Chemistry of Taste - ACS Publications - American Chemical Society

Chapter 5

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 19, 2016 | http://pubs.acs.org Publication Date: July 19, 2002 | doi: 10.1021/bk-2002-0825.ch005

Genetic Markers, Taste Responses, and Food Preferences Adam Drewnowski Nutritional Sciences Program, School of Public Health and Community Medicine, University of Washington, Seattle, W A 98195

Genetic sensitivity to the bitter taste of 6-n-propylthiouracil (PROP) has been linked with reduced acceptance of other bitter compounds and a dislike of some bitter foods. PROP tasters and "supertasters" were more likely to dislike black coffee, grapefruit juice, green tea, Brussels sprouts and some salad greens. Antioxidant phytochemicals such as flavonoids in citrus fruit, polyphenols in tea and red wine, glucosinolates in cruciferous vegetables, and isoflavones in soy products are almost always bitter. Many of these chemoprotective compounds have been linked to reduced risk of cancer and coronary heart disease. Consumer acceptance of these beneficial phytochemicals may be influenced by genetic taste factors.

Low concentrations of phenylthiocarbamide (PTC) and 6-w-propylthiouracil (PROP) taste bitter to some people but are tasteless to others (J). The ability to taste dilute solutions of PTC/PROP is a heritable trait, thought to be determined by a dominant gene (2). Although phenotypic taste responses to

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© 2002 American Chemical Society Given and Paredes; Chemistry of Taste ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


53 PTC/PROP are well-described in the literature, the gene responsible for this trait has not been described and its exact location is unknown. The earliest studies used taste responses to PTC crystals or to P T C impregnated filter paper to separate tasters from nontasters. PROP filter papers, used to test for genetic taste blindness, were distributed by the American Genetic Association as early as 1931. The proportion of PTC/PROP tasters in a Caucasian population was estimated at 70%. Later studies relied on the bimodal distribution of PROP detection thresholds, determined using the method of solutions (2-4). Women were likely to be more PROP-sensitive than were men. Following on Kalmus' observation (2) that PROP tasters showed a wide range of sensitivity to PROP, Bartoshuk (1994) suggested that the tasters group might include a smaller subsample of "supertasters", identified by low PROP thresholds and a high ratio of perceived PROP bitterness to the perceived saltiness of salt solutions. The speculation was that nontasters had two recessive alleles (tt); medium tasters were hétérozygotes with one dominant allele (Tt); while supertasters had two dominant alleles (TT) (5). Recent genetic studies in humans have provisionally linked the ability to taste PROP with a locus at 5pl5 (6). PROP-tasting was associated with enhanced sensitivity to some, but not all, bitter tastes. Early studies showed that PTC/PROP tasters rated caffeine and quinine, though not urea, as more bitter (7,8). PROP tasters also rated saccharin solutions, at concentrations found in diet soft drinks, as more bitter than did nontasters (9). A more recent time-intensity study (10) found that PROP tasters gave higher aftertaste intensity ratings to a low concentration of caffeine (0.018mol/L) than did nontasters. One interpretation of such findings was that PTC/PROP tasters would avoid coffee and diet soft drinks sweetened with saccharin (11). Later reports that PROP tasting was associated with enhanced oral burn of capsaicin, the active ingredient of hot peppers, led to the suggestion that PROP tasters might also avoid hot and spicy foods (12). PROP tasters were also said to be more sensitive to the trigeminal irritation by ethanol, and reportedly avoided alcoholic beverages (13). Whether PROP-tasting influenced the perceived sweetness or reported liking for sugar solutions remains unclear. In some studies, low concentrations of sucrose and neohesperidin dihydrochalcone (NHDC), an intense sweetener, tasted sweeter to tasters than to nontasters (9,14). PROP tasters were also reported to dislike sweet sucrose solutions (15). However, other studies found no effect of PROP taster status on taste responses to sucrose solutions across a wider range of concentrations (16) or on self-reported preferences for sweet foods (17). Whether the ability to taste PROP predicts sensory response to dietary fats is another unresolved question. Two studies reported that PROP supertasters gave higher "fatness" ratings to a single sample of unsweetened heavy cream

