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FLUID REPLACEMENT AND HEAT STRESS Fluid Replacement and Heat Stress, 1993 Pp. 161-167. Washington, D.C. National Academy Press 13 Palatability and Fluid Intake Barbara J. Rolls1 INTRODUCTION People consume fluids in response to a variety of physiological, psychological, and environmental stimuli. In this paper, I discuss some of the physiological changes that can affect fluid intake in man and why people drink spontaneously when they have free access to water. The sensations accompanying dehydration are also considered, as is rehydration and its accuracy in restoring fluid deficits. Finally, the effects of palatability of the availabel fluids on thirst satisfaction and consumption are discussed. PHYSIOLOGICAL THIRST STIMULI During fluid restriction, both the cellular and extracellular body fluid compartments are depleted. Changes in both compartments are associated with thirst and drinking. There is clear experimental evidence that dehydration of the cellular compartment is a potent thirst stimulus. For example, the effects of double-blind intravenous infusions of hypertonic saline (0.45 M) and isotonic saline (0.15 M) were compared in seven healthy young men (Phillips et al., 1985b). Only the hypertonic saline significantly increased 1 Barbara J. Rolls, The Pennsylvania State University, 104 Benedict House, University Park, PA 16802-2311
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FLUID REPLACEMENT AND HEAT STRESS plasma sodium and osmolality, subjective ratings of thirst, and water intake. The main sensations associated with the infusions were a dry and unpleasant-tasting mouth. The increase in plasma sodium (4.2 meq/liter) associated with thirst and drinking was well within the physiological range and was similar to the changes seen after exercise, thermal dehydration, and water deprivation. The evidence for a role for hypovolemia in dehydration-induced drinking is more equivocal. Fluid deprivation decreases plasma volume, and such volume changes would be detected by receptors in and around the heart and kidneys. When the kidneys detect hypovolemia, renin is released, and it acts on a substrate in the plasma to increase the formation of angiotensin. Angiotensin II has been found to be a potent thirst stimulus in most species (Rolls and Rolls, 1982), but we have found that it stimulates drinking in humans only at supraphysiological levels (Phillips et al., 1985a). More experiments are needed to define the situations in which hypovolemia is a thirst stimulus in humans and to determine its mechanism of action. SENSATIONS ASSOCIATED WITH DEHYDRATION The idea that thirst is associated with unpleasant oral sensations dates back to the ancient Greeks. However, only recently have there been systematic attempts to characterize these sensations. Visual analog scales, which are 100 mm lines on which subjects indicate how they currently feel in relation to a very specific question (e.g., how dry is your mouth now?) have been critical in this assessment. We found, for example, that after 24 h without fluids, healthy young men showed a marked increase in ratings of thirst, how pleasant it would be to drink water, dryness of the mouth, and unpleasantness of the taste in the mouth. The changes in the sensations were significantly correlated with subsequent water intake (Rolls et al., 1980). Recently, there has been a detailed examination of the sensations accompanying graded levels of dehydration induced by restricting food and fluid intake and imposing a regimen of moderate heat-exercise stress. The number and intensity of unpleasant sensations reported increased with the level of dehydration. Sensations that showed a significant linear trend in intensity were a dry and irritated mouth; a bad and chalklike taste in the mouth; a dry, scratchy, and warm throat; chapped lips; feeling weary, dizzy, lightheaded, sleepy, tired, irritable, and thirsty; having a headache and loss of appetite; and thinking of drinking (Engell et al., 1987). It is not clear from this study whether the more general sensations are associated with a lack of caffeine that would result from fluid restriction (i.e., no coffee or tea). This
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FLUID REPLACEMENT AND HEAT STRESS study also showed that these thirst sensations contributed to the differential fluid intake that followed the various levels of deprivation. AD LIBITUM THIRST AND DRINKING Although increased thirst and fluid intake follow depletions of body fluids, it is not clear whether, when liquids are freely availabel, people would wait for a significant depletion before drinking. To investigate whether thirst and drinking during free access to water occur in response to body fluid deficits, blood samples and visual analog-scale thirst ratings were obtained from five young men at hourly intervals and when they were thirsty during a normal working day. Although there were significant increases in ratings of thirst, pleasantness of drinking water, mouth dryness, and unpleasantness of the taste in the mouth when subjects were thirsty enough to drink compared with the ratings during intervening intervals, there were no concomitant changes in body fluid variables. Subjects drank mainly in association with eating. The results indicated that during free access to water, humans become thirsty and drink before body fluid deficits develop, perhaps in response to subtle oropharyngeal cues (Phillips et al., 1984a). Several other recent studies have confirmed that when food and a variety of fluids are consumed freely, most of the fluid intake is associated with eating (de Castro, 1988; Engell, 1988). In one of these studies, de Castro (1988) concluded that in such situations, spontaneous fluid intake is in excess of requirements and is determined primarily by eating, with excess fluid being eliminated by the kidneys. REHYDRATION Although fluid intake is normally closely associated with food intake, most of the controlled studies of rehydration have been conducted with no food availabel. This does not negate the importance of these studies but indicates that they should be extended to encompass the more naturalistic situation in which food and fluid are both availabel. It is likely that during the early stage of rehydration, little food would be consumed, but that as rehydration continues, the consumption of food would encourage further fluid intake. It was found that following acute thirst stimuli, such as hypertonic saline infusions or 24 h fluid deprivation, healthy young individuals drank sufficient room temperature tap water in an hour to restore the body fluids to prestimulus levels (Phillips et al., 1985b; Rolls et al., 1980). It is
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FLUID REPLACEMENT AND HEAT STRESS important to note that drinking slowed before plasma dilution had become significant. The attenuation of drinking was attributed to changes in sensations, such as stomach fullness and decreased mouth dryness (Rolls et al., 1980). In contrast to these young men, healthy elderly individuals do not experience normal thirst following fluid deprivation and do not rapidly restore their body fluids to predeprivation levels (Phillips et al., 1984b). In future studies, special consideration should be given to those factors that influence rehydration in the elderly. A number of studies indicate that prolonged dehydration in situations such as intense exercise or thermal dehydration in a desert environment is associated with voluntary dehydration in which insufficient fluid is consumed to restore body fluids (Hubbard et al., 1984). It is clear, however, that the taste and temperature of the availabel fluids can influence the rate of rehydration. TASTE OF DRINKS The taste of the availabel drinks is a major determinant of the amount consumed. There are many factors that influence how good a particular drink tastes to an individual. For example, the cultural background of the individual (Rozin and Vollmecke, 1986), previous experience with the drink (Pliner, 1982), and the time of day (Birch et al., 1984) can affect palatability. Even within an individual, palatability is not constant. Dehydration can increase the pleasantness of fluids, whereas rehydration decreases the pleasantness (Rolls and Rolls, 1982). The decrease in pleasantness can, however, be specific to the particular fluid being consumed, so if the goal is to increase fluid intake, switching to a different drink will help to maintain consumption (Rolls, 1986). It has been shown that variety can increase fluid consumption. Nondeprived subjects, under the pretext of a tasting experiment, consumed three drinks successively, with a 10-min period allowed for each drink, under three different conditions: three different flavors (low-calorie orange, lemon, and lime drinks), one flavor only, or water alone with no flavor. More (22%) was consumed in the three-flavor condition than in the no-flavor condition, and more (99%) was consumed in the one-flavor condition than in the no-flavor condition (Rolls and Rolls, 1982). It is well known that adding sweeteners to drinks leads to increased fluid intake (Rolls, 1987). Soft drinks are consumed in large quantities in the absence of physiological need. This excess fluid does not matter as long as the kidneys are functioning normally, but palatable drinks can potentially induce dangerous overhydration when kidney function is impaired (Rolls et al., 1978). Surprisingly little is known about the effects of soft drinks on the
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FLUID REPLACEMENT AND HEAT STRESS satisfaction of thirst. This lack of information meant that claims such as “sugary soft drinks can leave you thirstier than ever” made in the press (Brody, May 13 and 20, 1987) could not be confirmed or denied. We (Rolls et al., 1990) have recently determined the effects on ratings of thirst of two volumes (8 and 16 oz.) of three drinks: lemonade sweetened with sucrose (8.3%), lemonade sweetened with aspartame (0.045%), and tap water, all of which were served at 9°C. Fourteen male subjects were tested in all six conditions as well as in a no-drink condition. The drinks were consumed with a lunch of sandwiches. Surprisingly, the subjects ate the same amount of food regardless of the type or volume of drink availabel. The drinks did, however, have differential effects on ratings of thirst measured immediately after the end of the meal. The suppression of thirst was greater with the 16-oz. drinks than with the 8-oz. drinks. The type of drink availabel also affected the ratings of thirst, in that the water and aspartame-sweetened drinks were equally effective in reducing thirst and were both more effective than the same volume of the sucrose-sweetened drink. The subjects were unable to tell the difference between the aspartame-and sucrose-sweetened drinks in a sensory evaluation test at the end of the study. It is not clear how these different effects on thirst would affect subsequent fluid intake. A recent study indicates that adding too much sugar to beverages can decrease