The participants will undergo a 7-day observational period and acute postprandial tests with a standardized meal. Before the study initiation, all participants will undergo a screening visit for the assessment of inclusion and exclusion criteria. Participants meeting the inclusion criteria will be instructed to collect a stool sample and will be scheduled for the experimental visit (day 0). At day 0, each participant will arrive at the diabetes clinic in the morning, after a 10-hour fasting, to undergo:
* venous blood sampling for biochemical determinations
* anthropometrics and blood pressure measurements
* body composition evaluation (bioimpedance analysis)
* basal metabolic rate evaluation (indirect calorimetry)
* administrations of questionnaires for the evaluation of lifestyle habits
* positioning of a subcutaneous sensor for continuous glucose monitoring (CGM).
Information regarding characteristics of bowel movements in the day of stool sampling will be collected according to the Bristol stool scale.
The participants will be instructed to consume at home a mixed-nutrient meal at breakfast on the second day and on the last day of the observational period. PGR will be assessed by CGM.
Over the following 7 days, participants will undergo CGM while keeping stable their own dietary and lifestyle habits and will be asked to record the following information by using a dedicated app:
* a 7-day food diary reporting all foods and drinks consumed, including dressing, portions by household measures (cup, spoons, etc.) or weight, and providing as much details as possible (i.e., cooking methods, brands names). Seven-day food records will be discussed with a skilled dietitian to check potential mistakes and missing information. Energy intake and dietary composition (nutrients and non-nutrients), and food groups consumption will be calculated using the Food Composition Database for epidemiological Studies in Italy of the European Institute of Oncology;
* any stressful event or drug assumption.
In parallel, during the 7-day observational period, data on physical activity levels and sleep habits will be collected by using an accelerometer (Actigraph medical device).
At the 8th day, a subgroup of 60 participants (20 with prevalent impairment of insulin sensitivity, 20 with prevalent impairment of insulin secretion, and 20 with prevalent visceral obesity - as determined by anthropometrics, fasting C-peptide, and plasma triglycerides) will return to the hospital and consume the same standardized meal consumed at home. Before and over 4 hours after the meal, venous blood samples will be collected at fixed time points for biochemical evaluations.
CGM data from 30 min before meal to 6 h after meal will be analyzed. Postprandial blood glucose changes will be calculated with the trapezoidal method as the incremental area under the curve above the baseline value (iAUC). Predictors of early blood glucose response (iAUC0-3h), late blood glucose response (iAUC3-6h), total blood glucose response (iAUC0-6h), and, as indicator of time-course of the blood glucose changes, the difference between late and early response (iAUC3-6h minus 0-3h) will be evaluated using mixed-effect linear regression considering the patient's identification number (ID) as a random effect. To evaluate the significance of the random-effect factor, ANOVA will be used to test for the difference between the model with and without the random effect.
