Mastication and Energy Expenditure in Normal Weight and Obese People

The objective of the study is to analyze the specific role of physical properties of food and mastication in the cephalic phase reflex of thermogenesis, and the involvement of the autonomic nervous system in the implementation of this reflex.

The hypothesis of the study is that oral stimulation elicited by food properties during mastication can generate changes in the amplitude of variation of thermogenesis, and that these changes are distinct from those attributable to the thermal effect of food during digestion.

A group of 12 subjects presenting a normal weight and a group of 12 obese subjects, with a good general health and a good dental state will be constituted. They will participate to 5 different recording sessions during which they will be invited to chew and spit out foods and chewing gums of different hardness. During recording sessions with mastication and no-mastication periods, gas exchange (O2 concentration), cardiac activity (electrocardiography) and muscular activity of masticatory muscles (electromyography) will be recorded. Blood samples will be collected during one of the sessions to analyse the kinetic of appearance of some hormones in the blood compartment during or just after mastication. The selected hormones are involved in the eating behaviour and its regulation (ghrelin, peptide C, peptide YY and glucose-like peptide 1). From these recordings, energy expenditure, heart rate variability, intensity of masticatory muscle contraction, and hormone blood appearance will be analysed in link with the nature and hardness of the food chewed. Since the food bolus is spit out after mastication (without any swallowing = sham-feeding), the changes observed in theses variables will be directly attributed to sensory information coming from oral cavity during mastication. Food choice and sport activity of each volunteer will be taken into account to compare the results obtained for the two groups (normal weight and obese).

The work will produce more data and evidence on the contribution of oral signals attributable to the food in the regulation of the energy balance. The results will also bring more knowledge on the involvement of the autonomic nervous system. Comparison of the results obtained for the two groups (normal weight and obese people), will bring some positive outcomes which could be useful in the fight against obesity.

Eligibility Criteria:
Inclusion criteria :

19 \< body mass index \< 25 kg/m2 for the normal weight group 30 \< body mass index \< 35 kg/m2 for the obese group Good general health

Exclusion criteria :

* sport activity
* Smoking in the 6 months before the beginning of the study
* Regular alcohol consumption (more than 4 unit/day)
* Medication that may interfere with the results of the study (beta-blockers, corticoids, anti-inflammatories, thyroid hormone, insulin, antiarrhythmics) or that may have an effect on salivation or muscular activity
* Oral or dental pathology (including pain), current oral care
* Orthodontic treatment in the 3 years before the study
* Weigh gain or weight loss of more than 10% in the 3 last months
* Thyroid disturbance
* High blood pressure
* Cardiac activity anomaly
* Anomaly for one of the biological elements : complete blood count, fasting blood glucose, glycated haemoglobin, thyroid hormone (TSH), sedimentation rate \> 8mm, CRP rate \> 10ml/L
* Allergies, intolerance or disgust for one of the food tested