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Bitter and sweet components of ethanol taste in humans
Drug and Alcohol Dependence 60 (2000) 199 – 206 www.elsevier.com/locate/drugalcdep
Bitter and sweet components of ethanol taste in humans Anna Scinska a,d, Eliza Koros c,d, Boguslaw Habrat b, Andrzej Kukwa a, Wojciech Kostowski c,e, Przemyslaw Bienkowski c,d,* a Department of Otolaryngology, Warsaw Medical Uni6ersity, Warsaw, Poland Department of Pre6ention and Treatment of Addictions, Institute of Psychiatry and Neurology, ul. Sobieskiego 1 /9, PL-02957 Warsaw, Poland c Department of Pharmacology and Physiology of the Ner6ous System, Institute of Psychiatry and Neurology, ul. Sobieskiego 1 /9, PL-02957 Warsaw, Poland d Clinical Psychopharmacology Laboratory, National Centre of Sport Medicine, Warsaw, Poland e Department of Experimental and Clinical Pharmacology, Warsaw Medical Uni6ersity, Warsaw, Poland b
Received 27 August 1999; received in revised form 26 October 1999; accepted 10 December 1999
Abstract This study examined taste descriptions elicited by ethanol and by other tastants in humans. All subjects described 10% ethanol as bitter and :30% of the subjects described it as sweet and/or sour. Highly significant correlations were found between sweetness of some sucrose solutions (0.6–1%) and intensity of the taste of ethanol. In another experiment, quinine (bitter) solutions were rated as similar to 10% ethanol taste and this effect was potentiated by the addition of sucrose. In contrast, citric acid (sour) tended to decrease similarity ratings when added to the quinine solutions. Taken together, these findings suggest that: (1) in humans ethanol tastes both bitter and sweet; and (2) the relationship between sucrose and ethanol intakes previously found in animals and humans may result, at least partially, from similar taste responses elicited by sucrose and ethanol. © 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Ethanol; Taste perception; Bitter; Sweet; Sour; Humans
1. Introduction It has been repeatedly reported that rodents with a high preference for sweet substances (e.g. saccharin or sucrose) consume more ethanol (alcohol) than rats or mice with a low preference for these substances (Gosnell and Krahn, 1992; Bachmanov et al., 1996; for review, see Kampov-Polevoy et al., 1999). The above correlation has been reported for both genetically selected alcohol preferring (Sinclair et al., 1992; Stewart et al., 1994; Kampov-Polevoy et al., 1996) and outbred strains of rats (Gosnell and Krahn, 1992; Koros et al., 1998; Kampov-Polevoy et al., 1999). The interaction between the intake of sweet substances and the preference for ethanol seems not to be limited to rodents. An increased ethanol intake among adolescents was correlated with an increased intake of sugars and sweets * Corresponding author. Tel.: +48-22-8427644; fax: + 48-226425375.
(Yamamoto et al., 1991). More recently, KampovPolevoy et al. (1997) have found that compared to controls, more alcoholic subjects preferred high sucrose concentrations. At least two hypothesis could explain the mechanism of association between sweets consumption and alcohol drinking. First, one could speculate that a common neurochemical substrate in the CNS mediates rewarding properties of sweets and ethanol (Kampov-Polevoy et al., 1999). For example, there is a growing body of evidence linking excessive consumption of sweets and alcohol to the brain opioid system (Lynch, 1986; Gosnell and Majchrzak, 1989; Herz, 1997; Bienkowski et al., 1999). The second hypothesis deals with peripheral taste factors. Ethanol-associated taste cues have been repeatedly proposed to determine ethanol drinking behaviour especially in alcohol-naive individuals (Richter, 1941; Fromme and Samson, 1983; Di Lorenzo et al., 1986). In line with the above suggestions, Samson et al. (1989)
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have shown that first exposure to ethanol taste is aversive in more than 40% of men and women and that this aversive response tends to predict future abstinence. Electrophysiological and behavioural studies in non-human subjects have suggested that ethanol taste has both bitter and sweet components (Di Lorenzo et al., 1986; Kiefer and Lawrence, 1988; Hellekant et al., 1997). Also preliminary experiments with human subjects have revealed that low ethanol concentrations may produce some sweet sensations (Wilson et al., 1973). Given the above, one could assume that the relationship between sweets preference and ethanol intake results, at least partially, from similar taste responses to sweets and alcohol (for recent discussion, see Bachmanov et al., 1996). The purpose of the present study was twofold. First, we aimed to examine descriptive responses to ethanol taste in healthy human volunteers. For this reason two experiments were done. In experiment 1, participants described taste sensations produced by different concentrations of ethanol. In experiment 2, the subjects assessed similarity of tastant mixtures to ethanol. The second aim of the present study was to search for correlations between responses to sucrose and ethanol taste. Thus, in experiment 1 reactivity to sucrose taste was also analysed.
2. Methods
2.1. Subjects Twenty healthy volunteers (eight males, 12 nonpregnant females) participated in the study. All subjects consumed at least one standard drink per week and were classified as social drinkers. Eight participants were also regular cigarette smokers. Their mean (9 S.D.) Fagerstro¨m Tolerance Questionnaire (Fagerstro¨m, 1978) score was 3.5 ( 91.2). Table 1 presents basic characteristics of the participants including parameters of cigarette consumption in the smoking group. Candidates for the study were recruited from a Warsaw university community and staff of the Institute of Psychiatry and Neurology through poster advertisements and word-of-mouth referrals. Candidates were
excluded if they had a significant medical or psychiatric problem other than nicotine dependence. Care was taken to eliminate individuals with disorders known to alter gustatory function (Naumann, 1993; Cullen and Leopold, 1999). The participants read and signed an informed consent form prior to the initiation of the experiments. All experimental procedures were reviewed and approved by a local Ethics Committee.
