Human neural synergy when combining stevia with a flavour modifier and the neural effects of sucrose vs stevia

There is a drive to increase the acceptability of sweeteners such as stevia in foods and reduce any off tastes or bitterness. One way to do this is to add flavour modifiers. We have shown previously that adding a flavour modifier to the artificial sweetener sucralose resulted in synergistic neural activity which could underpin improved taste sensation. Therefore, the aim of this study was to see if adding a flavour modifier to stevia would also reveal synergistic neural effects. Healthy adults (N = 34, Mean age 25 yrs.) participated in a within-subjects study examining the neural effects of stevia plus a flavour modifier vs stevia alone and vs sucrose. We examined whole brain data, and insula, postcentral gyrus, and hypothalamus ROIs identified from meta-analysis on brain responses to sweet tastes. We also examined the ROIs NAcc and amygdala given their role in reward and aversion processing. We examined the relationships between brain activity and the subjective ratings of pleasantness, bitterness and mouth fullness for the tastes. Super-additive neural effects to stevia plus modifier were found in the postcentral gyrus, parietal cortex and the occipital gyrus at the whole brain level, p < 0.05 Family Wise Error corrected threshold. In the ROI analysis we found reduced activation of the hypothalamus to the stevia plus modifier condition vs stevia p = 0.008 and higher postcentral gyrus activation to sucrose vs stevia p = 0.01. We found the insula tracked the pleasantness of the stevia conditions. The hypothalamus tracked the pleasantness and mouth fullness of the stevia plus modifier but not the stevia and the NAcc tracked the mouth fullness of the stevia plus modifier more than stevia. Further the amygdala tracked the bitterness of the stevia but not the stevia plus modifier. This study offers the first evidence that combining stevia with a flavour modifier reveals synergistic neural activity in brain areas associated with taste sensation, intensity and multisensory integration. Further we show that adding a modifier to stevia could increase unconscious desirability for stevia outside of subjective awareness by masking its bitterness and increasing its mouth fullness.