The aim of this observational study is to evaluate the impact of raw cornstarch supplementation on postprandial glycemic response in adult patients with Glycogen Storage Disease type I (GSD I), using continuous glucose monitoring (CGM) systems.GSD I is a rare inherited metabolic disorder characterized by impaired glucose homeostasis during fasting, leading to recurrent hypoglycemia and metabolic abnormalities. Nutritional therapy, based on frequent carbohydrate intake and raw cornstarch supplementation, represents the cornerstone of treatment. However, the optimal timing of raw cornstarch administration in relation to meals and its effect on postprandial glycemic control remain unclear.The main question this study aims to answer is: "Does raw cornstarch intake during meals affect postprandial glycemic response in adult patients with Glycogen Storage Disease type I?". Additional objectives include evaluating whether dose and timing of cornstarch intake influences glucose profile, glycemic variability, hypoglycemic events, body composition, energy expenditure, and lipid profile. Participants will include adults with genetically confirmed GSD Ia or Ib who routinely use CGM systems and regularly consume raw cornstarch as part of their dietary management. This is a real-life observational study, and participants will continue their routine clinical care without receiving additional experimental treatments. After providing informed consent, participants will undergo clinical, metabolic, and nutritional assessments, including:
* anthropometric measurements; * body composition analysis by bioelectrical impedance; * indirect calorimetry for resting energy expenditure; * handgrip strength evaluation; * biochemical analyses, including fasting glucose, HbA1c, lipid profile, liver enzymes, creatinine, uric acid, and lactate.
CGM-derived parameters collected during the 14 days preceding the visit will be analyzed, including Time in Range (TIR), Time Above Range (TAR), Time Below Range (TBR), coefficient of variation (CV%), Glucose Management Indicator (GMI), and frequency of hypoglycemic episodes. Participants will also complete a 7-day food diary reporting meal timing, food intake, and raw cornstarch consumption. Investigators will compare meals consumed with and without raw cornstarch supplementation and evaluate postprandial glycemic trends. Postprandial glycemic response will be assessed using dynamic indices, including glucose peak, nadir, time to peak, time to nadir, glucose excursion rates, and incremental area under the curve (iAUC). Glucose values will be analyzed every 5 minutes for up to 4 hours after meals using CGM data. Inclusion criteria include: genetically confirmed GSD Ia or Ib, age \>18 years, routine CGM use, and habitual raw cornstarch consumption.Exclusion criteria include psychiatric disorders, pregnancy, celiac disease, dialysis treatment, nocturnal enteral feeding, and severe acute or chronic illnesses. Approximately 8 adult patients are expected to be enrolled. Risks for participants are considered minimal, as all procedures are part of routine clinical follow-up. Potential benefits include improved personalization of nutritional therapy and optimization of carbohydrate and raw cornstarch administration, potentially improving glycemic control and reducing metabolic complications in patients with GSD I.
confirmed genetic diagnosis of GSD Ia or Ib;
age >18 years;
routine CGM use;
habitual raw cornstarch consumption.
psychiatric disorders;
pregnancy, celiac disease;
dialysis treatment;
nocturnal enteral feeding;
Adult patients with genetically confirmed Glycogen Storage Disease type Ia or Ib routinely followed at the Unit of Diabetology of the University Hospital
Case-only
Prospective
Not listed
Incremental Area Under the Glucose Curve (iAUC)
Incremental area under the postprandial glucose curve (iAUC) following meals with different nutritional composition, with or without raw cornstarch supplementation, assessed using continuous glucose monitoring (CGM). Postprandial glucose excursions will be quantified as incremental area under the glucose curve (iAUC; mg·min/dL) calculated from CGM data collected at 5-minute intervals. iAUC will be calculated from 30 minutes before meal consumption up to 4 hours after the meal
Postprandial Glucose Excursion Magnitude
The magnitude of postprandial glucose excursions will be evaluated using CGM-derived glucose concentration parameters, including: Glucose Peak (mg/dL); Glucose Nadir (mg/dL). These measures will be derived from CGM recordings collected at 5-minute intervals during the period extending from 30 minutes before meal consumption to 4 hours after meal consumption.
Postprandial Glucose Excursion Timing
The temporal characteristics of postprandial glucose excursions will be evaluated using: * Time to glucose peak (minutes); * Time to glucose nadir (minutes). * Difference between time to peak and time to nadir (minutes) These measures will be derived from CGM recordings collected at 5-minute intervals during the period extending from 30 minutes before meal consumption to 4 hours after meal consumption.
Continuous Glucose Monitoring (CGM) Parameters
Longitudinal evaluation of CGM-derived glucometric parameters to assess overall glycemic control, mean glucose levels, glucose variability, and time in range. CGM-derived parameters will be evaluated in relation to dietary habits, raw cornstarch intake, and self-reported menstrual cycle phase.
Body Composition
Evaluation of body composition parameters assessed by bioelectrical impedance analysis (BIA) using a bioimpedance analyzer, including fat mass (%), fat-free mass (%),skeletal muscle mass (%), Body Cell Mass (%) and total body water (%), in relation to dietary habits and levels of raw cornstarch intake. All parameters will be reported in percentage (%).
Postprandial Tiglycerides Response
This outcome will be evaluated by assessing the postprandial lipid profile, expressed as postprandial triglyceride levels, following meals with different nutritional composition, with or without raw cornstarch supplementation.Triglyceride concentrations (mg/dL) will be assessed at baseline and at 60, 120 and 180 minutes after meal.
Resting Energy Expenditure
Evaluation of resting energy expenditure assessed by indirect calorimetry under standardized resting conditions. Results will be reported in kilocalories per day (kcal/day).
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