Experiment 2 - Minimally Invasive 13C-Glucose Breath Test to Measure the Utilization of Different Exogenous Carbohydrate Sources in Healthy Controls and Subjects with Glycogen Storage Disease Type I For initial set of studies, 10 healthy children (5-18 y) and 10 healthy adults (19-35 y) will undergo 13C-GBT four times. Then, 3 children and 5 adults with GSD I will be recruited in 4 separate study days from our Pediatric and Adult metabolic clinic at BC Children's and Vancouver General Hospital, respectively (if recruitment is a challenge, we plan to approach clinics across Canada). After fast (~ 4 h for healthy controls and 2 h for GSD I) subjects will receive on study day 1- UCCS (2 g/kg/day), study day 2- U-13C-glucose (75 mg for adults; 1.5 mg/kg for children) (99 atom% 13C enrichment, Cambridge Isotope Laboratories Inc., Andover, MA) with UCCS (2 g/kg/day), study day 3- the extended release waxy maize cornstarch Glycosade® (2 g/kg/day) (Vitaflo International Ltd, Liverpool, England), and on study day 4- U-13C-glucose (75 mg for adults; 1.5 mg/kg for children) with Glycosade® (2 g/kg/day) dissolved in sterile water. On each study day, breath samples will be collected every 20 min for 240 min. The rate of CO2 production will be measured at 120 min after oral dose using indirect calorimetry. Blood glucose will be measured using glucometer and finger-prick blood samples will be assessed at the beginning and hourly until the end of the study day. The percentage of test carbohydrates exhaled, as 13CO2 will be measured over a 4 h period.