Postprandial glycemia (PPG) is a relevant determinant of glucose control in people with type 2 diabetes (T2D). Epidemiological and pathophysiological studies indicate that PPG is a better risk predictor for cardiovascular disease and all-cause mortality than fasting plasma glucose. Therefore, both fasting and postprandial glycemia should be targeted to achieve optimal glycemic control and, thus, prevent or reduce the risk of diabetes complications.

Post-prandial glucose response (PGR) cannot be predicted based solely on the meals' carbohydrate content. Recent research using continuous glucose monitoring (CGM) systems has identified different patterns of PGR to a standard meal among both healthy people and individuals with type 1 diabetes. Different contributors to the PGR have emerged, including genotype, hormonal and metabolic factors, phenotype, gut microbiota composition, background diet, sleep habits, physical activity levels.

The present project aims at exploring the PGR in a real-life setting in a cohort of people with T2D, and identifying person-specific factors associated with different postprandial glucose patterns.

To this purpose, 144 individuals with T2D on treatment with diet alone or diet plus metformin will be characterized for their anthropometric, metabolic, and gut-microbiome features and will undergo a one-week observational period through CGM system, while properly recording their food intake, physical activity, and sleep habits. A mixed-nutrient standardized meal will be consumed at home in two occasions by each participant to investigate the intra-individual variability of the PGR. Moreover, in a subgroup of participants (n=60), divided according to anthropometric and metabolic features, hormonal and metabolic response to the standardized meal will be evaluated at the hospital, to explore the contribution of different T2D phenotypes to the PGR.

A further step will be developing a prediction algorithm of PGR based on the intra- and inter-individual factors shown to influence postprandial glucose, able to further optimize the management of T2D with precision therapeutic strategies.

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.

Inclusion Criteria:

• Men and post-menopausal women
• HbA1c ≤ 7.5%
• Body Mass Index (BMI) 20-39.9 kg/m2
• Ability to comply with the study protocol activities.

Exclusion Criteria:

• Any acute or chronic condition possibly influencing the interpretation of the results or possibly worsened because of study participation
• Pregnancy or breast-feeding
• Use of antibiotics, probiotics, or prebiotics during the last three months.