2.2. Procedures 2.2.1. Experiment 1 — intensity and pleasantness of sucrose and ethanol taste All subjects participated first in experiment 1 which was designed as a single taste test. The taste tests were conducted between 10:00 and 13:00 h in a quiet room. The participants were asked to refrain from eating, drinking or smoking for at least 1 h prior to the test session. Before the test, the subjects were familiarised with all procedures and scales. Sucrose (0.3, 0.6, 1, 3, 10, 30%; Krasnystaw Sugar Refinery, Krasnystaw, Poland) solutions were prepared with distilled water. Ethanol solutions (0.3, 0.6, 1, 3, 6, 10%, v/v) were prepared from 96% ethanol (Rectified Spirit, commercially available; PPH Jurbo-Agro, Wroclaw, Poland) and distilled water. All solutions were made on the day of administration and stored at room temperature. Increasing concentrations of sucrose or ethanol were administered in a volume of 1 ml on the anterior tongue from single-use syringes. [Preliminary experiments revealed that the above volume and concentrations of ethanol samples were enough to produce clear-cut taste responses without significant olfactory and burning sensations (see also Wilson et al., 1973).] The subject was asked to taste each sample within the entire oral cavity and to rate intensity (‘How intense is the taste?’) and pleasantness (‘How pleasant is the taste?’) on 100-mm lines labelled at the ends for intensity ‘not at all’ and ‘extremely’ (scored 0–100) and for pleasantness ‘extremely unpleasant’ and ‘extremely pleasant’ (scored − 50 to 50). In addition, each participant was required to describe the sample using four basic taste categories (bitter, salty, sour, and sweet). The subject was instructed to use as many categories as necessary. The testing of each sample was separated by 60 s during which the subjects filled response forms,
Table 1 Baseline characteristics of subjectsa
Age (years) Cigarettes/day Years smoked a
All participants (n= 20)
Males (n =8)
Females (n = 12)
Non-smokers (n =12)
Smokers (n= 8)
23.99 2.8
23.99 3.3
24.0 92.4
24.2 93.2
23.6 9 2.1 8.7 9 5 5.4 9 2.4
Data presented as mean 9 S.D.
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rinsed their mouths with distilled water, and waited for the next sample. The order of ethanol and sucrose administration (i.e. ethanol – sucrose or sucrose– ethanol) was randomised across all subjects.
The Spearman R-test was employed to search for possible correlations between responses to sucrose and ethanol taste. Due to a risk of a Type I error, P B0.01 was considered significant in experiment 1.
2.2.2. Experiment 2 — taste similarity assessment The procedure used in experiment 2 consisted of two phases (ethanol/water taste discrimination and taste similarity tests). The aim of the discrimination phase was to familiarise the subjects with ethanol taste. Before the discrimination phase the participants received 1-ml samples of 10% ethanol and distilled water and were informed about the actual content of the syringes. Then, ethanol or water was given in a random order and the subjects were asked to identify the samples. The subjects continued to be trained under these conditions until they exhibited a discrimination criterion that was defined as correct recognition of four consecutive samples. Immediately after the discrimination phase the taste similarity tests started. In the similarity test the subjects received 1-ml samples of different tastants and were asked to assess its similarity to the taste of 10% ethanol on a 100-mm line [labelled ‘not similar at all’ (0) and ‘identical’ (100)]. Six different test samples were given in a random order. To facilitate similarity assessment the participants were required to recognise 10% ethanol and water before administration of each test sample. To validate the whole procedure, similarity of different ethanol solutions (0, 1, 3, and 10%) to the taste of 10% ethanol was first examined. To confirm specificity of the procedure physiological saline (0.9% NaCl; Polfa, Lublin, Poland) was administered. (Preliminary experiments revealed that 0.9% NaCl and 10% ethanol elicited comparable intensity ratings.) Other test samples were selected on the basis of experiment 1. Three quinine hydrochloride solutions (bitter tastant; 0.001, 0.002, 0.005%; Polfa, Warsaw, Poland) were given either alone or in combination with 3% sucrose (sweet) or 0.1% citric acid (sour; Libella, Kotyn, Poland). Each tastant (or mixture of tastants) was administered in a volume of 1 ml to a group of 6 – 9 subjects. Some of the subjects participated in more than one similarity assessment session.
2.3.2. Experiment 2 An analysis of variance (ANOVA) with repeated measures where appropriate was used to analyse the data from the taste similarity tests. Newman–Keuls test was chosen for post hoc comparisons. The Student’s t-test was employed when the data from only two groups were compared.
2.3. Statistics 2.3.1. Experiment 1 Frequencies of assignment of taste descriptors (bitter, sour, salty, sweet) were compared at each concentrations of ethanol, with equal frequencies of all four descriptors being the expected values, using Fisher exact probability test. Frequencies of assignment of each descriptor were also compared between ethanol concentrations.
