Indirect Calorimetry Fasting Challenge

The indirect calorimetry fasting challenge measures metabolic flexibility by tracking the shift in respiratory exchange ratio (RER) from carbohydrate oxidation (RER approaching 1.0) to fat oxidation (RER approaching 0.7) during a controlled 16-24 hour fast in metabolic cages equipped with oxygen and carbon dioxide analyzers. Metabolic flexibility—the ability to efficiently switch between fuel substrates in response to nutrient availability—is a hallmark of metabolic health that is impaired in obesity, type 2 diabetes, and mitochondrial dysfunction. The key derived metric is the time required for the RER to drop below 0.75, termed the flexibility index, which quantifies how rapidly an animal transitions from carbohydrate to fat oxidation after food removal and serves as an integrated measure of hepatic glycogen depletion rate, lipolytic capacity, and mitochondrial beta-oxidation efficiency.

During the fed baseline period preceding the fast, mice consuming standard chow typically exhibit RER values of 0.85-0.95, reflecting a mixed fuel substrate with predominant carbohydrate oxidation during the dark (feeding) phase. Upon food removal, healthy lean mice show a progressive decline in RER that crosses the 0.75 threshold within 8-12 hours, whereas diet-induced obese mice or those with impaired fatty acid oxidation show delayed transitions, elevated nadir RER values, or blunted diurnal RER oscillations. The protocol simultaneously captures oxygen consumption (VO2), carbon dioxide production (VCO2), energy expenditure, and locomotor activity, enabling dissection of whether metabolic inflexibility arises from reduced energy demand, impaired substrate mobilization, or defective mitochondrial oxidation.

ConductMaze orchestrates the fasting challenge by automating food access control within the metabolic cage system and continuously streaming gas exchange data at 1-3 minute intervals throughout the fed baseline and fasting phases. The software computes real-time RER with automatic detection of the 0.75 crossover point, generates substrate oxidation curves (grams of carbohydrate and fat oxidized per hour using the Weir and Frayn stoichiometric equations), and integrates locomotor activity from infrared beam breaks to calculate activity-corrected energy expenditure. A built-in welfare module monitors weight loss and activity levels during the fast, triggering automatic alerts or protocol termination if predefined humane endpoints are reached.