The present study demonstrated that recognition thresholds for sucrose and salt in healthy subjects were significantly lower during caloric deprivation than after caloric loading. At the same time reactivity of taste to bitter solutions was not affected by food deprivation and satiety. In this study we used the two-alternative forced-choice method for taste threshold measurements. It has been demonstrated that this technique measures the sensitivity of sensory process in a bias-free manner that is not influenced by the properties of the decision process [10
]. The meal-related changes in the values of taste thresholds in our study model can not be attributed to any confounding effects of the order of testing and measurement conditions, as the values of taste thresholds were not affected by an order of presentation but depended on conditions of testing. Therefore it is likely that food-related changes in the values of taste thresholds in this study could reflect the modulatory effect of caloric deprivation and satiety on the sensitivity of the gustatory system.
Several mechanisms might be important for the modulation of taste sensitivity in hunger and satiety states. Firstly, systemic activation of the brain during food motivation or caloric satiety might alter sensitivity of the central structures involved in perception of taste stimuli [12
]. However Scott et al. have demonstrated that in satiety state food does not reduce responsiveness to taste stimuli of the brain areas devoted to the food analysis but in the areas concerned with motivation and reinforcement [13
]. A study conducted on primates also indicated the possibility of variations in taste sensitivity which was not related to variations in taste nerve signals from the peripheral structures [14
]. Secondly, the efferent influences on gustatory receptors evoked by hunger or satiety might affect sensitivity of the gustatory receptors. Such possibility has been demonstrated by several authors. Plata-Salaman has showed that impulsation from gastric mechanoreceptors and osmoreceptors during sensory satiety state contributed to both short-term satiety signals and to efferent control of sensory responses of the gustatory receptors [15
]. Such centrifugal "tuning" influences on the taste receptors may take place through the efferent neurons of the glossopharyngeal and lingual nerves. It has also been shown that activation of the gastric mechanoreceptors or osmoreceptors through the vagus and nucleus solitarius, and further through the efferent neurons in the lingual nerve, inhibits sensory responses of the gustatory receptors and therefore increases taste thresholds [16
]. Budilina et al. has demonstrated in deprived animals that the pattern of glossopharyngeal nerve discharge can be modulated by irritation of the stomach with the rubber balloon which was reflected in alteration of perception of the taste stimuli [17
]. Thirdly, alteration of the autonomic nervous system activity during fasting state might contribute to modulation of perception of taste stimuli [18
Selective modulation of sensitivity of the gustatory system might reflect different biological importance of salty, sweet and bitter qualities of taste. While sweet and salty tastes are indicators of eatable substances and trigger consumption, bitter taste indicates substances which are not suitable for consumption and should be rejected [20
]. Therefore the relatively constant and high level of sensitivity of the gustatory system to a bitter substance found in our present study might be an important determinant of the high ability of the taste system to detect substances potentially dangerous for consumption in both satiety and hunger states. It can also be important for the limitation of bad-tasting food consumption in fasting state which has been shown by some authors [21
]. A recent study conducted by Pasquet et al. has demonstrated the relationship between bitter sensitivity to 6-n-propylthiouracil (PROP) and feeding behaviour and food preferences [22
]. Low threshold tasters of PROP had a tendancy to use fewer food items and rated food preferences higher than other groups of subjects. In addition, they had high perceived unpleasantness of NaCl, a stimulus which is generally rated as aversive. Fisher et al. have described a positive correlation between bitter sensitivity and the percentage of food dislike [23
]. Other studies have demonstrated that super-tasters of bitter substances exhibited a reduced acceptability of foods containing plant-based toxins which may reflect the avoidance of potentially toxic food [24
]. The biological significance of substances of nutritional value declines after a meal. Therefore a decrease in sensitivity of the gustatory system to sweet and salty substances reflects the shift of attention from nutritional to non-nutritional factors during satiety state.