Experiment 1 - Pilot Study to Establish the Use of U-13C-Glucose Breath Test (13C-GBT) in Healthy Controls A. Pre-study Day Protocol Written informed consent will be obtained from participants before participating in the study. All participants will be requested to fast overnight (~ 12 hours) and will be asked to come to the Clinical Research and Evaluation Unit for a preliminary assessment (pre-study day) to measure basic anthropometry (weight and height), body composition and resting energy expenditure (REE). Weight and height will be measured by using a digital scale and a stadiometer, respectively. REE will be measured by continuous, open-circuit indirect calorimetry (CareFusion Vmax Encore, VIASYS), which will be calibrated prior to use. Total energy expenditure (TEE) and basal metabolic rate (BMR) are associated to fat free mass (FFM); therefore body composition will be determined using bioelectrical impedance analysis (BIA) (Quantum IV RJL systems). BIA measures resistance, reactance and impedance by applying electrodes placed at the right wrist and right ankle. Three readings for resistance, reactance and impedance will be taken for each participant - then the mean of these readings will be used to determine FFM and fat mass (FM). FFM and FM will be calculated using the manufacturer's software system (RJL Systems, Body Composition Analysis V.2.1). A general questionnaire will be used during the pre-study day to give a clear idea about medical history, nutritional status, supplement intake and physical activity. Finally, 3-day dietary record sheets will be distributed to the participants to record food consumption for any two days during the week and one day on the weekend.