Psychophysics of sweet and fat perception in obesity: problems, solutions and new perspectives

Abstract

Psychophysical comparisons seem to show that obese individuals experience normal sweet and fat sensations, they like sweetness the same or less, but like fat more than the non-obese do. These psychophysical comparisons have been made using scales (visual analogue or category) that assume intensity labels (e.g. extremely) which denote the same absolute perceived intensity to all. In reality, the perceived intensities denoted by labels vary because they depend on experiences with the substances to be judged. This variation makes comparisons invalid. Valid comparisons can be made by asking the subjects to rate their sensory/hedonic experiences in contexts that are not related to the specific experiences of interest. Using this methodology, we present the evidence that the sensory and hedonic properties of sweet and fat vary with body mass index. The obese live in different orosensory and orohedonic worlds than do the non-obese; the obese experience reduced sweetness, which probably intensifies fat sensations, and the obese like both sweet and fat more than the non-obese do. Genetic variation as well as taste pathology contribute to these results. These psychophysical advances will impact experimental as well as clinical studies of obesity and other eating disorders.

Figure 1

Differences between obese and underweight individuals for the liking of sugar, sweet foods and fat foods. This figure illustrates one kind of error resulting from the erroneous assumption that the perceived intensities denoted by descriptors like ‘most intense’ do not differ across groups. (a) Shows the data obtained with the hedonic gLMS. Significance of t-tests following ANOVA are: sugar, p<0.05; sweet foods, p<0.01; and fat foods, p<0.0000001. (b) Shows how the data would look if ‘most intense’ liking for foods is erroneously assumed to be equal for both groups (see text for explanation): sugar and sweet foods no longer show significant differences between thin and obese subjects; for fat foods, p<0.0000001.

Figure 2

Sweetness of a piece of candy (tasted) plotted against the liking for sugar (remembered). The subjects are divided by BMI into underweight (below 18.5), normal weight (18.5–24.9), overweight (25.0–29.9) and obese (30.0 and above). Note that as BMI increases, the correlation coefficients increase. The correlation coefficients for the underweight and normal weight subjects was significantly less than that for the obese subjects (p<0.01); the comparisons of the correlations coefficients for the overweight and obese subjects approached significance (p=0.06). r=correlation coefficient.