Extended indirect calorimetry with isotopic CO(2) sensors for prolonged and continuous quantification of exogenous vs. total substrate oxidation in mice

Indirect calorimetry (InCa) estimates whole-body energy expenditure and total substrate oxidation based on O(2) consumption and CO(2) production, but does not allow for the quantification of oxidation of exogenous substrates with time. To achieve this, we incorporated (13)CO(2) and (12)CO(2) gas sensors into a commercial InCa system and aimed to demonstrate their performance and added value. As a performance indicator, we showed the discriminative oscillations in (13)CO(2) enrichment associated with food intake in mice fed diets containing naturally low (wheat) vs high (maize) (13)C enrichment. To demonstrate the physiological value, we quantified exogenous vs total carbohydrate and fat oxidation continuously, in real time in mice varying in fat mass. Diet-induced obese mice were fed a single liquid mixed meal containing (13)C-isotopic tracers of glucose or palmitate. Over 13 h, ~70% glucose and ~48% palmitate ingested were oxidised. Exogenous palmitate oxidation depended on body fat mass, which was not the case for exogenous glucose oxidation. We conclude that extending an InCa system with (13)CO(2) and (12)CO(2) sensors provides an accessible and powerful technique for real-time continuous quantification of exogenous and whole-body substrate oxidation in mouse models of human metabolic physiology.