3. Results
3.1. Experiment 1 3.1.1. Intensity and pleasantness of sucrose and ethanol taste — effects of gender and smoking status The ANOVA did not reveal any effect of gender or smoking status on responses to either ethanol or sucrose solutions (all FsB 1, Ps\0.2; data not shown). Fig. 1 presents mean intensity (A,B) and pleasantness (C,D) of the taste of different sucrose and ethanol concentrations. In addition, number of subjects assigning highest pleasantness score to a given concentration is shown in parentheses. 3.1.2. Subjecti6e e6aluation of ethanol taste All subjects identified bitter sensations in the taste of ethanol samples. For example, 10% ethanol, i.e. the concentration used in experiment 2 (see below), was described as bitter only, bitter and sweet, bitter and sour, and bitter and sour and sweet by nine (45%), five (25%), four (20%) and two (10%) participants, respectively (Table 2). Fisher exact test indicated that frequencies of bitterness descriptions were significantly higher than expected (PB 0.01) for 6 and 10% ethanol. A similar trend (0.01B PB 0.05) was found for 1 and 3% ethanol. The level of saltiness descriptions was significantly lower (PB 0.01) than expected for 10% ethanol. A similar trend (0.01 B PB 0.05) was found for 1 and 3% ethanol. Frequencies of assignment of each descriptor were also compared between ethanol concentrations. Saltiness descriptions were significantly less frequent after 10% ethanol than after 0.6% ethanol administration. In addition, frequency of bitterness descriptions tended to be higher (0.01B PB 0.05) after 6 and 10% ethanol than after 0.6% ethanol administration. On the basis of the above findings (see also Table 2) bitter–sweet and bitter–sour mixtures were selected for experiment 2.
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Fig. 1. Mean ( 9 S.E.M.) intensity (A, B) and pleasantness (C, D) ratings of sucrose and ethanol taste in healthy human volunteers (n= 20). Number of subjects assigning highest pleasantness score to a given concentration is shown in parentheses.
3.1.3. Correlations between indi6idual responses to ethanol and sucrose taste Perceived intensities of lower sucrose concentrations ( 5 3%) were significantly and positively correlated with intensities of ethanol taste (Table 3). This relationship was particularly strong (P B 0.0001) for 0.6 – 1% sucrose and 3–10% ethanol. Notably, responses to higher sucrose concentrations did not predict intensity of ethanol taste. Fig. 2 presents examples of significant and nonsignificant correlation between sucrose and ethanol taste intensity. Hedonic responses to sucrose and ethanol taste (pleasantness) were not correlated at all (data not shown). 3.2. Experiment 2 3.2.1. Taste similarity tests — 6alidation of the procedure All subjects participating in experiment 2 learned to discriminate the taste of ethanol from the taste of water during the first 4–6 trials of the discrimination phase.
Table 2 Descriptions of the taste of different ethanol concentrationsa Ethanol concentration (%)
Bitter alone Sour alone Salty alone Sweet alone Bitter and sour Bitter and salty Bitter and sweet Salty and sour Salty and sweet Sour and sweet Bitter and salty and sour Bitter and sour and sweet
0.3
0.6
1
3
6
10
8 1 0 3 2 0 2 1 1 2 0 0
7 1 1 1 2 0 2 4 1 1 0 0
11 3 0 1 2 0 1 0 1 1 0 0
7 1 0 1 3 0 5 0 1 0 1 1
11 0 0 0 3 2 2 0 0 2 0 0
9 0 0 0 4 0 5 0 0 0 0 2
a Only those taste descriptions which were reported by at least one participant are considered; values are numbers of participants reporting a given taste sensation after administration of ethanol (total n =20 subjects).
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Table 3 Correlations between reported intensity of sucrose and ethanol tastea Ethanol
Sucrose 0.3%
0.3% 0.6% 1% 3% 6% 10% a
0.6%
1.0%
3%
10%
30%
R
P-value
R
P-value
R
P-value
R
P-value
R
P-value
R
P-value
0.69 0.56 0.60 0.55 0.52 0.52
B0.01 0.045 0.011 0.045 0.06 0.06
0.61 0.57 0.41 0.80 0.70 0.79
B0.01 0.02 0.11 B0.0001 B0.001 B0.0001
0.64 0.49 0.36 0.80 0.73 0.81
B0.01 0.06 0.16 B0.0001 B0.001 B0.0001
0.43 0.72 0.61 0.26 0.45 0.57
0.08 B0.001 0.01 0.26 0.05 B0.01
0.36 0.60 0.43 0.13 0.20 0.42
0.15 0.01 0.09 0.58 0.37 0.07
0.39 0.40 0.39 0.20 0.27 0.38
0.12 0.12 0.11 0.38 0.23 0.10
R, correlation coefficient; PB0.01 was considered significant in experiment 1 (see Section 2.3); n =20 subjects.
Fig. 2. (A) Significant positive correlation (R= 0.81, PB 0.0001) between intensity ratings of 1% sucrose and 10% ethanol taste. (B) Non-significant correlation (R =0.20, P\ 0.35) between intensity ratings of 10% sucrose and 6% ethanol (n = 20 subjects).
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Not surprisingly, taste sensations produced by 10% ethanol in the taste similarity tests were rated as highly similar to the taste of 10% ethanol (similarity ratings exceeded 80% for every subject). Distilled water did not mimic ethanol taste (similarity ratings did not exceed 20%). The ANOVA revealed that increasing concentrations of ethanol produced concentration-dependent ethanollike responses [F(3,25) =33.59, P B 0.001; Fig. 3]. In contrast, saline did not mimic the taste of ethanol (P\ 0.4; Student’s t-test; Fig. 3).
