Physiology & Behavior, Vol. 49, pp. 387-391. ©Pergamon Press plc, 1991. Printed in the U.S.A.
0031-9384/91 $3.00 + .00
Influence of Experience on Response to Bitter Taste ISRAEL RAMIREZ
Monell Chemical Senses Center, 3500 Market Street, Philadelphia, PA 19104-3308 R e c e i v e d 20 April 1990
RAMIREZ, I. Influence of experience on response to bitter taste. PHYSIOL BEHAV 49(2) 387-391, 1991.--Sucrose octaacetate, which tastes bitter to humans, can reduce the energy intake of rats when added to their diet. The reduction in energy intake is transient, lasting no more than 5 weeks. Rats that no longer reduce intake in response to sucrose octaacetate still avoid food containing it in choice tests, although to a lesser degree than rats having no previous experience with sucrose octaacetate. The ability of sucrose octaacetate to reduce preference without reducing long-term intake accounts for the previous finding that a wet diet containing sucrose octaacetate can stimulate hyperphagia yet be less preferred than the control diet. Sucrose octaacetate was more effective in reducing intake of a wet than of a dry diet. This last observation indicates that adding water to a food makes it easier for an animal to taste its food. Sucrose octaacetate
Taste
Dietary hyperphagia
Wet diets
only one food, rats ate more and gained more weight when fed the nonpreferred wet diet. A goal of the present experiments was to get better insight into how a diet could stimulate intake yet not be preferred in a choice test.
THE most direct approach to analyzing the role of the chemical senses in the control of food intake is to examine the effects of adding artificial flavors to foods. In general, studies employing artificial flavors have not provided much evidence that artificial flavors can alter long-term intake (2, 4, 8, 9, 12, 15-18). Some flavors that do affect ingestion (e.g., quinine) appear to do so by virtue of their postingestive effects (2-4, 17). Other flavors (e.g., sucrose octaacetate, curry) decrease long-term intake only when given at concentrations that would seem excessive to a human (17,18). Recent studies showed that the effects of saccharin on longterm energy intake depend on the animals' previous experience and the water content of the diet. Adding saccharin to the diet increased energy intake only for rats that had no previous experience with either saccharin or the experimental diet. Rats that had an opportunity to ingest either the plain diet or saccharin in water before getting sweetened diet did not overeat when given the sweetened diet (13). The importance of water content was shown by the finding that rats fed a diet high in water (80%) consumed more energy when fed a sweetened diet, whereas rats fed a diet containing less water (60%) did not consume more food when fed a sweetened diet (13). The present experiments were undertaken, in part, to determine if prior experience and moisture content have an equally strong effect on response to another flavor, sucrose octaacetate (SOA). This substance is aversive to rats (2-4, 6, 12, 16-18), and tastes intensely bitter to humans (5), but according to the papers cited, probably is not very toxic. Whether SOA can be regarded as completely nontoxic at the concentrations used in previous studies (up to 20% of the diet) is unknown. Earlier studies in my laboratory with SOA emphasized the ability of this substance to reduce food preference without blocking the hyperphagic effects of wet diets (12). Rats preferred a plain dry diet over a SOA-adulterated wet diet, yet when given
GENERALMETHOD
Animals A total of 69 female CD rats (Crl:CD®BR, Charles River, Wilmington, MA) were used. There were always 9-10 rats/group. The rats were maintained on a 12-h light/dark cycle at approximately 23°C, fed food and water ad lib, and individually housed in hanging stainless steel cages.
Diets Rats were fed Purina Laboratory Chow (No. 5001) for the first week after arriving in the laboratory. They were then fed modifications of the basic AIN-76 formula (10-12) (see Table 1). Xanthan gum (0.94%) was added to prevent settling of wet diets (11). BHT was added to the corn oil (0.01% of oil) in all diets to retard spoilage. Wet diets were made fresh daily by mixing 289.6 g diet with 710.4 ml deionized water. The concentration of SOA using 0.145% of wet diet (0.5% of solids) was chosen on the basis of pilot and previously published experiments (12). Food was offered in glass jars. Food intake was measured by weighing the food jars daily for the wet diets and three times a week for the dry diets. Spillage was collected and weighed. An extra jar was placed in an empty cage to estimate evaporative water loss for the wet diets. Additional details on diet formulation and food intake measurement can be found in previous publications (10-12).
