Influence of Linoleic Acid on Taste Intensity
Mary Holland Brumbach
Virginia Clyburn
Hailey Hughes
Sean Patterson
Krysta Webster
Submitted as partial requirement
of the psychology major at Wofford College
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Effect of Linoleic Acid on Human Taste Intensity: An Experimental Study - Prof. David W. P, Papers of Psychology
An experiment aimed at measuring the effect of linoleic acid on human taste intensity when added to sweet stimuli using a sip and spit method. The study also explores the relationship between taste intensity, bmi, percentage of body fat, and prop sensitivity. Linoleic acid's impact on taste receptor cells and its potential association with dietary fat perception are also examined.
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Influence of Linoleic Acid on Taste Intensity
Mary Holland Brumbach Virginia ClyburnHailey Hughes Sean Patterson Krysta Webster
of the psychology major at Wofford CollegeSubmitted as partial requirement
Abstract Human and rat have an innate preference for fat which may lead to obesity. This preference may be driven by the taste component linoleic acid, the principle free fatty acid found in corn oil. It has been shown that rats are capable of detecting linoleic acid within a solution. It is speculated that rats also perceive an increase in intensity when linoleic acid is added to the solution. This experiment sought to measure the effect on human taste intensity when linoleic acid was added to sweet stimuli. Using a sip and spit method, subjects rated the intensity of a triad, each containing a different amount of linoleic acid (0, 88, or 352 μM) mixed with either sucrose or sucralose (31, 62, 125, 250, 500 mM). There was no effect of linoleic acid on the perceived intensity of the sweet stimuli. However, there was a difference in perceived intensity between the two sweet stimuli. Sucrose was perceived as more intense than sucralose at the 125 and 500 mM concentrations. Body mass index (BMI) and percentage of body fat are two measures of obesity. If taste plays a role in producing obesity, BMI and percentage of body fat would have a positive correlation with taste intensity of sweet stimuli. No correlation was found between either percentage of body fat or BMI and the intensity of the sweet stimuli. Although the current methodology did not measure an effect of linoleic acid, different threshold and discrimination procedures such as forced choice rating system may produce more conclusive data.
variety of other tastes stimuli to lead to taste cell depolarization (Gilbertson, 1999). The DRK channels are not sensitive to all fatty acids; however, conjugated linoleic acid was discovered to be moderately effective. Subsequent research suggests that a subtype DRK channel, Shaker Kv1.5, may be sensitive to inhibition by fatty acids.
Behavioral research in a rodent model supports the theory that linoleic acid may increase the depolarization of taste receptor cells in response to taste stimuli. Pittman (2003) discovered when linoleic acid was added to sucrose greater than 62 mM, rats increased their licking as if the sweet stimuli were more intense. When linoleic acid was added to concentrations of NaCl, an aversive stimulus, the licking response decreased significantly. These responses led researchers to believe that the tastants were perceived as more intense when linoleic acid was present. Subsequent research by Pittman discovered that rats could develop a conditioned taste aversion to linoleic acid. This provides evidence that rats possess the ability to detect linoleic acid and avoid it when paired with an aversive stimulus. When the chorda tympani nerve was severed, rats were unable to develop a conditioned taste aversion to linoleic acid. This data suggests that the chorda tympani nerve needs these signals for the detection of linoleic acid (Pittman, 2003). Arthur L. Fox discovered that some of the population can detect PROP, called tasters, while some of the population cannot detect PROP, called nontasters (Bartoshuk et al., 1994). PROP, a thyroid medication, allows tasting of nontaster, taster, and supertaster status (Fox, 1931). An incomplete dominant allele appears to determine PROP taste thresholds. Variations in PROP sensitivity appears to arise form underlying anatomical differences, since ratings of PROP intensity are highly correlated with the
density of fungiform papillae on the tongue (Bartoshuk et al., 1994). Therefore, the tongue anatomy of tasters is different from nontasters, with supertasters tending to have the most taste pores and fungiform papillae. PROP sensitivity is very important, because sensitivity determines overall perception of certain taste stimuli. The ability to taste PROP positively correlates with increased taste intensity of the primary taste qualities; sweet, sour, bitter and salt (Bartoshuk et al. , 1994). Taster status can determine the type of food that individuals decide to eat. Supertasters may choose foods that nontasters would not choose because of their sensitivity to foods. Some suggest that factors such as PROP sensitivity can affect the perception of dietary fat. One study uncovered a possible association between fat perception based on taste and PROP