SENSILINS: Impact of Cephalic Phase Insulin Release Induced by an Environmental Food Odor Stimulus on Glucose Homeostasis According to Insulin Sensitivity Level

This single-center, randomized, single-blind, 2-period crossover interventional study will evaluate whether exposure to a pleasant food odor 10 minutes before a 75 g oral glucose tolerance test (OGTT) modifies glucose homeostasis in adults with different metabolic phenotypes. Participants will undergo two experimental conditions in random order: food odor stimulation and control condition without odor, separated by a 4-week washout. The main objective is to quantify the within-subject effect of food odor stimulation on the incremental area under the glucose curve (iAUC) from 0 to 120 minutes during OGTT and to assess whether this effect differs according to metabolic status. Two predefined groups will be enrolled: adults without overweight and without insulin resistance, and adults with class I obesity and low-to-moderate insulin resistance. Secondary objectives include characterization of cephalic phase insulin release (CPIR), C-peptide and GLP-1 responses, glycemic kinetics, associations between CPIR and metabolic responses, and participant acceptability of the test environment and olfactory stimulation. A plasma biobank will be constituted from part of the collected samples for future research.

Inclusion Criteria * Age 18 to 50 years inclusive * Stable body weight during the previous 3 months (±5% of total body weight) * Willing to comply with the full study protocol * Sedentary lifestyle or stable regular physical activity, with agreement to keep this unchanged throughout the study * Able to understand study information, read and write French, and provide written informed consent * Affiliated with a social security scheme or equivalent * Non-smoker and non-vaper * Willing not to take dietary supplements, probiotics, prebiotics, or laxatives for 10 days before each visit * For women of childbearing potential: negative serum pregnancy test; not pregnant and not breastfeeding * Mean score between 1 and 2 on the 3 specific CiTAS questionnaire statements used as inclusion criteria * ETOC flash olfactory screening: able to detect the odor-containing vial among 4 presented vials for all 7 odors tested * Able to identify the madeleine odor used in the study * Rated pleasantness/appetence of the madeleine odor above 1/9 * For the no-overweight group: BMI 19 to \