Continuous Glucose Monitoring in Adult Patients With Glycogen Storage Disease Type I and the Impact of Raw Cornstarch Addition to Meals on Postprandial Glycemic Response

The study is designed as an observational study. Participants will be enrolled at the Unit of Diabetology of the University Hospital "Federico II" in Naples, Italy. After providing written informed consent, each participant will undergo the following assessments, all of which are part of clinical management of patients with hepatic glycogen storage disease and will be performed at the Unit of Diabetology of the University Hospital "Federico II" in Naples:

• anthropometric measurements: body weight, height, waist circumference, and hip circumference;
• body composition assessment by bioelectrical impedance analysis (BIA) using the QUANTUM V Segmental bioimpedance analyzer to obtain resistance, reactance, and phase angle. These parameters will subsequently be analyzed using dedicated software to estimate fat-free mass (FFM), fat mass (FM), total body water (TBW), intracellular and extracellular water, and skeletal muscle mass;
• assessment of resting energy expenditure and substrate oxidation by indirect calorimetry (COSMED calorimeter) using a ventilated hood system equipped with an infrared analyzer for carbon dioxide production (VCO₂) and a zirconium cell analyzer for oxygen consumption (VO₂);
• handgrip strength test for the evaluation of muscle strength;
• biochemical profile assessment, including fasting plasma glucose, glycated hemoglobin (HbA1c), triglycerides, LDL cholesterol, serum and urinary creatinine, uric acid, aspartate aminotransferase (AST/GOT), alanine aminotransferase (ALT/GPT), lactate, and complete blood count.

The following glucometric parameters recorded during the 14 days preceding the study visit will be collected through continuous glucose monitoring (CGM) systems and downloaded from dedicated platforms:

• Glucose Management Indicator (GMI);
• coefficient of variation (CV%);
• Time in Range (TIR: glucose ≥70 and ≤140 mg/dL);
• Time Above Range (TAR: level 1 hyperglycemia >140 and ≤180 mg/dL; level 2 hyperglycemia >180 mg/dL);
• Time Below Range (TBR: level 1 hypoglycemia ≥54 and <70 mg/dL; level 2 hypoglycemia <54 mg/dL);
• number of hypoglycemic events.

Dietary intake of calories, macronutrients, and micronutrients will be assessed through a 7-day food diary. Participants will complete the food diary according to routine clinical practice, as patients with glycogen storage disease regularly use food diaries during nutritional follow-up. To ensure the quality and consistency of the collected data, participants will receive standardized training from a dietitian prior to diary completion. Instructions will include meal timing, food description, portion size estimation, dose and timing of raw cornstarch administration, and accurate recording of meal initiation and completion times. Following completion, food diaries will be reviewed and harmonized by the same dietitian involved in routine nutritional follow-up. This process will include verification of diary completeness, correction of omissions or inconsistencies through direct patient clarification, and conversion of reported quantities into standardized units (grams, milliliters, equivalent portions). Dietary data will be entered into the Metadieta software database, based on CREA Food Composition Tables. For each food item, the software automatically calculates energy intake, macronutrients (carbohydrates, proteins, lipids), and micronutrients (vitamins and minerals). The software also allows automated calculation of average daily nutrient and caloric intake for both quantitative and qualitative dietary analyses. Meals extracted from the food diaries will be categorized as "meals with cornstarch" in patients routinely consuming raw cornstarch during main meals according to medical prescription, and "meals without cornstarch" in patients not routinely consuming cornstarch during main meals. To evaluate the effect of meals consumed with or without concomitant raw cornstarch supplementation on postprandial glycemic response, the following dynamic glycemic indices will be considered: