Sugar is routinely used to comfort neonatal children undergoing painful procedures and has been shown to increase cold pain thresholds in animals7,9,10. However, it is not known whether sweet taste can also affect hot pain thresholds or percepts in adults. This study was designed to test this by investigating whether the anticipation of a solution containing sucrose could change both warm detection and heat pain thresholds in young healthy adult participants as well as brain responses to food-related odors.
Participants were asked to taste solutions containing 10% sucrose and 0.016% sucralose, a mixture that is close in sweetness to table sugar (but without the calories!). They were then tested using a two-ramp thermal protocol to measure warm detection and heat pain thresholds. Each condition was repeated three times and then the results were analyzed. We expected that a simple anticipatory manipulation of sucrose would increase both thresholds. However, this was not the case: the thresholds remained unchanged. This is a surprising result, since previous studies have found that the anticipation of a sweet substance increases both pain thresholds and tolerance to pain in adults, whereas changes in cold perception are confounded by motivational or affective factors6,11.
We also wanted to determine if differences in sensitivities to sweet and savory foods were associated with these brain responses and food preference. Participants were therefore split into high and low sweet sensitive groups based on a modified taste strip test. Brain responses to odors that varied in taste quality (sweet or savory) and energy density (high or low fat) were then compared. The HS group, compared to the LS group, showed stronger brain activation of the frontal operculum and insula in response to sweet odors and high-fat food odors.
The sensitivity to sweet and savory flavors was also correlated with the participants’ liking scores for these foods. These data suggest that different genetic variants of the TAS1R3 gene, which controls for taste receptor expression, influence individual perception of both sweet and savory flavors and may account for differences in sensitivities to both sweet and hot stimuli.
In addition, this study demonstrated that the analgesic effect of sucrose in adults is associated with brain activation in the prefrontal cortex and insula, suggesting that hedonic effects play an important role. Furthermore, the data suggest that these hedonic effects of sweet tastes are modulated by the dopamine neurotransmitter system. This is consistent with previous observations that dopamine acts directly to inhibit the nociceptive flexion reflex in mice32,33. Future research will explore how these findings can be extended to other types of pain and to non-sweet substances. Lastly, these results are relevant to the health context, as both obesity and addiction to sweets are associated with increased risk of chronic pain conditions such as diabetes.