acceptability. Trained athletes reported that glucose-electrolyte drinks containing 12% glucose caused significantly more nausea and fullness than either 6% glucose or water. Because of the possibility of stomach upset, they were less likely to choose the more concentrated drink during training or competition (Davis et al., 1988). TEMPERATURE OF DRINKS As with the taste of drinks, the preferred temperature depends on a number of factors, which include culture and learning (Zellner et al., 1988) and the physiological state of the individual (Sandick et al., 1984). Several studies indicate that finding the optimal temperature for availabel fluids will improve rehydration. In a French study (Boulze et al., 1983), men were dehydrated by mountain climbing or sweating in a vaporarium. During rehydration, when offered water from 0 to 50°C, subjects drank the most when the water was 15°C; this was the preferred temperature. In an American study (Sandick et al., 1984), subjects drank the most of the coldest water (5°C) during rehydration after exercise. In a study in which the effects of both the flavor and temperature of the availabel beverages on rehydration after a simulated desert walk were examined, it was found that consumption of cold, flavored, iodinated water elicited over twice the percent rehydration
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FLUID REPLACEMENT AND HEAT STRESS (80.5% versus 37%) than when warm iodinated water was ingested (Hubbard et al., 1984). There is a need not only for more studies on temperature and intake, but the effects of the temperature of various fluids on the rate of rehydration should also be determined. CONCLUSIONS Although the termination of drinking ultimately depends on the restoration of fluid and electrolyte losses, the early slowing of drinking during rehydration is attributed to a reduction of the sensations associated with thirst (Rolls et al., 1980; Sandick et al., 1984). It is known that the taste and temperature of beverages can affect these sensations, but as yet, this information has not been translated into the development of optimal drinks that will ensure fluid balance in different environments. REFERENCES Birch, L.L., J. Billman, and S.S. Richards. 1984 Time of day influences food acceptability. Appetite 5:109-116. Boulze, D., P. Montastruc, and M. Cabanac. 1983 Water intake, pleasure and water temperature in humans. Physiol. Behav. 30:97-102. Brody, J.E. May 13, 1987. Hot-weather drinks of dubious value. The New York Times. p. 20. Brody, J.E. May 20, 1987. Seeking a beneficial thirst quencher. The New York Times. Davis, J.M., W.A. Burgess, C.A. Slentz, W.P. Bartoli, and R.R. Pate. 1988 Effects of ingesting 6% and 12% glucose/electrolyte beverages during prolonged intermittent cycling in the heat. Eur. J. Appl. Physiol. 57:563-569. de Castro, J.M. 1988 A microregulatory analysis of spontaneous fluid intake by humans: evidence that the amount of liquid ingested and its timing is mainly governed by feeding. Physiol. Behav. 43:705-714. Engell, D. 1988 Interdependency of food and water intake in humans. Appetite 10:133-141. Engell, D.B., O. Maller, M.N. Sawka, R.N. Francesconi, L. Drolet, and A.J. Young. 1987 Thirst and fluid intake following graded hypohydration levels in humans. Physiol. Behav. 40:229-236. Hubbard, R.W., B.L. Sandick, W.T. Mattew, R.P. Francesconi, J.B. Sampson, M.J. Durkot, O.Maller and D.B. Engell. 1984 Voluntary dehydration and alliesthesia for water. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 57:868-875.
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FLUID REPLACEMENT AND HEAT STRESS Phillips, P.A., B.J. Rolls, J.G.G. Ledingham, and J.J. Morton. 1984a Body fluid changes, thirst and drinking in man during free access to water. Physiol. Behav. 33:357-363. Phillips, P.A., B.J. Rolls, J.G.G. Ledingham, M.L. Forsling, J.J. Morton, M.J. Ce, and L. Wollner. 1984b Reduced thirst after water deprivation in healthy elderly men. N. Engl. J. Med. 311:753-759. Phillips, P.A., B.J. Rolls, J.G.G. Ledingham, J.J. Morton, and M.L. Forsling. 1985a Angiotensin II-induced thirst and vasopressin release in man. Clin. Sci. 68:669-674. Phillips, P.A., B.J. Rolls, J.G.G. Ledingham, M.L. Forsling, and J.J. Morton. 1985b Osomotic thirst and vasopressin release in humans: a double-blind crossover study. Am. J. Physiol. 248:R645-R650. Pliner, P. 1982 The effects of mere exposure on liking for edible substances. Appetite 3:283-290. Rolls, B.J. 1986 Sensory-specific satiety. Nutr. Rev. 44:93-101. Rolls, B.J. 1987 Sweetness and satiety. Pp. 161-173 in Sweetness, J. Dobbing, ed. Springer-Verlag, London. Rolls, B.J., and E.T. Rolls. 1982 Thirst. Cambridge University Press, Cambridge. Rolls, B.J., R.J. Wood, and R.M. Stevens. 1978 Palatability and body fluid homeostasis. Physiol. Behav. 20:15-19. Rolls, B.J., R.J. Wood, E.T. Rolls, H. Lind, W. Lind, and J.G.G. Ledingham. 1980 Thirst following water deprivation in humans. Am. J. Physiol. 239:R476-R482. Rolls, B.J., S. Kim, and I.C. Fedoroff. 1990 Effects of drinks sweetened with sucrose or aspartame on hunger, thirst and food intake in men. Physiol. Behav. 48:19-26 Rozin, P., and T.A. Vollmecke. 1986 Food likes and dislikes. Annu. Rev. Nutr. 6:433-456. Sandick, B.L., D.B. Engell, and O. Maller. 1984 Perception of drinking water temperature and effects for humans after exercise. Physiol. Behav. 32:851-855. Zellner, D.A., W.F. Stewart, P. Rozin, and J.M. Brown. 1988 Effect of temperature and expectations on liking for beverages. Physiol. Behav. 44:61-68.
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