3.2.2. Taste similarity tests — effects of bitter– sweet and bitter–sour mixtures All quinine solutions produced significant ethanollike responses [F(3,26) = 4.65, P B0.01; Fig. 4]. Addition of sucrose to quinine solutions moderately but significantly increased similarity ratings [F(1,26) = 8.94, P B0.01]. Notably, 3% sucrose solution produced significant ethanol-like effects (Fig. 4A). Thus, the interaction between quinine and sucrose taste seemed to be hypoadditive. Although 0.1% citric acid induced some ethanol-like taste sensations it tended to decrease ethanol-like effects of quinine (Fig. 4B). This observation was partially confirmed by the ANOVA which indicated a significant Quinine concentration×Citric acid interaction [F(3,20)= 3.40, P B 0.05]. 4. Discussion In agreement with al., 1980; Hyde and but see also Conner major differences in
previous reports (Weizenbaum et Feller, 1981; Kunka et al., 1981; and Booth, 1988), there were no responses to sweet taste between
Fig. 3. Mean (9 S.E.M.) similarity ratings after administration of different ethanol concentrations or physiological saline (0.9% NaCl). Subjects trained to discriminate ethanol taste from water assessed similarity of different ethanol concentrations and saline to the taste of 10% ethanol. **PB 0.01 versus water (n= 6–9 subjects).
Fig. 4. Mean ( 9S.E.M.) similarity ratings produced by quinine given alone or in combination with 3% sucrose (A) or 0.1% citric acid (B). Subjects trained to discriminate ethanol taste from water assessed similarity of quinine, quinine + sucrose or quinine + citric acid solutions to the taste of 10% ethanol. **PB 0.01 versus water; c PB 0.05 versus 0.005% quinine; suc, sucrose; cit, citric acid (n =6 – 9 subjects).
males and females in the present study. Similarly, we did not find any differences in sweet taste reactivity between smokers and non-smokers. Several investigators have shown that smoking status does not alter sweet taste intensity (Peterson et al., 1968; Redington, 1984; Perkins et al., 1990; Pomerleau et al., 1991). In contrast, studies on hedonics of sweet taste in smokers has brought conflicting results (Rodin, 1987; Perkins et al., 1990; Pomerleau et al., 1991). According to our knowledge this is the first paper reporting no effects of gender and smoking status on subjective responses to ethanol taste. Large-scale epidemiological studies have indicated that more males that females meet the criteria of harmful alcohol consumption (York and Welte, 1994; Fergusson et al., 1995; Single et al., 1995). Similarly, cigarette smoking
A. Scinska et al. / Drug and Alcohol Dependence 60 (2000) 199–206
increases the risk of developing abuse or dependence on alcohol (for review see Shiffman and Balabanis, 1996; Sobell and Sobell, 1996). The results of the present study would suggest that taste factors may not be responsible for the above interactions. However, more experiments with higher ethanol concentrations are needed to resolve this issue. Besides, it is possible that only long and heavy cigarette smoking may alter perception of ethanol taste. (The subjects in the present study were at best moderately dependent on nicotine.) Intensity of ethanol taste was significantly and positively correlated with intensity (sweetness) of some sucrose solutions. Interestingly, this relationship was particularly strong when responses to the highest concentrations of ethanol were compared with responses to low (0.6–1%) sucrose concentrations. On the other hand, there was no correlation between sweetness of the highest sucrose concentrations (10 – 30%) and intensity of ethanol taste. Thus, it seems that the positive relationship between intensities of sucrose and ethanol taste did not result from higher reactivity of some subjects to every tastant. The above findings would rather suggest that in humans ethanol activates some nerve fibres sensitive to low sugar concentrations. Bearing in mind frequencies of taste descriptions found in experiment 1, one could hypothesise that any effects of ethanol on sugar-sensitive fibres may be too weak to induce sweetness descriptions but sufficiently strong to determine intensity of ethanol taste. All subjects identified the bitter component in ethanol taste. For example, descriptions of the taste of 10% ethanol belonged in four categories: ‘bitter’, ‘bitter +sour’, ‘bitter+ sweet’ and ‘bitter +sour + sweet’. Results of several preclinical studies have suggested the existence of both bitter and sweet components in ethanol taste (Di Lorenzo et al., 1986; Hellekant et al., 1997). Wilson et al. (1973) have suggested that the taste of ethanol was primarily sweet for their human subjects. However, it seems that the magnitude of the sweet component might be overestimated in this latter report. Although the authors stated that: ‘…sometimes, however, the taste of the alcohol mixture may be described as being like an almond, or slightly bitter…’ (Wilson et al., 1973), percentages of different taste responses were not shown. In experiment 2, another procedure was used to examine similarity between ethanol taste and tastes of other substances administered singly or in mixtures. In line with some animal studies (Di Lorenzo et al., 1986; Kiefer and Lawrence, 1988) and the results of experiment 1, quinine solutions were rated as similar to 10% ethanol. Addition of sucrose moderately increased similarity of quinine to 10% ethanol. Thus, it seems that the taste of ethanol in humans may include both bitter and sweet components. In contrast, citric acid tended to decrease ethanol-like effects induced by quinine. Nota-
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bly, some subjects in experiment 1 identified a sour component in ethanol taste. However, when these individuals were interviewed after the session they tended to describe the sour component of ethanol taste as mild burning sensations. Di Lorenzo et al. (1986) have reported that rats with learned taste aversion to ethanol solutions generalised this aversion to bitter–sweet and to a lesser extent to sweet–sour mixtures. Although obvious species differences limit direct comparison between the studies, one could hypothesise that citric acid given in combination with sucrose (and not quinine) would more closely mimic ethanol taste in our subjects. Future experiments might also determine if addition of sucrose and/or quinine increases 10% ethanol-appropriate responses induced by lower ethanol concentrations. Taken together, the results of the present study suggest that: (1) in humans the taste of ethanol has both bitter and sweet components; and (2) the relationship between sucrose and ethanol intake previously found in animal and human studies may result from similar taste responses elicited by sucrose and ethanol taste.
