Purpose: Glycogen storage disease type I (GSD I), also known as Von Gierke disease is caused by deficiency of glucose-6-phosphatase (G6Pase) enzyme affecting 1:100,000 births worldwide. Therefore, deficiency of this enzyme results in accumulation of glycogen in liver and inadequate production of glucose leading to fasting hypoglycemia. Dietary treatment for GSD I is based on supplying complex carbohydrates with small frequent meals, and use of uncooked cornstarch (UCCS). Glycosade® (modified cornstarch) has shown promise in maintaining normal blood glucose concentrations in subjects with GSD I. However, the primary dilemma in the dietary treatment of GSD I is the choice of the exogenous carbohydrate sources to achieve a desirable metabolic control and prevent hypoglycemia. In addition, the previous studies were invasive, which required several blood samples, long study days (~10 h) and could not offer mechanistic details of glucose metabolism in subjects with GSD I. Thus, there is a need to develop a minimally-invasive method to examine glucose metabolism and measure the utilization of different carbohydrate sources directly in subjects with GSD I and healthy controls. This simple breath test will help individualize treatment and tailor carbohydrate supply in subjects with GSD I.