Meeting Abstract
Moderate intensity exercise in humans consistently causes increases in metabolic rates, and thankfully, also increased rates of lipid oxidation. Interestingly, it is not uncommon for a standard laboratory mouse consuming a diet with >5% lipid content to voluntarily run as much as 3 km per day when provided with a running wheel. Changes in RER are often used to infer shifts in oxidative substrates, but such changes are difficult to reliably quantify during short term bursts of activity (e.g., 20-60 seconds) on a running wheel. In order to reliably document changes in oxidative fuels we designed a treadmill respirometry system with reduced time constant to document rapid changes in respiratory gases. Subadult mice were switched from a control diet to one that was supplemented with a stable isotope labeled fatty acid (13C-palmitic acid; 4g/kg chow) for one week in order to enrich their body lipids. Mice were fasted for 1 h prior to experimental trials to minimize the assimilation (and subsequent oxidation) of recently ingested nutrients. We then measured the metabolic rates (VCO2) as well as the isotopic enrichment (δ13C) of mice for 5 minutes prior to exercise and during a 7-min bout of exercise at a speed of 15 m/min, and found that VCO2 increased by over 50% during the first minute of exercise and leveled out during continued exercise. Rates of oxidation of endogenous palmitic acid (calculated from the (δ13C in the exhaled breath) increased by 40% during the first minute of exercise; however, these values rapidly decreased, and during the last minute of exercise they were not statistically different from the resting state. These results suggest that unlike humans, mice do not fuel sustained, short-term locomotor activity using endogenous lipid stores.
