C. Study day Protocol For pilot study, 10 healthy adults (19-35 y) will undergo 13C-GBT protocols in two separate study days as a proof-of-principle, once without oral isotope dose and once with oral isotope dose (U-13C-glucose), each study separated by ≥ 1 week. Thus, 10 healthy adults will be studied for a total of n = 20 studies. After obtaining informed consent from the subject, the subject will be enrolled in the study. The participant will arrive for the study day after fast (~4 h) to standardize measurements at the Clinical Research and Evaluation Unit (CREU) at BC Children's Hospital. Basic anthropometric measurements (body weight and height) will be collected and a brief study day questionnaire will be administered to collect information on medical, diet and physical activity history. 2 Baseline breath samples will be collected to determine natural background 13C abundance. Subjects will receive on study day 1 glucose (75 g/day) dissolved in sterile water, study day 2 U-13C-glucose (75 mg/day) (99 atom% 13C enrichment, Cambridge Isotope Laboratories Inc., Andover, MA) with glucose (75 g/day) dissolved in sterile water.