Acknowledgements This study was supported by PARPA (grant Alc 18/99) and the State Committee for Scientific Research (KBN grant no. 4 PO5A 083 17).
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Bitter and sweet components of ethanol taste in humans Anna Scinska a,d, Eliza Koros c,d, Boguslaw Habrat b, Andrzej Kukwa a, Wojciech Kostowski c,e, Przemyslaw Bienkowski c,d,* a Department of Otolaryngology, Warsaw Medical Uni6ersity, Warsaw, Poland Department of Pre6ention and Treatment of Addictions, Institute of Psychiatry and Neurology, ul. Sobieskiego 1 /9, PL-02957 Warsaw, Poland c Department of Pharmacology and Physiology of the Ner6ous System, Institute of Psychiatry and Neurology, ul. Sobieskiego 1 /9, PL-02957 Warsaw, Poland d Clinical Psychopharmacology Laboratory, National Centre of Sport Medicine, Warsaw, Poland e Department of Experimental and Clinical Pharmacology, Warsaw Medical Uni6ersity, Warsaw, Poland b
Received 27 August 1999; received in revised form 26 October 1999; accepted 10 December 1999
Abstract This study examined taste descriptions elicited by ethanol and by other tastants in humans. All subjects described 10% ethanol as bitter and :30% of the subjects described it as sweet and/or sour. Highly significant correlations were found between sweetness of some sucrose solutions (0.6–1%) and intensity of the taste of ethanol. In another experiment, quinine (bitter) solutions were rated as similar to 10% ethanol taste and this effect was potentiated by the addition of sucrose. In contrast, citric acid (sour) tended to decrease similarity ratings when added to the quinine solutions. Taken together, these findings suggest that: (1) in humans ethanol tastes both bitter and sweet; and (2) the relationship between sucrose and ethanol intakes previously found in animals and humans may result, at least partially, from similar taste responses elicited by sucrose and ethanol. © 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Ethanol; Taste perception; Bitter; Sweet; Sour; Humans
1. Introduction It has been repeatedly reported that rodents with a high preference for sweet substances (e.g. saccharin or sucrose) consume more ethanol (alcohol) than rats or mice with a low preference for these substances (Gosnell and Krahn, 1992; Bachmanov et al., 1996; for review, see Kampov-Polevoy et al., 1999). The above correlation has been reported for both genetically selected alcohol preferring (Sinclair et al., 1992; Stewart et al., 1994; Kampov-Polevoy et al., 1996) and outbred strains of rats (Gosnell and Krahn, 1992; Koros et al., 1998; Kampov-Polevoy et al., 1999). The interaction between the intake of sweet substances and the preference for ethanol seems not to be limited to rodents. An increased ethanol intake among adolescents was correlated with an increased intake of sugars and sweets * Corresponding author. Tel.: +48-22-8427644; fax: + 48-226425375.
(Yamamoto et al., 1991). More recently, KampovPolevoy et al. (1997) have found that compared to controls, more alcoholic subjects preferred high sucrose concentrations. At least two hypothesis could explain the mechanism of association between sweets consumption and alcohol drinking. First, one could speculate that a common neurochemical substrate in the CNS mediates rewarding properties of sweets and ethanol (Kampov-Polevoy et al., 1999). For example, there is a growing body of evidence linking excessive consumption of sweets and alcohol to the brain opioid system (Lynch, 1986; Gosnell and Majchrzak, 1989; Herz, 1997; Bienkowski et al., 1999). The second hypothesis deals with peripheral taste factors. Ethanol-associated taste cues have been repeatedly proposed to determine ethanol drinking behaviour especially in alcohol-naive individuals (Richter, 1941; Fromme and Samson, 1983; Di Lorenzo et al., 1986). In line with the above suggestions, Samson et al. (1989)
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have shown that first exposure to ethanol taste is aversive in more than 40% of men and women and that this aversive response tends to predict future abstinence. Electrophysiological and behavioural studies in non-human subjects have suggested that ethanol taste has both bitter and sweet components (Di Lorenzo et al., 1986; Kiefer and Lawrence, 1988; Hellekant et al., 1997). Also preliminary experiments with human subjects have revealed that low ethanol concentrations may produce some sweet sensations (Wilson et al., 1973). Given the above, one could assume that the relationship between sweets preference and ethanol intake results, at least partially, from similar taste responses to sweets and alcohol (for recent discussion, see Bachmanov et al., 1996). The purpose of the present study was twofold. First, we aimed to examine descriptive responses to ethanol taste in healthy human volunteers. For this reason two experiments were done. In experiment 1, participants described taste sensations produced by different concentrations of ethanol. In experiment 2, the subjects assessed similarity of tastant mixtures to ethanol. The second aim of the present study was to search for correlations between responses to sucrose and ethanol taste. Thus, in experiment 1 reactivity to sucrose taste was also analysed.
2. Methods
2.1. Subjects Twenty healthy volunteers (eight males, 12 nonpregnant females) participated in the study. All subjects consumed at least one standard drink per week and were classified as social drinkers. Eight participants were also regular cigarette smokers. Their mean (9 S.D.) Fagerstro¨m Tolerance Questionnaire (Fagerstro¨m, 1978) score was 3.5 ( 91.2). Table 1 presents basic characteristics of the participants including parameters of cigarette consumption in the smoking group. Candidates for the study were recruited from a Warsaw university community and staff of the Institute of Psychiatry and Neurology through poster advertisements and word-of-mouth referrals. Candidates were
excluded if they had a significant medical or psychiatric problem other than nicotine dependence. Care was taken to eliminate individuals with disorders known to alter gustatory function (Naumann, 1993; Cullen and Leopold, 1999). The participants read and signed an informed consent form prior to the initiation of the experiments. All experimental procedures were reviewed and approved by a local Ethics Committee.