Statistical Analysis Statistical methods were similar to those used in previous
387
388
RAMIREZ
TABLE 1
o-
Plain
- "-
SOA
COMPOSITION OF DIETS*
Ingredient
Plain
SOA
200 650 50 50 35 10 3 2 0.9375 0
200 650 50 50 35 10 3 2 0.9375 5
Total weight
1000.9375
1005.9375
Total energy
3850
3850
21 68 12
2I 68 12
a
13
450
Casein Cornstarch Corn oil (with 0.01% BHT) Cellulose AIN minerals AIN vitamins DL-Methionine Choline bitartrate Xantban gum Sucrose octaacetate (SOA)
*The diets are identical to those previously described in (7, 8, 11). Casein, corn starch, methionine and minerals were procured from Teklad, Madison, WI. Alphacel (cellulose), vitamins and choline were procured from ICN Biochemicals, Cleveland, OH. Xanthan gum and sucrose octaacetate were obtained from Sigma Chemical, St. Louis, MO. Other items were obtained locally: Crisco Shortening (Procter & Gamble, Cincinnati, OH) and Mazola corn oil (CPC International, Englewood Cliffs, NJ). Wet diets were prepared by mixing 289.6 g of dry diet with 710.4 ml of deionized water.
studies (10-12). Analysis of covariance, using initial body weight as the covariate, was used for energy intake and weight gain in Experiments 1, 2, and 4. In all other cases, conventional analysis of variance was used. In experiments involving more than two groups, significant F-tests were followed up with t-tests (using the pooled error variance) to identify the sources of the overall significance. Results are given as means---SEM. EXPERIMENT
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Percent of energy as: Protein Carbohydrate Fat
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Weeks FIG. l. Weekly energy intake and cumulative weight gain in rats fed unflavored (Plain) or diet containing 0.5% SOA (SOA) in Experiment 1. Intake at week 0 represents the last week both groups were fed plain wet diet before the SOA group was switched to the SOA diet. Means - SEM. "The two groups differed, p
0031-9384/91 $3.00 + .00
Influence of Experience on Response to Bitter Taste ISRAEL RAMIREZ
Monell Chemical Senses Center, 3500 Market Street, Philadelphia, PA 19104-3308 R e c e i v e d 20 April 1990
RAMIREZ, I. Influence of experience on response to bitter taste. PHYSIOL BEHAV 49(2) 387-391, 1991.--Sucrose octaacetate, which tastes bitter to humans, can reduce the energy intake of rats when added to their diet. The reduction in energy intake is transient, lasting no more than 5 weeks. Rats that no longer reduce intake in response to sucrose octaacetate still avoid food containing it in choice tests, although to a lesser degree than rats having no previous experience with sucrose octaacetate. The ability of sucrose octaacetate to reduce preference without reducing long-term intake accounts for the previous finding that a wet diet containing sucrose octaacetate can stimulate hyperphagia yet be less preferred than the control diet. Sucrose octaacetate was more effective in reducing intake of a wet than of a dry diet. This last observation indicates that adding water to a food makes it easier for an animal to taste its food. Sucrose octaacetate
Taste
Dietary hyperphagia
Wet diets
only one food, rats ate more and gained more weight when fed the nonpreferred wet diet. A goal of the present experiments was to get better insight into how a diet could stimulate intake yet not be preferred in a choice test.