Given and Paredes; Chemistry of Taste ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


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(18) and to one high-fat salad dressing (19). Other studies, based on more than one stimulus, found no effect of PROP taster status on the perceived sweetness, creaminess, or acceptability of sweetened dairy products (20). There is no convincing evidence at this point that PROP tasters dislike high-fat foods. Some studies have even made the claim that the ability to taste PROP protects against obesity, given that some PROP tasters appeared to have lower body mass indices than did nontasters (18,19). However, those studies were limited to very small samples of college students. Epidemiological studies using regression models to account for covariates have failed to find a connection between PROP tasting and body mass. Most researchers agree that PROP tasters are more responsive than nontasters to a number of other bitter compounds and are more likely to dislike some, but not all, bitter foods. Early studies on PROP tasters and nontasters focused on the perceived bitterness of urea, potassium benzoate, or quinine hydrochloride (8,9), though sometimes with inconsistent results. However, these classic stimuli of taste psychophysics have no particular relevance to everyday food choices and eating habits. Instead, we chose to examine the perceived bitterness of dietary phytochemicals (21), including those found in vegetables and fruit. Plant-based phenols and polyphenols, tannins, flavonoids, isoflavones, and glucosinolates are, almost without exception, bitter, acrid or astringent (22). Clinical and epidemiological studies increasingly suggest that such phytochemicals have antioxidant and chemoprotective activity and successfully reduce the risk of cancer and other chronic disease (23,24). Increasing consumption of vegetables and fruit is a major dietary strategy for disease prevention (25). According to the psychosocial literature on dietary choice, taste is the major barrier against the adoption of healthful eating habits (17). Arguably, genetic taste factors that influence food preferences and food choices might reduce dietary exposure to substances known to affect cancer risk. However, since the ability to taste PROP does not confer increased responsiveness to all bitter compounds, it is important to know which compounds are perceived by PROP tasters as more bitter and which are not. A study of genetic taste markers would thus have implications for chronic disease prevention and public health (22).

PROP Tasters and Nontasters

Individual sensitivity to PROP solutions is generally determined using the detection threshold procedure (26). Such procedures have used a series of 15 PROP solutions, ranging in concentration from 1.0 χ 10-6 mol/L to 3.2 χ 10-3 mol/L PROP (8). The highest concentration, solution no. 15, contained 0.5446



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g/L PROP; the next concentration contained 0.3064 g / L , and so on (2,3). In our studies of women (16,17), each participant was first presented with the least concentrated solution of PROP (solution 1), and then with increasingly higher solutions, until she reported detecting a taste distinct from that of water. She was then presented with two identical cups; one containing the detected concentration of PROP and the other containing deionized water. The water was at the same temperature and was stored in the same location as the PROP solution. She was asked to judge which of the two samples had the bitter taste (4, 26-28). Wrong answers led to the presentation of the more concentrated PROP solution, while correct answers led to a second presentation of the same solution. Two consecutive correct answers at the same concentration led to presentation of less concentrated PROP solutions. Reversal points were defined as the concentration at which a series of correct responses turned to an incorrect response or vice versa (26). Detection thresholds were based on a mean of five reversal points. Participants rinsed thoroughly with deionized water after tasting each PROP stimulus. For bitterness intensity scaling, the same respondents tasted and rated 5 more solutions of PROP at concentrations of 0.032, 0.1, 0.32, 1.0, and 3.2 mmol/L (solutions 1,9, 11, 13, and 15). Bitterness of each stimulus was rated using 9-point category scales, where 1 = "not at all bitter" and 9 = "extremely bitter". Respondents also ranked each stimulus along a 9-point hedonic preference scale (29), that ranges from 1 = "dislike extremely" to 9 = "like extremely," with a neutral point at 5 ("neither like nor dislike"). Consistent with previous studies (2), the distribution of PROP detection thresholds was bimodal. Figure 1 summarizes combined data from several studies for a total of 538 women, ranging in age from 18 to 80 years. The antimode fell at solution 9. PROP tasters had thresholds below 0.1 mmol/L (solution 9) and nontasters had thresholds in excess of 0.2 mmol/L (solution 10). Cases with thresholds between 9 and 10 are usually rejected as unclassifiable (11). Rating the intensity of 5 PROP solutions along a category scale is a faster and much less cumbersome procedure than the classic threshold detection method. As shown in Figure 2, summed bitterness intensity ratings for the 5 PROP solutions also showed a bimodal distribution. Summed bitterness ratings were inversely linked to the detection threshold (r=-0.55; pO.01). Summed bitterness intensity and summed hedonic ratings for the 5 PROP solutions, were strongly and inversely linked (r=-0.83; pO.01). As shown in Figure 3, greater perceived bitterness was linked to a progressively increasing dislike of bitter taste. The same relationship between perception and hedonic response was obtained for tasters and nontasters. Consistent with past studies,



