Physiology & Behavior, Vol. 20, pp. 5 4 7 - 5 5 2 . Pergamon Press and Brain Research Publ., 1978. Printed in the U.S.A
Development of Carbohydrate Preference During Water Rationing: A Specific Hunger? 1 DAVID T. COREY, 2 ALLAN WALTON AND NElL I. WIENER
York University, Department of Psychology, 4700 Keele Street, Downsview, Ontario, Canada, M3J 1P3 (Received 18 November 1977) COREY, D. T., A. WALTON AND N. I. WIENER. Development of carbohydrate preference during water rationing: A specific hunger? PHYSIOL. BEHAV. 20(5) 547-552, 1978. - Weanling rats maintained on a water restricted schedule increased their preference for a high-carbohydrate diet versus an isocaloric high-protein diet over a period of six days. This preference disappeared immediately following a period of ad lib water access. In the second experiment rats that had been water restricted prior to their exposure to the test diets developed a clear carbohydrate preference within six hours of exposure, Moreover, the preference was closely linked to the short-term availability of water. Experiment 3 demonstrated that partially desalivated rats preferred high-carbohydrate diets even with ad lib access to water. These results indicated that dry mouth alone is sufficient to produce the dietary selection observed in Experiments 1 and 2 and a specific hunger need not be hypothesized to explain the phenomenon. Carbohydrate preference Dietary selection Specific hunger Ease of swallowing Partial desalivation Habit
THAT animals in their natural environment obtain foods consonant with their needs is, of course, a truism. Laboratory self-selection experiments have also demonstrated that animals who are deficient in some substance will, under some conditions, preferentially ingest significant amounts of the required nutrient [6]. Typically, these animals become deficient through the removal of the target nutrient from their diet. Extra-dietary factors, such as environmental temperature and activity level, can also mediate dietary selection [ 1,4]. Similarly, the availability of water can influence dietary selection [10,11]. SchmidtNeilson et al. [ 11 ] noted that desert rodents provided with a diet of dry mesquite beans would, under conditions of severe water deprivation, leave uneaten those portions of the bean highest in protein content. Overmann and Yang [5] have reported that weanling rats will also avoid a nutritious, high-protein diet while maintained on a water restricted regimen. These animals gradually developed a preference for an isocaloric high-carbohydrate diet and the preference persisted as long as the animals remained water restricted. The development of a carbohydrate preference during water shortages is highly adaptive since protein releases half as much water of metabolism as carbohydrate (.6 ml/g vs .3 ml/g) as a result of the increased urinary water loss required to excrete the nitrogenous products of protein. Thus, the ability to select the more appropriate diet would confer a decided advantage to a water-starved organism.
Dry mouth
Oropharyngeal mechanisms
The carbohydrate preference observed by Overmann and Yang took a number of days to develop and occurred more rapidly between diets more distinct in taste and texture. The authors inferred the presence of an associative mechanism from these characteristics. Such a mechanism could have taken the form of a conditioned taste aversion or preference [9]. However, some preferences, such as sodium appetite following sodium depletion can develop without any specific learning [3,13 ]. A spontaneous, nonassociative mechanism such as this was not considered by Overmann and Yang because of the gradual development of the preference, but it should be pointed out that during prolonged water restriction there are at least two other changes: weight loss (relative to ad lib controls) and a cumulative thirst increment. Either or both of these changes could be responsible for a gradual development of carbohydrate preference. The present experiments were designed to further investigate the mechanism(s) responsible for the development of carbohydrate preference during water restriction. EXPERIMENT 1 The purpose of the first experiment was to partially replicate the phenomenon reported by Overmann and Yang and to test some of the predictions consistent with a learning interpretation. Specifically, we looked for evidence of retention and extinction of the carbohydrate preference.