2.2. Procedures 2.2.1. Experiment 1 — intensity and pleasantness of sucrose and ethanol taste All subjects participated first in experiment 1 which was designed as a single taste test. The taste tests were conducted between 10:00 and 13:00 h in a quiet room. The participants were asked to refrain from eating, drinking or smoking for at least 1 h prior to the test session. Before the test, the subjects were familiarised with all procedures and scales. Sucrose (0.3, 0.6, 1, 3, 10, 30%; Krasnystaw Sugar Refinery, Krasnystaw, Poland) solutions were prepared with distilled water. Ethanol solutions (0.3, 0.6, 1, 3, 6, 10%, v/v) were prepared from 96% ethanol (Rectified Spirit, commercially available; PPH Jurbo-Agro, Wroclaw, Poland) and distilled water. All solutions were made on the day of administration and stored at room temperature. Increasing concentrations of sucrose or ethanol were administered in a volume of 1 ml on the anterior tongue from single-use syringes. [Preliminary experiments revealed that the above volume and concentrations of ethanol samples were enough to produce clear-cut taste responses without significant olfactory and burning sensations (see also Wilson et al., 1973).] The subject was asked to taste each sample within the entire oral cavity and to rate intensity (‘How intense is the taste?’) and pleasantness (‘How pleasant is the taste?’) on 100-mm lines labelled at the ends for intensity ‘not at all’ and ‘extremely’ (scored 0–100) and for pleasantness ‘extremely unpleasant’ and ‘extremely pleasant’ (scored − 50 to 50). In addition, each participant was required to describe the sample using four basic taste categories (bitter, salty, sour, and sweet). The subject was instructed to use as many categories as necessary. The testing of each sample was separated by 60 s during which the subjects filled response forms,
Table 1 Baseline characteristics of subjectsa
Age (years) Cigarettes/day Years smoked a
All participants (n= 20)
Males (n =8)
Females (n = 12)
Non-smokers (n =12)
Smokers (n= 8)
23.99 2.8
23.99 3.3
24.0 92.4
24.2 93.2
23.6 9 2.1 8.7 9 5 5.4 9 2.4
Data presented as mean 9 S.D.
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rinsed their mouths with distilled water, and waited for the next sample. The order of ethanol and sucrose administration (i.e. ethanol – sucrose or sucrose– ethanol) was randomised across all subjects.
The Spearman R-test was employed to search for possible correlations between responses to sucrose and ethanol taste. Due to a risk of a Type I error, P B0.01 was considered significant in experiment 1.
2.2.2. Experiment 2 — taste similarity assessment The procedure used in experiment 2 consisted of two phases (ethanol/water taste discrimination and taste similarity tests). The aim of the discrimination phase was to familiarise the subjects with ethanol taste. Before the discrimination phase the participants received 1-ml samples of 10% ethanol and distilled water and were informed about the actual content of the syringes. Then, ethanol or water was given in a random order and the subjects were asked to identify the samples. The subjects continued to be trained under these conditions until they exhibited a discrimination criterion that was defined as correct recognition of four consecutive samples. Immediately after the discrimination phase the taste similarity tests started. In the similarity test the subjects received 1-ml samples of different tastants and were asked to assess its similarity to the taste of 10% ethanol on a 100-mm line [labelled ‘not similar at all’ (0) and ‘identical’ (100)]. Six different test samples were given in a random order. To facilitate similarity assessment the participants were required to recognise 10% ethanol and water before administration of each test sample. To validate the whole procedure, similarity of different ethanol solutions (0, 1, 3, and 10%) to the taste of 10% ethanol was first examined. To confirm specificity of the procedure physiological saline (0.9% NaCl; Polfa, Lublin, Poland) was administered. (Preliminary experiments revealed that 0.9% NaCl and 10% ethanol elicited comparable intensity ratings.) Other test samples were selected on the basis of experiment 1. Three quinine hydrochloride solutions (bitter tastant; 0.001, 0.002, 0.005%; Polfa, Warsaw, Poland) were given either alone or in combination with 3% sucrose (sweet) or 0.1% citric acid (sour; Libella, Kotyn, Poland). Each tastant (or mixture of tastants) was administered in a volume of 1 ml to a group of 6 – 9 subjects. Some of the subjects participated in more than one similarity assessment session.
2.3.2. Experiment 2 An analysis of variance (ANOVA) with repeated measures where appropriate was used to analyse the data from the taste similarity tests. Newman–Keuls test was chosen for post hoc comparisons. The Student’s t-test was employed when the data from only two groups were compared.
2.3. Statistics 2.3.1. Experiment 1 Frequencies of assignment of taste descriptors (bitter, sour, salty, sweet) were compared at each concentrations of ethanol, with equal frequencies of all four descriptors being the expected values, using Fisher exact probability test. Frequencies of assignment of each descriptor were also compared between ethanol concentrations.