sensitivity (Tepper & Nurse, 1997). Medium tasters and supertasters possessed the ability to discriminate between 40% fat and 10% fat salad dressing, while the nontasters could not (Tepper and Nurse, 1997). We speculate that sensitivity to tastants is correlated with obesity. BMI and the precent of body fat are two indicators of obesity. The ability to correctly assess human taste intensity can be largely influenced by an assessment tool. The Labeled Magnitude Scale (LMS) minimizes ceiling effects and therefore is sufficient in discriminating sensitive tasters from nontasters. Labeled Magnitude Scale (LMS) with intensity adjectives spaced so that the scale would have appropriate ratio properties. The LMS has repeatedly been proven to b a valid instrument to classify individuals as tasters or nontasters (Lucchina, 1998). The top of the scale is labeled ‘strongest imaginable.’ The LMS rests on the assumption that ‘strongest imaginable’ refers to the same perceived intensity on average, across non- tasters, medium tasters and supertasters (Bartoshuk et al., 1998). Tasters report the
Chemical Stimuli The perceived intensities of two sweet stimuli, sucrose and sucralose (SPLENDA) were measured. According to the manufacturer directions, splenda volume was matched to sucrose. Five concentrations of each sweet stimulus produced 10 concentration-specific groupings of 31, 62, 125, 250, and 500 mM. Stimuli were grouped in triads based on concentration and stimulus. Within each triad, the given concentration of splenda or sucrose contained either 0, 88, or 352 μM linoleic acid. Each stimulus sample contained approximately 10 ml of solution. All solutions were mixed in 5 mM ethanol to facilitate the emulsion of linoleic acid. In addition to the 10 triads of sweet stimuli, there was a triad containing only 5 mM ethanol with or without 88 and 352 μM linoleic acid. All solutions were mixed within 90 minutes of the start of the experiment. All of the solutions were maintained at room temperature. The sensitivity to PROP was assessed using a 0.32 mM PROP solution. Tasting Procedure Subjects were given 90 seconds to taste the three stimuli in each triad and rate the stimulus intensity on a labeled magnitude scale. Subjects sampled each taste stimulus using a sip and spit method. Subjects rinsed with water between each stimulus. Between each triad there was a 60 second rest period. After the perceived intensity was measured for all 11 groups, the sensitivity to PROP was assessed using the same procedure and labeled magnitude scale. Statistics The results of our data were analyzed using SPSS. A univariant ANOVA test determined the significant main effects and interactions as well as significant group
LINOLEIC CHART
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Fig. 1. The perceived intensity of ethanol with or without linoleic acid.
differences between the independent variables. Post hoc testing with the least significant difference (LSD) test determined the source of any statistically significant effects. Results were deemed significant if p<0.01. RESULTS No measurable effect was found in the perceived intensities of linoleic acid at concentrations of 0, 88, or 352 μM. Figure 1 demonstrates the LMS ratings of the different concentrations of linoleic acid presented independently of another stimulus. There was no evidence for detection of differences between the different concentrations of linoleic acid because the mean ratings of
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Fig. 2. The perceived intensity of sucrose with either 0, 88, or 352 μM of linoleic acid.
Figure 4 demonstrates that as the sucrose and sucralose concentrations increased, the LMS ratings of the perceived intensities increased as well (F4,687 = 132.345, p<0.01). Additionally, there was a significant overall effect between the perceived intensity of sucrose and sucralose (F1,687 = 23.925, p<0.01). There was also a significant interaction between the stimulus concentration (F4,687 = 8.968, p<0.01). Post-hoc tests with the LSD statistic revealed a significant increase in the perceived intensity of sucrose compared to sucralose at the 125 and 500 mM concentrations (F9,687 = 65.453, p<0.01). At concentrations of 125 and 500 mM, sucrose is significantly higher than sucralose. The mean value for sucrose verses surcralose is 23.89 and 14.61 respectively for the 125 mM concentration and 49.57 and 32.03 respectively for the 500 mM concentration. These were only revealed at 125 and 500 mM concentrations and not at 31, 62, or 250 mM. This demonstrates that the differences (^100) were concentration-specific. 2030
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(E) Sucrose R2 = 0.1211Splenda R2 = 0.
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Sucrose R2 = 0.0111Splenda R2 = 0.0177
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(A) Sucrose R2 = 0.0216Splenda R2 = 0.
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(^0 20) LMS Rating of 62 mM Concentration 40 60 80 100
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(B) Sucrose R2 = 0.0221Splenda R2 = 0.
Fig. 5. The relationship between PROPintensity and the perceived intensity of sucrose and sucralose at (a) 31, (b) 62, (c)125, (d) 250 and (e) 500 mM concentrations.