i This research was supported by National Research Council Grant No. APA 301 to Neil I. Wiener. A preliminary version of this paper was presented by the second author to the Canadian Psychological Association meeting in Vancouver, June, 1977. The authors wish to acknowledge the invaluable assistance of K. Duncan and the advice and comments of L. Boulter and B. Kohn. 2Requests for reprints should be sent to D.T. Corey, Department of Psychology, York University, 4700 Keele Street, Downsview, Ontario, Canada M3J IP3. 547
548
COREY, WALTON AND WEINER METHOD
A n imals
Twenty, male, 21-day old, Wistar rats (Woodlyn Farms, Guelph, Ontario) were randomly assigned to two, equal sized groups. Vivarium humidity was maintained at about 20% and temperature ranged between 21 ° and 24°C. The room lights were set on a 12 hr light/dark cycle. Upon receipt the animals were housed individually and fed from two, round, glass containers (40 mm high x 80 mm dia.), secured to the front of the cage. The containers were filled daily with powdered diets mixed from purified ingredients. The containers held enough food for approximately three days consumption and at no time was there less than one day's food supply available. Water was available in 100 ml graduated cylinders with metal spouts containing ball bearings to reduce leakage. Diets
The two test diets were mixed from purified powdered ingredients supplied by ICN Pharmaceuticals Incorporated. The composition of the high-carbohydrate diet was as follows: dextrin (267 g), dextrose (267 g), sucrose (267 g), and casein (137g). The high protein diet comprised: dextrin (80 g), dextrose (80 g), sucrose (80 g) and casein (700 g). Both diets contained 40 g of Salt Mixture (8) and 30 g of Vitamin Diet Fortification Mixture. The diets were nutritionally adequate for normal growth and were isocaloric by weight. To reduce the high preference for the sweeter highcarbohydrate diet, non-nutritive, .08% sucrose octaacetate (W/W) was added, giving the diet a distinctive bitter aftertaste. Pro cedure
and its data were eliminated from the analysis. The carbohydrate diet preference data (carbohydrate/carbohydrate + protein x 100) are displayed in Fig. 1. During the initial eight days of presentation, carbohydrate preference declined substantially in both groups as expected, F(6,108) = 7.05, p
Development of Carbohydrate Preference During Water Rationing: A Specific Hunger? 1 DAVID T. COREY, 2 ALLAN WALTON AND NElL I. WIENER
York University, Department of Psychology, 4700 Keele Street, Downsview, Ontario, Canada, M3J 1P3 (Received 18 November 1977) COREY, D. T., A. WALTON AND N. I. WIENER. Development of carbohydrate preference during water rationing: A specific hunger? PHYSIOL. BEHAV. 20(5) 547-552, 1978. - Weanling rats maintained on a water restricted schedule increased their preference for a high-carbohydrate diet versus an isocaloric high-protein diet over a period of six days. This preference disappeared immediately following a period of ad lib water access. In the second experiment rats that had been water restricted prior to their exposure to the test diets developed a clear carbohydrate preference within six hours of exposure, Moreover, the preference was closely linked to the short-term availability of water. Experiment 3 demonstrated that partially desalivated rats preferred high-carbohydrate diets even with ad lib access to water. These results indicated that dry mouth alone is sufficient to produce the dietary selection observed in Experiments 1 and 2 and a specific hunger need not be hypothesized to explain the phenomenon. Carbohydrate preference Dietary selection Specific hunger Ease of swallowing Partial desalivation Habit
THAT animals in their natural environment obtain foods consonant with their needs is, of course, a truism. Laboratory self-selection experiments have also demonstrated that animals who are deficient in some substance will, under some conditions, preferentially ingest significant amounts of the required nutrient [6]. Typically, these animals become deficient through the removal of the target nutrient from their diet. Extra-dietary factors, such as environmental temperature and activity level, can also mediate dietary selection [ 1,4]. Similarly, the availability of water can influence dietary selection [10,11]. SchmidtNeilson et al. [ 11 ] noted that desert rodents provided with a diet of dry mesquite beans would, under conditions of severe water deprivation, leave uneaten those portions of the bean highest in protein content. Overmann and Yang [5] have reported that weanling rats will also avoid a nutritious, high-protein diet while maintained on a water restricted regimen. These animals gradually developed a preference for an isocaloric high-carbohydrate diet and the preference persisted as long as the animals remained water restricted. The development of a carbohydrate preference during water shortages is highly adaptive since protein releases half as much water of metabolism as carbohydrate (.6 ml/g vs .3 ml/g) as a result of the increased urinary water loss required to excrete the nitrogenous products of protein. Thus, the ability to select the more appropriate diet would confer a decided advantage to a water-starved organism.
