Regional differences in suprathreshold intensity for bitter and umami stimuli

Abstract

The sense of taste is often referred to as a 'nutritional gatekeeper', thought to have evolved to indicate energy sources and prevent ingestion of potential toxins. Fungiform papillae are structures on the anterior tongue in which taste buds are situated. They are concentrated at the tongue's tip and they can provide a useful estimate of overall taste bud density for taste research. Some reports suggest taste perception may differ subtly across tongue regions, irrespective of FP number. Other data show an association between taste intensity perception for the bitter compound 6-n-propylthiouracil (PROP) and FP density. However, contradictions exist in the literature, with more recent, larger studies suggesting little or no association between FP number and perceived taste intensity. Much research has examined the relation between FP density and PROP perception, while other tastes have been less thoroughly studied. Here, in a cohort of mainly Caucasian individuals, aged 18-45, recruited from the campus of a large rural university, we examined regional and whole-mouth taste intensities, and FP density using an updated method of a digital still photography method first described in 2005. We found regional differences in suprathreshold intensity. Although all taste sensations were experienced all over the tongue, once again disproving the mythical tongue map, we also observed bitter and umami taste perception to be significantly greater on the posterior tongue than on the anterior tongue. In contrast, there were no regional differences observed for sweet, salty or sour tastes. The relation of FP density to whole-mouth intensity of 6-n-propylthiouracil, and to the intensity of saltiness of NaCl, sweetness from sucrose or from Acesulfame-K, bitterness of quinine, or burning from capsaicin delivered to different regions of the tongue are also discussed.

Keywords: Fungiform papilla (FP); Project GIANT-CS; bitter; regional differences; spatial taste test; tongue map; umami.

Figure 1

Image of blue-dyed tongue between two plastic slides and a paper circle for size indication. The approximate midline of the tongue is shown with the straight dashed white line. Two areas, 0.283 cm², one from each side of the tongue midline, were used to count the FP number.

Figure 2

Mean intensity ratings on the gLMS for sweet (2.0 M sucrose), sour (112 mM citric acid), salty (1.12 M NaCl), bitter (2mM quinine) and umami (50:50 mixture of 200mM monosodium glutamate and 100 mM inosine monophosphate). Solutions were swabbed on four areas of the tongue, front tip, left and right (Areas 1 and 2) and back circumvallate papillae, left and right (Areas 3 and 4). The graphs show the mean ratings for each taste on each of the four areas. Dotted lines depict gLMS labels; NS = no sensation, BD = barely detectable, W = weak, M = moderate.

Figure 3

Mean intensity ratings on the front and the back of the tongue for sweet (2.0 M sucrose), sour (112 mM citric acid), salty (1.12 M NaCl), bitter (2mM quinine) and umami (50:50 mixture of 200mM monosodium glutamate and 100 mM inosine monophosphate). Dotted lines show the relative positions of the labeled descriptors on the gLMS. Significantly different mean values (p < 0.001) between the front and back areas for a taste are denoted with **.

Figure 4

Gender differences in FP density. Men had a significantly lower number of FP than women.

Figure 5

FP across age in men (blue) and women (red). The regression line for men is denoted by the dotted line, the solid line denotes women. There was no association between age and FP in men or in women in this cohort.