![]() All participants came to the test laboratory between 7 AM and 11 AM after a 12-hour fast and had abstained from any form of physical activity (other than walking). After the minimum amount of home breath sessions were collected, participants were scheduled for the study laboratory measurement day. They were instructed to perform Lumen metabolic measurements for at least 30 sessions, with each session consisting of 3 breath maneuvers, and to complete 3 sessions at different time points each day. ![]() Participants practiced the Lumen breathing technique while supervised and took the device home for a further familiarization period in order to show proficiency with the device and app. The Lumen device was paired and synchronized to the participant’s smartphone together with the Lumen app. If they met the BMI criteria, they were provided their own Lumen device, which was labeled with their unique identification number. ![]() Participants were recruited and their height and weight were measured using a stadiometer and Seca scale (Seca). The use of a smartphone app enables the user to track metabolic status outside of physiologic test laboratories. For fat oxidation, less carbon dioxide is produced. For carbohydrate oxidation, more carbon dioxide is produced relative to the consumption of oxygen. This concept is based on the fact that oxygen consumption is stable under resting conditions thus, a change in metabolic fuel use will generally be represented by changes in CO 2 production. The % CO 2 in the exhaled volume of air is determined from a specific breathing maneuver with a breath hold of 10 seconds. The device indirectly measures metabolic fuel usage via a CO 2 sensor and a flow sensor to determine the rate of CO 2 production from a single breath maneuver. Metaflow Ltd developed Lumen, a novel metabolic fuel utilization breathalyzer, which is a personalized handheld device that provides an individual’s metabolic state in real time by measuring CO 2 from exhaled breath ( Figure 1). Though indirect calorimetry is not invasive, this method is time-consuming (up to 40 minutes), only available in test laboratory settings, and requires technical and physiological expertise for handling the metabolic cart and interpretation of the metabolic data obtained. ![]() The ratio between VCO 2 and VO 2 is the respiratory exchange ratio (RER), which provides insight into the relative contribution of carbohydrates and lipids to overall energy expenditure. Indirect calorimetry (metabolic cart), which is currently the preferred method for determining metabolic fuel utilization, measures the carbon dioxide produced (VCO 2) and oxygen consumed (VO 2) when breathing. ![]()
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