Intranasal Insulin Boosts Gustatory Sensitivity

Intranasal insulin has been the subject of attention not only with respect to enhancing memory processes, but also for its anorexic effects, as well as its effects on olfactory sensitivity. In the present study, the influence of intranasal insulin on gustatory sensitivity was investigated using intranasal applications of insulin or placebo in a double-blind manner alongside a control condition without any application. We hypothesised that, because it mediates satiety, intranasal insulin alters gustatory sensitivity, whereas placebo application and the control should not alter gustatory sensitivity. We did not expect the sensitivity to the different taste solutions to differ. Sweet, salty, bitter and sour liquids in four concentrations each were sprayed onto the tongue of healthy male subjects. Additionally, water with no taste was applied to enable calculation of taste sensitivity in terms of parameter d' of signal detection theory. The task of the subject was to identify the quality of the respective tastant. Gustatory sensitivity and blood parameters were evaluated using repeated-measures ANOVAs. Gustatory sensitivity (implying all tastants) improved significantly after intranasal insulin application compared to the application of placebo, although it did not reach significance compared to the control condition. Subjects performed best when detecting the sweet taste and worst when detecting the bitter taste. The blood parameters glucose, insulin, homeostatic model assessment and leptin did not differ with respect to insulin or placebo condition, nor did they differ regarding measurements preceding or following intranasal application, in confirmation of preserved peripheral euglycaemia during the experiment. Thus, it can be concluded that the application of intranasal insulin led to an improved gustatory sensitivity compared to placebo.