Summed bitterness intensity ratings for 5 PROP solutions

Figure 2. Distribution of summed bitterness ratings for 5 PROP solutions for 538female respondents.



57 PROP tasting was unrelated to the perception of saltiness intensity of the 5 N a C l solutions. In past studies, PROP "supertasters" were identified by a combination of PROP detection thresholds and intensity scaling of PROP relative to NaCl solutions (5, 11). Other studies, based on PROP-impregnated filter paper, identified those subjects who gave intensity scores of 8 or 9 on a 9-point scale as "high responders" or potential supertasters. We used summed intensity ratings for the 5 PROP solutions to assign respondents to 3 taste categories. Scores below 21 indicated nontasters, scores in the range 21 to 35 indicated regular tasters, while scores of 36 and above indicated supertasters. PROP responses for each group are summarized in Figure 4. As expected, bitterness intensity and hedonic curves were mirror images of each other, since increased perception of bitterness was associated with a greater dislike of PROP solutions. Furthermore, the shape of the bitterness intensity curve roughly indicated the location of the detection threshold for each taster group.

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Summed bitterness ratings for 5 PROP solutions

Figure 3. Summed hedonic ratings for PROP solutions as a function of summed bitterness. Data are shown separately for PROP tasters (closed circles) and PROP nontasters (open circles).



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Figure 4. Bitterness intensity (top panel) and hedonic ratings (bottom panel) for 5 PROP solutions by PROP taster status for 538 women, aged 18-80 y.


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PROP Tasting and Other Bitter Compounds The ability to taste PROP was linked to higher bitterness ratings and lower preferences for naringin, a bioactive flavonoid that is the principal bitter component of grapefruit juice (28). PROP supertasters gave significantly lower hedonic ratings to naringin solutions than did regular tasters or nontasters (26). Naringin is not present in orange juice. A crosstab analysis of mean reported preferences for grapefruit juice showed that 19 out of 84 PROP tasters, as compared to only 3 out of 37 nontasters strongly disliked grapefruit juice (giving scores of 1 or 2 on a 9-point scale). No such differences by PROP taster status were observed for orange juice, oranges, or apples. PROP tasting was also linked to an increased dislike of concentrated infusions of Japanese green tea (30). In addition, PROP tasters gave lower selfreported acceptability ratings to soy foods, such as miso and tofu, on a food preference checklist (30). Soy products, especially fermented ones, contain bitter isoflavones, genistein and dadzein (31). Consistent with some past data PROP tasting was also associated with increased perceived bitterness of caffeine solutions. We had previously found (17) that PROP tasters gave lower acceptability ratings to coffee, instant coffee, and espresso. Respondents who expressed a decided dislike for coffee (ratings

Chemistry of Taste - ACS Publications - American Chemical Society

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