THE most direct approach to analyzing the role of the chemical senses in the control of food intake is to examine the effects of adding artificial flavors to foods. In general, studies employing artificial flavors have not provided much evidence that artificial flavors can alter long-term intake (2, 4, 8, 9, 12, 15-18). Some flavors that do affect ingestion (e.g., quinine) appear to do so by virtue of their postingestive effects (2-4, 17). Other flavors (e.g., sucrose octaacetate, curry) decrease long-term intake only when given at concentrations that would seem excessive to a human (17,18). Recent studies showed that the effects of saccharin on longterm energy intake depend on the animals' previous experience and the water content of the diet. Adding saccharin to the diet increased energy intake only for rats that had no previous experience with either saccharin or the experimental diet. Rats that had an opportunity to ingest either the plain diet or saccharin in water before getting sweetened diet did not overeat when given the sweetened diet (13). The importance of water content was shown by the finding that rats fed a diet high in water (80%) consumed more energy when fed a sweetened diet, whereas rats fed a diet containing less water (60%) did not consume more food when fed a sweetened diet (13). The present experiments were undertaken, in part, to determine if prior experience and moisture content have an equally strong effect on response to another flavor, sucrose octaacetate (SOA). This substance is aversive to rats (2-4, 6, 12, 16-18), and tastes intensely bitter to humans (5), but according to the papers cited, probably is not very toxic. Whether SOA can be regarded as completely nontoxic at the concentrations used in previous studies (up to 20% of the diet) is unknown. Earlier studies in my laboratory with SOA emphasized the ability of this substance to reduce food preference without blocking the hyperphagic effects of wet diets (12). Rats preferred a plain dry diet over a SOA-adulterated wet diet, yet when given
GENERALMETHOD
Animals A total of 69 female CD rats (Crl:CD®BR, Charles River, Wilmington, MA) were used. There were always 9-10 rats/group. The rats were maintained on a 12-h light/dark cycle at approximately 23°C, fed food and water ad lib, and individually housed in hanging stainless steel cages.
Diets Rats were fed Purina Laboratory Chow (No. 5001) for the first week after arriving in the laboratory. They were then fed modifications of the basic AIN-76 formula (10-12) (see Table 1). Xanthan gum (0.94%) was added to prevent settling of wet diets (11). BHT was added to the corn oil (0.01% of oil) in all diets to retard spoilage. Wet diets were made fresh daily by mixing 289.6 g diet with 710.4 ml deionized water. The concentration of SOA using 0.145% of wet diet (0.5% of solids) was chosen on the basis of pilot and previously published experiments (12). Food was offered in glass jars. Food intake was measured by weighing the food jars daily for the wet diets and three times a week for the dry diets. Spillage was collected and weighed. An extra jar was placed in an empty cage to estimate evaporative water loss for the wet diets. Additional details on diet formulation and food intake measurement can be found in previous publications (10-12).
Statistical Analysis Statistical methods were similar to those used in previous
387
388
RAMIREZ
TABLE 1
o-
Plain
- "-
SOA
COMPOSITION OF DIETS*
Ingredient
Plain
SOA
200 650 50 50 35 10 3 2 0.9375 0
200 650 50 50 35 10 3 2 0.9375 5
Total weight
1000.9375
1005.9375
Total energy
3850
3850
21 68 12
2I 68 12
a
13
450
Casein Cornstarch Corn oil (with 0.01% BHT) Cellulose AIN minerals AIN vitamins DL-Methionine Choline bitartrate Xantban gum Sucrose octaacetate (SOA)
*The diets are identical to those previously described in (7, 8, 11). Casein, corn starch, methionine and minerals were procured from Teklad, Madison, WI. Alphacel (cellulose), vitamins and choline were procured from ICN Biochemicals, Cleveland, OH. Xanthan gum and sucrose octaacetate were obtained from Sigma Chemical, St. Louis, MO. Other items were obtained locally: Crisco Shortening (Procter & Gamble, Cincinnati, OH) and Mazola corn oil (CPC International, Englewood Cliffs, NJ). Wet diets were prepared by mixing 289.6 g of dry diet with 710.4 ml of deionized water.
studies (10-12). Analysis of covariance, using initial body weight as the covariate, was used for energy intake and weight gain in Experiments 1, 2, and 4. In all other cases, conventional analysis of variance was used. In experiments involving more than two groups, significant F-tests were followed up with t-tests (using the pooled error variance) to identify the sources of the overall significance. Results are given as means---SEM. EXPERIMENT
a
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o ,¢
350
,T. . . . . ~
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±
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0
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,
1
2
3
4
5
6
7
1
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$
4
5
6
7
A
~
Percent of energy as: Protein Carbohydrate Fat
a
®
75
C
,~
.>
5o
25
.6.-'
Weeks FIG. l. Weekly energy intake and cumulative weight gain in rats fed unflavored (Plain) or diet containing 0.5% SOA (SOA) in Experiment 1. Intake at week 0 represents the last week both groups were fed plain wet diet before the SOA group was switched to the SOA diet. Means - SEM. "The two groups differed, p