3. Results
3.1. Experiment 1 3.1.1. Intensity and pleasantness of sucrose and ethanol taste — effects of gender and smoking status The ANOVA did not reveal any effect of gender or smoking status on responses to either ethanol or sucrose solutions (all FsB 1, Ps\0.2; data not shown). Fig. 1 presents mean intensity (A,B) and pleasantness (C,D) of the taste of different sucrose and ethanol concentrations. In addition, number of subjects assigning highest pleasantness score to a given concentration is shown in parentheses. 3.1.2. Subjecti6e e6aluation of ethanol taste All subjects identified bitter sensations in the taste of ethanol samples. For example, 10% ethanol, i.e. the concentration used in experiment 2 (see below), was described as bitter only, bitter and sweet, bitter and sour, and bitter and sour and sweet by nine (45%), five (25%), four (20%) and two (10%) participants, respectively (Table 2). Fisher exact test indicated that frequencies of bitterness descriptions were significantly higher than expected (PB 0.01) for 6 and 10% ethanol. A similar trend (0.01B PB 0.05) was found for 1 and 3% ethanol. The level of saltiness descriptions was significantly lower (PB 0.01) than expected for 10% ethanol. A similar trend (0.01 B PB 0.05) was found for 1 and 3% ethanol. Frequencies of assignment of each descriptor were also compared between ethanol concentrations. Saltiness descriptions were significantly less frequent after 10% ethanol than after 0.6% ethanol administration. In addition, frequency of bitterness descriptions tended to be higher (0.01B PB 0.05) after 6 and 10% ethanol than after 0.6% ethanol administration. On the basis of the above findings (see also Table 2) bitter–sweet and bitter–sour mixtures were selected for experiment 2.
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Fig. 1. Mean ( 9 S.E.M.) intensity (A, B) and pleasantness (C, D) ratings of sucrose and ethanol taste in healthy human volunteers (n= 20). Number of subjects assigning highest pleasantness score to a given concentration is shown in parentheses.
3.1.3. Correlations between indi6idual responses to ethanol and sucrose taste Perceived intensities of lower sucrose concentrations ( 5 3%) were significantly and positively correlated with intensities of ethanol taste (Table 3). This relationship was particularly strong (P B 0.0001) for 0.6 – 1% sucrose and 3–10% ethanol. Notably, responses to higher sucrose concentrations did not predict intensity of ethanol taste. Fig. 2 presents examples of significant and nonsignificant correlation between sucrose and ethanol taste intensity. Hedonic responses to sucrose and ethanol taste (pleasantness) were not correlated at all (data not shown). 3.2. Experiment 2 3.2.1. Taste similarity tests — 6alidation of the procedure All subjects participating in experiment 2 learned to discriminate the taste of ethanol from the taste of water during the first 4–6 trials of the discrimination phase.
Table 2 Descriptions of the taste of different ethanol concentrationsa Ethanol concentration (%)
Bitter alone Sour alone Salty alone Sweet alone Bitter and sour Bitter and salty Bitter and sweet Salty and sour Salty and sweet Sour and sweet Bitter and salty and sour Bitter and sour and sweet
0.3
0.6
1
3
6
10
8 1 0 3 2 0 2 1 1 2 0 0
7 1 1 1 2 0 2 4 1 1 0 0
11 3 0 1 2 0 1 0 1 1 0 0
7 1 0 1 3 0 5 0 1 0 1 1
11 0 0 0 3 2 2 0 0 2 0 0
9 0 0 0 4 0 5 0 0 0 0 2
a Only those taste descriptions which were reported by at least one participant are considered; values are numbers of participants reporting a given taste sensation after administration of ethanol (total n =20 subjects).
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Table 3 Correlations between reported intensity of sucrose and ethanol tastea Ethanol
Sucrose 0.3%
0.3% 0.6% 1% 3% 6% 10% a
0.6%
1.0%
3%
10%
30%
R
P-value
R
P-value
R
P-value
R
P-value
R
P-value
R
P-value
0.69 0.56 0.60 0.55 0.52 0.52
B0.01 0.045 0.011 0.045 0.06 0.06
0.61 0.57 0.41 0.80 0.70 0.79
B0.01 0.02 0.11 B0.0001 B0.001 B0.0001
0.64 0.49 0.36 0.80 0.73 0.81
B0.01 0.06 0.16 B0.0001 B0.001 B0.0001
0.43 0.72 0.61 0.26 0.45 0.57
0.08 B0.001 0.01 0.26 0.05 B0.01
0.36 0.60 0.43 0.13 0.20 0.42
0.15 0.01 0.09 0.58 0.37 0.07
0.39 0.40 0.39 0.20 0.27 0.38
0.12 0.12 0.11 0.38 0.23 0.10
R, correlation coefficient; PB0.01 was considered significant in experiment 1 (see Section 2.3); n =20 subjects.
Fig. 2. (A) Significant positive correlation (R= 0.81, PB 0.0001) between intensity ratings of 1% sucrose and 10% ethanol taste. (B) Non-significant correlation (R =0.20, P\ 0.35) between intensity ratings of 10% sucrose and 6% ethanol (n = 20 subjects).
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Not surprisingly, taste sensations produced by 10% ethanol in the taste similarity tests were rated as highly similar to the taste of 10% ethanol (similarity ratings exceeded 80% for every subject). Distilled water did not mimic ethanol taste (similarity ratings did not exceed 20%). The ANOVA revealed that increasing concentrations of ethanol produced concentration-dependent ethanollike responses [F(3,25) =33.59, P B 0.001; Fig. 3]. In contrast, saline did not mimic the taste of ethanol (P\ 0.4; Student’s t-test; Fig. 3).
