Central Integration of Dietary Protein Signalling

Protein is accepted as the nutrient with the most satiating effect. During digestion it acts at different levels. In the stomach it delays gastric emptying and maintains the effect of gastric distension on satiety. In the gut amino acids (AA) and peptides are detected and this leads to an activation of the vagus nerve and the release of gut hormones. The vagus nerve then transmits the signals to the dorsal vagal complex (DVC), responsible for meal termination. Circulating gut peptides and absorbed AA are also detected in the hypothalamus, which is the main structure controlling energy balance. The exact mechanisms by which dietary protein influences hunger and satiety signalling are not yet fully understood. To gain insight, in this thesis we carried out 3 projects. Firstly, concerning transmission of protein signals by the vagus nerve, our study showed that vagal capsaicin sensitive fibres are not necessary for high protein (HP) diet induced hypophagia, and thus that compensatory mechanisms probably exist which enable adaptation. Secondly we compared the activity of DVC regions in response to a protein or carbohydrate load. Using 3D reconstruction neurones were found in distinct though partially overlapping positions in the NTS. This suggests that short-term protein signalling is specifically transmitted to the DVC. In the third project, we discovered that mice fed a HP diet for 6 weeks had altered body composition but not food intake or body weight. These results suggest an effect of protein on energy metabolism independent from the satiating effect. Further studies may help to understand the role of protein in meal size control, energy balance and body weight regulation.