The interaction between oral and gastric signals is an important part of food intake regulation. Previous studies suggest that bypassing oral stimulation diminishes the suppression of hunger and increases gastric emptying rate. However, the role of appetite hormones, like cholecystokinin-8 and ghrelin, in this process is still unclear. Our objective was to determine the contributions of gastric and oral stimulation to subsequent appetite and hormone responses and their effect on ad libitum intake. Fourteen healthy male subjects (age 24.6 ± 3.8y, BMI 22.3 ± 1.6 kg/m2) completed a randomized, single-blinded, cross-over experiment with 3 treatment-sessions: 1) Stomach distention: naso-gastric infusion of 500 mL/0 kJ water, 2) Stomach distention with caloric content: naso-gastric infusion of 500 mL/1770 kJ chocolate milk, and 3) Stomach distention with caloric content and oral exposure: oral administration of 500 mL/1770 kJ chocolate milk. Changes in appetite ratings and plasma glucose, insulin, cholecystokinin-8, and active and total ghrelin concentrations were measured at fixed time-points up to 30 min after infusion or oral administration. Subsequently, subjects consumed an ad libitum buffet meal. Oral administration reduced appetite ratings more than both naso-gastric infusions (P < 0.0001). Gastric infusion of a caloric load increased insulin and cholecystokinin-8 and decreased total ghrelin concentrations more than ingestion (all P < 0.0001). No differences in active ghrelin response were observed between conditions. Ad libitum intake did not differ between oral and gastric administration of chocolate milk (P > 0.05). Thus, gastric infusion of nutrients induces greater appetite hormone responses than ingestion does. These data provide novel and additional evidence that bypassing oral stimulation not only affects the appetite profile but also increases anorexigenic hormone responses, probably driven in part by faster gastric emptying. This confirms the idea that learned associations between sensory characteristics and associated metabolic consequences serve to adapt hormone responses to nutrient content. These findings underscore the importance of oral stimulation in the regulation of food intake.
Alex’s Notes: Eating is a complex interaction between smell, taste, and oral stimulation. When you attempt to eat something that tastes… not the best, we tend to hold our breath or plug our noses so that we don’t have to smell the food while eating it. This makes the food blander because smell is an important part of eating. Similarly, the mere oral stimulation of chewing food is enough to help reduce appetite even if you spit it out afterwards, and the longer the food is in the mouth the fuller you become.
Not everything is about hunger and taste, however. The role of oral stimulation in appetite regulation is also reflected in hormonal responses, among others, several studies have shown that oral stimulation has an important role in glucose homeostasis. The purpose of the study at hand was to further elaborate upon the interaction of how food is consumed and the subsequent appetite, blood glucose regulation, hormonal response, and food intake.
Subjects were healthy and normal weight males aged 18-35 years. They were randomized to one of three experimental groups:
- Stomach distention through infusion of water (control group)
- Stomach distention through infusion of chocolate milk (Gas-Cal)
- Stomach distention through drinking of chocolate milk (Oral-Cal)
As we can see, all three groups had their stomachs filled by an equal volume of liquid. However, the Gas-Cal group had a liquid that supplied calories, and the Oral-Cal group had that same caloric liquid but drank it for oral stimulation.
Not surprisingly, the group that drank the chocolate milk had greater appetite reduction than the other groups, as well as significantly reduced hunger and thirst ratings. The blood glucose response between the groups was not different between the two chocolate milk conditions, which makes sense since it was the same drink supplied differently. However, insulin was significantly higher at all time-points after the Gas-Cal compared to the Oral-Cal, supporting the notion that oral stimulation is important for controlling blood sugar. The authors summarize the findings through explaining that
“If oral signaling is bypassed, there is no possibility for anticipation and therefore a greater challenge in terms of glucose homeostasis as well as an apparent decrease in satiation. More generally speaking, our hormone results demonstrate that sensing of nutrients in the mouth and esophagus by way of learned associations between sensory characteristics and associated metabolic consequences does indeed provide a means for the body to anticipate on imminent metabolic events and constitutes a distinct part of the regulation of digestion which interacts with later nutrient signaling from the gastro-intestinal tract, which is tuned to the actual macronutrient composition.”