma osmolality or blood volume. Further, elderly individuals experience a decrease in the ability of their kidneys to conserve water. This suggests that the elderly are predisposed to dehydration when illness increases water loss (that is, vomiting, diarrhea) or when physical incapacity prevents access to water.
Fluid and Environmental Characteristics
Many fluid characteristics stimulate or enhance drinking, during or after exposure to a hot environment. Fluid temperature (consumption is greatest at 14 to 16°C, reduced above 37°C), turbidity, sweetness, fruit flavorings (for example, cherry, grape, orange, lemon), addition of citric acid which imparts a citrus flavor, and addition of sodium chloride or other minerals are examples. These components enhance palatability and increase fluid consumption. The addition of a small amount of salt (sodium chloride), besides enhancing palatability, may result in thirst and increased drinking, due to the specific action of sodium on fluid movements. An increased sodium concentration outside of cells causes water to leave cells via osmosis. The resulting cellular dehydration is an important stimulus for drinking. Increased beverage carbonation tends to reduce the palatability of a fluid as well as the volume of fluid consumed, without an increase in thirst. In addition, intakes of food and water are closely related. During 24-hour observations of fluid intake, most studies report that the majority of fluid (69 to 78 percent) is consumed during meals. The foregoing characteristics, therefore, tend to reduce the magnitude of voluntary dehydration.
Conversely, fluid characteristics may influence drinking behavior negatively, regardless of the intensity of thirst. Experiments conducted during mild prolonged exercise have shown that the following qualities are perceived as undesirable: nausea, bloating, an objectionable feeling in the mouth, excessive viscosity, and excessive sweetness (see Passe, 1996). Exercise and high ambient temperature may independently alter an individual's perception of fluid palatability. For example, drinking behavior increases when air temperature exceeds 25°C. Fluid consumption can also be enhanced by changing the shape of a fluid container, proximity of fluid containers to the drinker, volume of fluid that is available, and time allowed for drinking.
Societal customs may influence fluid consumption, as evidenced by cross-cultural differences in beverage preferences. Even rituals, such as accepting the friendly offer of a beverage in a social setting, may enhance fluid intake beyond that driven by physiological cues. These factors usually involve learned habits. Similarly, when people repeatedly drink fluids with initially unfamiliar flavors, the palatability of the fluids is enhanced.
Although a comprehensive theory of thirst and fluid balance eludes description, it is likely that the thirst drive increases and diminishes because multiple factors (for example, oral dryness, gastric distension, osmolality, volume, fluid qualities) are integrated concurrently by the brain's thirst center.
See also Alcohol; Appetite; Beer; Cocktails; Coffee; Grapes and Grape Juice; Sensation and the Senses; Tea; Water; Wine.
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Lawrence E. Armstrong