Dry mouth
Oropharyngeal mechanisms
The carbohydrate preference observed by Overmann and Yang took a number of days to develop and occurred more rapidly between diets more distinct in taste and texture. The authors inferred the presence of an associative mechanism from these characteristics. Such a mechanism could have taken the form of a conditioned taste aversion or preference [9]. However, some preferences, such as sodium appetite following sodium depletion can develop without any specific learning [3,13 ]. A spontaneous, nonassociative mechanism such as this was not considered by Overmann and Yang because of the gradual development of the preference, but it should be pointed out that during prolonged water restriction there are at least two other changes: weight loss (relative to ad lib controls) and a cumulative thirst increment. Either or both of these changes could be responsible for a gradual development of carbohydrate preference. The present experiments were designed to further investigate the mechanism(s) responsible for the development of carbohydrate preference during water restriction. EXPERIMENT 1 The purpose of the first experiment was to partially replicate the phenomenon reported by Overmann and Yang and to test some of the predictions consistent with a learning interpretation. Specifically, we looked for evidence of retention and extinction of the carbohydrate preference.
i This research was supported by National Research Council Grant No. APA 301 to Neil I. Wiener. A preliminary version of this paper was presented by the second author to the Canadian Psychological Association meeting in Vancouver, June, 1977. The authors wish to acknowledge the invaluable assistance of K. Duncan and the advice and comments of L. Boulter and B. Kohn. 2Requests for reprints should be sent to D.T. Corey, Department of Psychology, York University, 4700 Keele Street, Downsview, Ontario, Canada M3J IP3. 547
548
COREY, WALTON AND WEINER METHOD
A n imals
Twenty, male, 21-day old, Wistar rats (Woodlyn Farms, Guelph, Ontario) were randomly assigned to two, equal sized groups. Vivarium humidity was maintained at about 20% and temperature ranged between 21 ° and 24°C. The room lights were set on a 12 hr light/dark cycle. Upon receipt the animals were housed individually and fed from two, round, glass containers (40 mm high x 80 mm dia.), secured to the front of the cage. The containers were filled daily with powdered diets mixed from purified ingredients. The containers held enough food for approximately three days consumption and at no time was there less than one day's food supply available. Water was available in 100 ml graduated cylinders with metal spouts containing ball bearings to reduce leakage. Diets
The two test diets were mixed from purified powdered ingredients supplied by ICN Pharmaceuticals Incorporated. The composition of the high-carbohydrate diet was as follows: dextrin (267 g), dextrose (267 g), sucrose (267 g), and casein (137g). The high protein diet comprised: dextrin (80 g), dextrose (80 g), sucrose (80 g) and casein (700 g). Both diets contained 40 g of Salt Mixture (8) and 30 g of Vitamin Diet Fortification Mixture. The diets were nutritionally adequate for normal growth and were isocaloric by weight. To reduce the high preference for the sweeter highcarbohydrate diet, non-nutritive, .08% sucrose octaacetate (W/W) was added, giving the diet a distinctive bitter aftertaste. Pro cedure
and its data were eliminated from the analysis. The carbohydrate diet preference data (carbohydrate/carbohydrate + protein x 100) are displayed in Fig. 1. During the initial eight days of presentation, carbohydrate preference declined substantially in both groups as expected, F(6,108) = 7.05, p
