Background Based on stoichiometric assumptions, and real-time assessment of expired carbon dioxide (%CO 2 ) and flow rate, the Lumen device provides potential for consumers/athletes to monitor metabolic responses to dietary programs outside of laboratory conditions. However, there is a paucity of research exploring device efficacy. This study aimed to evaluate Lumen device response to: i) a high-carbohydrate meal under laboratory conditions, and ii) a short-term low- or high-carbohydrate diet in healthy volunteers. Methods Following institutional ethical approval, 12 healthy volunteers (age: 36 ± 4 yrs; body mass: 72.1 ± 3.6 kg; height: 1.71 ± 0.02 m) performed Lumen breath and Douglas bag expired air measures under fasted laboratory conditions and at 30 and 60 min after a high-carbohydrate (2 g·kg −1 ) meal, along with capilliarized blood glucose assessment. Data were analyzed using a one-way ANOVA, with ordinary least squares regression used to assess the model between Lumen expired carbon dioxide percentage (L%CO 2 ) and respiratory exchange ratio (RER). In a separate phase, 27 recreationally active adults (age: 42 ± 2 yrs; body mass: 71.9 ± 1.9 kg; height: 1.72 ± 0.02 m) completed a 7-day low- (~20% of energy intake [EI]; LOW) or high-carbohydrate diet (~60% of EI; HIGH) in a randomized, cross-over design under free-living conditions. L%CO 2 and derived Lumen Index (L I ) were recorded daily across morning (fasted and post-breakfast) and evening (pre/post meal, pre-bed) periods. Repeated measures ANOVA were employed for main analyses, with Bonferroni post-hoc assessment applied ( P ≤ 0.05). Results Following the carbohydrate test-meal, L%CO 2 increased from 4.49 ± 0.05% to 4.80 ± 0.06% by 30 min, remaining elevated at 4.76 ± 0.06% by 60 min post-feeding ( P < 0.001, η p 2 = 0.74). Similarly, RER increased by 18.1% from 0.77 ± 0.03 to 0.91 ± 0.02 by 30 min post-meal ( P = 0.002). When considering peak data, regression analysis demonstrated a significant model effect between RER and L%CO 2 (F = 5.62, P = 0.03, R 2 = 0.20). Following main dietary interventions, no significant interactions (diet × day) were found. However, main diet effects were evident across all time-points assessed, highlighting significant differences for both L%CO 2 and L I between LOW and HIGH conditions ( P < 0.003). For L%CO 2 , this was particularly noted under fasted (4.35 ± 0.07 vs. 4.46 ± 0.06%, P = 0.001), pre-evening meal (4.35 ± 0.07 vs. 4.50 ± ...