Detection of non-esterified fatty acids in the oral cavity
As evidence for the ability of humans to taste non-esterified fatty acids continues to grow, differences in threshold sensitivity are becoming apparent. A focus on the individual and environmental factors that influence these differences is warranted. Factors identified and directly examined over the course of three studies as primary hypotheses in this dissertation included: number of testing occasions, psychophysical testing methodology, sample preparation method, BMI, hunger and dietary fat exposure. Post-hoc analyses of additional factors included: sex and threshold performance reliability. The first study examined the effect of repeated testing and testing methodology on detection threshold sensitivity. Sixteen participants were tested 20 times beginning with either the ascending 3-alternative forced-choice method or the modified staircase or method. After 10 visits, participants were tested using the other method. Detection thresholds decreased (performance improved) with repeated testing. No differences between the median of the first 10 visits, second 10 visits, and overall median were observed between the two methods. Significant improvement from baseline was seen at visit 7 with the staircase method as well as visits 9 and 10, likely due to the fact that this method generated data with less variation. The variance at each visit using the ascending method increased over time, which suggests this method may be able to distinguish hypo and hypersensitive individuals, as the increasing variance suggests that sensitive individuals improved over time while less sensitive participants failed to demonstrate continued improvement. The most appropriate detection threshold testing method will depend on the goals of the researcher, but based on this study, the ascending method may be better at the identification of highly sensitive individuals, which is a common goal of psychophysical testing. The second study sought to reduce the amount of time it took (7 or more visits) to obtain significant improvement in detection thresholds from baseline. Previous work using other taste qualities demonstrated that brief exposure to stimuli over a 10-day period generated significant improvement in detection threshold sensitivity. We tested whether one exposure to oleic acid every day for 10 days would result in improved detection threshold sensitivity compared to baseline. Fifty-four participants performed baseline taste testing using the ascending, 3-alternative, forced choice method and were randomly assigned to either the oleic acid training group or the sucrose training group. The sucrose group served as an attention control. Both groups came into the lab each day for 10 days to swish, expectorate, and describe a 5 mL sample of their respective stimulus. Follow-up testing occurred on day 11 or 12. No significant improvement between baseline and follow-up was observed. Post-hoc analyses revealed no differences in sensitivity by sex or BMI (lean vs. overweight). Hunger was not associated with sensitivity. No correlations between visits were observed, which suggests that more than two visits are needed to observe significant improvements in oleic acid detection thresholds. The third study examined associations between BMI and fat taste sensitivity. Other groups have demonstrated associations between increased BMI and decreased sensitivity, but sample sizes were small and only one study had tested people more than once or twice. Forty-eight participants (lean: N=24; overweight: N=24) completed 7 detection threshold testing visits using the modified staircase method. The Block Rapid Fat Screener and the Automated Self-Assessment 24 were used to collect dietary fat intake information. The entire group improved over the course of 7 visits. This finding held for both lean and overweight but not for obese (N=13) individuals. Positive associations between dietary fat intake and detection thresholds (increased fat intake led to higher thresholds (decreased performance) were seen in the lean and overweight but not in the obese, a possible result of under-reporting of dietary fat intake in the obese. Further study examining how and why the obese fail to improve in their ability to detect oleic acid over time is necessary to explain these differences in this subgroup. In addition to the primary research conducted, three review papers were written. The first paper argues that fat taste should be considered a primary taste and outlines the criteria that might constitute a primary taste as no universally agreed upon criteria currently exist. The second review article discusses the cephalic phase physiological responses that occur during modified sham feeding (MSF) of fat and/or NEFA. MSF involves the oral presentation of a stimulus but does not permit swallowing; thus, post-ingestive effects are eliminated. The final review article examines possible sources of inter- and intra-individual variability that likely contribute to the differences in sensitivity reported in various fat taste studies.
Mattes, Purdue University.
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