3.2.2. Taste similarity tests — effects of bitter– sweet and bitter–sour mixtures All quinine solutions produced significant ethanollike responses [F(3,26) = 4.65, P B0.01; Fig. 4]. Addition of sucrose to quinine solutions moderately but significantly increased similarity ratings [F(1,26) = 8.94, P B0.01]. Notably, 3% sucrose solution produced significant ethanol-like effects (Fig. 4A). Thus, the interaction between quinine and sucrose taste seemed to be hypoadditive. Although 0.1% citric acid induced some ethanol-like taste sensations it tended to decrease ethanol-like effects of quinine (Fig. 4B). This observation was partially confirmed by the ANOVA which indicated a significant Quinine concentration×Citric acid interaction [F(3,20)= 3.40, P B 0.05]. 4. Discussion In agreement with al., 1980; Hyde and but see also Conner major differences in
previous reports (Weizenbaum et Feller, 1981; Kunka et al., 1981; and Booth, 1988), there were no responses to sweet taste between
Fig. 3. Mean (9 S.E.M.) similarity ratings after administration of different ethanol concentrations or physiological saline (0.9% NaCl). Subjects trained to discriminate ethanol taste from water assessed similarity of different ethanol concentrations and saline to the taste of 10% ethanol. **PB 0.01 versus water (n= 6–9 subjects).
Fig. 4. Mean ( 9S.E.M.) similarity ratings produced by quinine given alone or in combination with 3% sucrose (A) or 0.1% citric acid (B). Subjects trained to discriminate ethanol taste from water assessed similarity of quinine, quinine + sucrose or quinine + citric acid solutions to the taste of 10% ethanol. **PB 0.01 versus water; c PB 0.05 versus 0.005% quinine; suc, sucrose; cit, citric acid (n =6 – 9 subjects).
males and females in the present study. Similarly, we did not find any differences in sweet taste reactivity between smokers and non-smokers. Several investigators have shown that smoking status does not alter sweet taste intensity (Peterson et al., 1968; Redington, 1984; Perkins et al., 1990; Pomerleau et al., 1991). In contrast, studies on hedonics of sweet taste in smokers has brought conflicting results (Rodin, 1987; Perkins et al., 1990; Pomerleau et al., 1991). According to our knowledge this is the first paper reporting no effects of gender and smoking status on subjective responses to ethanol taste. Large-scale epidemiological studies have indicated that more males that females meet the criteria of harmful alcohol consumption (York and Welte, 1994; Fergusson et al., 1995; Single et al., 1995). Similarly, cigarette smoking
A. Scinska et al. / Drug and Alcohol Dependence 60 (2000) 199–206
increases the risk of developing abuse or dependence on alcohol (for review see Shiffman and Balabanis, 1996; Sobell and Sobell, 1996). The results of the present study would suggest that taste factors may not be responsible for the above interactions. However, more experiments with higher ethanol concentrations are needed to resolve this issue. Besides, it is possible that only long and heavy cigarette smoking may alter perception of ethanol taste. (The subjects in the present study were at best moderately dependent on nicotine.) Intensity of ethanol taste was significantly and positively correlated with intensity (sweetness) of some sucrose solutions. Interestingly, this relationship was particularly strong when responses to the highest concentrations of ethanol were compared with responses to low (0.6–1%) sucrose concentrations. On the other hand, there was no correlation between sweetness of the highest sucrose concentrations (10 – 30%) and intensity of ethanol taste. Thus, it seems that the positive relationship between intensities of sucrose and ethanol taste did not result from higher reactivity of some subjects to every tastant. The above findings would rather suggest that in humans ethanol activates some nerve fibres sensitive to low sugar concentrations. Bearing in mind frequencies of taste descriptions found in experiment 1, one could hypothesise that any effects of ethanol on sugar-sensitive fibres may be too weak to induce sweetness descriptions but sufficiently strong to determine intensity of ethanol taste. All subjects identified the bitter component in ethanol taste. For example, descriptions of the taste of 10% ethanol belonged in four categories: ‘bitter’, ‘bitter +sour’, ‘bitter+ sweet’ and ‘bitter +sour + sweet’. Results of several preclinical studies have suggested the existence of both bitter and sweet components in ethanol taste (Di Lorenzo et al., 1986; Hellekant et al., 1997). Wilson et al. (1973) have suggested that the taste of ethanol was primarily sweet for their human subjects. However, it seems that the magnitude of the sweet component might be overestimated in this latter report. Although the authors stated that: ‘…sometimes, however, the taste of the alcohol mixture may be described as being like an almond, or slightly bitter…’ (Wilson et al., 1973), percentages of different taste responses were not shown. In experiment 2, another procedure was used to examine similarity between ethanol taste and tastes of other substances administered singly or in mixtures. In line with some animal studies (Di Lorenzo et al., 1986; Kiefer and Lawrence, 1988) and the results of experiment 1, quinine solutions were rated as similar to 10% ethanol. Addition of sucrose moderately increased similarity of quinine to 10% ethanol. Thus, it seems that the taste of ethanol in humans may include both bitter and sweet components. In contrast, citric acid tended to decrease ethanol-like effects induced by quinine. Nota-
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bly, some subjects in experiment 1 identified a sour component in ethanol taste. However, when these individuals were interviewed after the session they tended to describe the sour component of ethanol taste as mild burning sensations. Di Lorenzo et al. (1986) have reported that rats with learned taste aversion to ethanol solutions generalised this aversion to bitter–sweet and to a lesser extent to sweet–sour mixtures. Although obvious species differences limit direct comparison between the studies, one could hypothesise that citric acid given in combination with sucrose (and not quinine) would more closely mimic ethanol taste in our subjects. Future experiments might also determine if addition of sucrose and/or quinine increases 10% ethanol-appropriate responses induced by lower ethanol concentrations. Taken together, the results of the present study suggest that: (1) in humans the taste of ethanol has both bitter and sweet components; and (2) the relationship between sucrose and ethanol intake previously found in animal and human studies may result from similar taste responses elicited by sucrose and ethanol taste.
Acknowledgements This study was supported by PARPA (grant Alc 18/99) and the State Committee for Scientific Research (KBN grant no. 4 PO5A 083 17).
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