The role of sweet and savoury taste in food intake and food preferences

Background and aim The sensory attributes of food play a key role in the selection and termination of meals and their rewarding properties. The majority of our foods are either sweet or savoury tasting. In addition, within our food range, savoury-tasting foods contain in general higher levels of protein. The effect of specific taste modalities on human food intake, however, requires further clarification. The primary aim of this thesis was to investigate the role of sweet and savoury taste in food intake and food preferences. The secondary aim was to provide more insight into the processes of explicit and implicit liking and wanting, to be able to identify underlying reward mechanisms involved in food intake behaviour. Methods We conducted series of experiments where healthy young adults participated. We started by investigating the difference between a sweet and savoury taste on satiation, independent of palatability, texture, energy density, and macronutrient composition (n=64). Next, the effect of sweet and savoury taste of a single meal on subsequent satiety and food preferences was investigated (n=61). To further explore the effect of taste in the context of a complete diet on satiety and food preferences, the effect of three 24-h diets that differed only in taste (predominantly sweet tasting, predominantly savoury tasting, or a mixture of sweet and savoury tasting) were compared (n=39). Next, we separated the influence of taste from within-meal protein content on satiety and food preferences, by comparing the effect of sweet and savoury high and low protein single meals (n=60). Finally, the effect of long-term protein status on satiety and food preferences was investigated by comparing the effect of two 14-d diets that differed in protein content (a low protein diet vs. a high protein diet) (n=37). Results Sweet and savoury taste, independent of palatability, texture, energy density, and macronutrient composition, did not differ in their effect on satiation and satiety in terms of subsequent ad libitum intake. Sweet and savoury taste did differ in their effect on subsequent food preferences. In general, after eating a food with a certain taste, appetite for foods with a similar taste was lower than for foods with a dissimilar taste, hence, a clear transfer effect of sensory specific satiety was demonstrated. This transfer effect was not equipotent for sweet and savoury taste; after eating a sweet single meal or sweet 24-h diet, preferences for sweet and savoury foods did not differ. Eating a savoury single meal or savoury 24-h diet, however, led to a clear preference for sweet foods. Neither sweet or savoury tasting single meals nor sweet or savoury 24-h diets shifted food preferences towards high or low protein foods. It was shown that protein content of a meal, independent of taste, did not have an effect on satiety and food preference. We did observe, however, an effect of protein status: after a 14-d low protein diet, there was an increase in ad libitum protein intake, compared to after a 14-d high protein diet, while total energy intake was not different. In addition, food preference for savoury high protein foods was increased. Regarding the different components of food reward it was demonstrated that in all studies both explicit and implicit measures correlated with several aspects of eating. It appeared that in a controlled setting, i.e. in the sensory booths, explicit processes played a stronger determining role in satiation (meal size) than implicit processes. Food choices appeared to be made on a more unconscious level. In a setting where subjects could behave more naturally (i.e. self-selection and serving of foods in a relaxed environment where subjects could sit and eat together), implicit, unconscious processes seemed to explain food intake behaviour more than explicit processes. When subjects experienced protein shortage, after the 14-d low protein diet, it appeared that implicit processes of wanting played a stronger determining role in decisions about what to eat. Conclusion Sweet and savoury taste do not differ in their effect on satiation or satiety in terms of subsequent ad libitum intake. The taste of a meal or diet does have a large effect on subsequent food preferences, thereby showing a clear transfer effect which is not equipotent for sweet and savoury taste. Savoury taste exerts a stronger modulating effect on subsequent food preferences than sweet taste. Sweet and savoury taste of a single meal or 24-h diet do not differ in their effect on food preferences for high or low protein foods. In addition, within-meal protein content seems not to influence satiety or food preferences. A low protein status, however, through selective reduction of dietary protein intake, elicits compensatory changes in food intake and food preferences to restore adequate protein status. It appears that both conscious (explicit) and unconscious (implicit) processes are involved in satiation and food choice. The role implicit motivational processes play in driving food choice is not static, but appears to vary. This is especially the case when homeostasis is challenged (by depleting macronutrient stores), where implicit processes of wanting appear to play a stronger determining role in decisions about what to eat.