Figure 5 (a) shows a negative slope between the 31 mM concentration and the LMS rating of PROP. The R^2 value for sucrose was 0.0216 and for sucralose was 0.0018. The negative slope was due to two outlying points that greatly skew the slope. Two subjects inappropriately rated sucrose and sucralose higher than the other 66 subjects did. Figure 5 (b) shows a positive correlation between the 62 mM concentration of sucrose/sucralose and PROP sensitivity. The R^2 value for sucrose was 0.0221 and for sucralose was 0.0031. Similarly, Figure 5 (c) shows a positive correlation between 125 mM concentration and LMS ratings of PROP sensitivity. The R 2 value for sucrose is 0.0111 and for sucralose is 0.0177. Figure 5 (d) also shows a positive correlation between 250 mM concentration and the LMS ratings of PROP sensitivity. The R^2 value for sucrose is 0. and for sucralose is 0.0267. Figure 5 (e) shows the association between PROP sensitivity and 500 mM sucrose with a R^2 value of 0. and sucralose with a R^2 value of 0.0676. The positive slope in Figures 5 (b-e) reveals the finding the tasters of PROP are more likely
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(^0 20) LMS Rating of 500 mM Sweet Stimuli 40 60 80 100
Percent Body Fat
Sucrose R2 = 0.0062Splenda R2 = 0.
Fig. 7. The relationship between B.M.I. and perceived intensity of the 500 mM sweet stimuli.
Fig. 6. The relationship between B.M.I. and percent body fat.
R 2 = 0.
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undistinguishable impact of linoleic acid on perception may be due to individual differences in the subjects or the rating scale that was implemented. Individual differences may explain the large amount of variation observed among the rated intensities. Future studies could prevent these confounds by gathering trained subjects thereby eliminating individual differences. Prior training of subjects would instruct them to accurately rate specific concentrations of solutions. Another suggestion for subsequent studies would be to employ a forced-choice scaling mechanism. A forced-choice rating would require subjects to place the stimuli in ascending or descending order based on the perceived intensities. The results revealed that the LMS ratings of the perceived intensity increased as the concentrations for sucrose and sucralose increased. Likewise, the perceived ratings increased with the increasing concentrations of sucralose. The transduction mechanism would presumably be the same for a specific solution regardless of the concentration. Furthermore, this increase in perception could be due to a more frequent signal sent during transduction. The same transduction mechanism probably occurs for each chemical. The more frequent signal rating could not be dependant on the amount of solution present, for all amounts of solutions provided were comparable. There was little variance in the LMS ratings among the subjects, demonstrating that all subjects perceived the concentrations similarly. Results revealed a significant increase in the perceived intensity of sucrose in comparison to sucralose. Manufacturers may present sucralose as an artificial sweetener with the same perceived sweetness as sucrose. The molecular weight varies greatly between the two sweeteners, and therefore volumetric measuring is necessary to create
solutions with equal intensities of sweetness. Food manufacturers assume that people use small concentrations of sucralose, and therefore these lower concentrations produce equivalent sweet perception when volumetrically matched to sucrose. However, high concentrations of sucrose are perceived as more intense than volumetrically equivalent amounts of sucralose, revealing that these two stimuli do not show a linear relationship. This variation in perception among the two stimuli may be due to a difference in their transduction mechanisms. Previously, it was assumed that all sweet stimuli had identical transduction mechanism. However, a recent study on mice demonstrated the possible existence of two different transduction mechanisms for sucrose and sucralose. This study showed a significant difference in the responses of the chorda tympani (CT) nerve to sucrose and sucralose (Inoue et al , 2001). At concentrations of 1000 mM, the CT response to sucrose was 160 percent when normalized to ammonium chloride, while the response to sucralose at 1000 mM was 40 percent. This evidence suggests varying transduction mechanisms between the two sweeteners in mice. If similar transduction mechanisms were found in humans, it would account for the differences in perceived intensities between sucrose and sucralose detected in this study. The results of this study showed that a higher PROP sensitivity led to a higher LMS ratings of sucrose. Tasters of PROP are more sensitive to many stimuli. This increase in sensitivity is probably due to a greater number of fungiform papillae, resulting in an increase of neural inputs. Greater numbers of fungiform papillae imply that greater numbers of neurons transmit information to the brain. Therefore, neural input for supertasters will be greater than for nontasters. Taster status can determine the type of food the individual decides to eat. Supertasters may choose foods that nontasters would
References Bartoshuk LM, Duffy VB, Miller IJ. (1994) PTC/ PROP tasting: anatomy,psychophysics, and sex effects. Physiol Behav. 56(6): 1165-1171.
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Lucchina LA, Curtis OF, Putnam P, Drewnowski A, Prutkin JM, Bartoshuk LM. (1998) Psychophysical measurement of 6-n-propylthiouracil (PROP) taste perception. Acad Sci. 855: 816-819. Mccormack D.N., Herzog P., Webster K.L., Pittman D.W. (2003). Gustatory detection of a free fatty acid linoleic acid, by rats. Abstract, 25th^ Annual Meeting of the Association for Chemoreception Sciences. Mela DJ. (1989) Bitter taste intensity. Chem. Senses. 14:131-135. Reed DR, Nanthakumar E, North M, Bell C, Bartoshuk LM, Price RA. (1999) Localization of a gene for bitter-taste perception to human chromosome 5p15. Am J Hum Genet. 64(5): 1478-1480. Tepper, B.J., Nurse, R.J.(1997) Fat perception is related to PROP taster status. Physiology & Behavior , 61, 949-954.