Patterns of fuel use during locomotion in mammals revisited: the importance of aerobic scope

Abstract

Fuel selection patterns during exercise are thought to be conserved among sea-level native mammals when intensity is expressed relative to maximum aerobic capacity (V · O2,max ). However, this claim is based on data from only a few species larger than rats, and has never been tested statistically. Thus, we investigated fuel use in a small mammal (Mus musculus, CD-1 strain), and combined these data with published data on rats, dogs, goats and humans to evaluate the robustness of the mammalian fuel selection model. We found that mice rely less on carbohydrates to power moderate intensity exercise at the same % V · O2,max than larger mammals. We suggest that this difference is due to a decline in aerobic scope (O 2 available for exercise above resting metabolism) as body size decreases. We propose a redefined fuel use model that reflects changes in fractional aerobic scope with body size. Our results indicate that exercise defined as percent aerobic scope is a better predictor of fuel use across a wide range of quadruped species from mice to dogs and running humans.

Acclimation to hypoxia increases carbohydrate use during exercise in high-altitude deer mice

“…To control for the effect of exercise intensity on fuel use (see Ref. 45), mice were exercised at a speed equivalent to~75% of V O2max. For a few mice, we lacked V O2max data and used average treadmill speeds for their acclimation group to perform submaximal running tests on these individuals.…”

“…There is a strong relationship between exercise intensity relative to maximum aerobic capacity and the proportional use of carbohydrates and lipids (45). Thus, we standardized exercise intensity by first determining running V O 2max for animals in their respective acclimation condition.…”

“…Animals living in these environments must be able to transport sufficient O 2 to support exercise but also allocate the appropriate metabolic substrates to sustain aerobic ATP production rates. The pattern of metabolic substrate use during exercise has been characterized in a number of mammals (10,41,45,55). In general, as metabolic demand increases with higher work rates, there is a predictable increase in the reliance on carbohydrates to support aerobic ATP production.…”

“…In general, as metabolic demand increases with higher work rates, there is a predictable increase in the reliance on carbohydrates to support aerobic ATP production. This pattern is remarkably conserved in low-altitude native mammals that vary in body size and aerobic capacity, suggesting the basis for a predictive model of exercise fuel use (45). Although exercise fuel use demonstrates phenotypic plasticity to chronic energetic (e.g., exercise training) and environmental (e.g., hypoxia) stress, the observed changes in rates of metabolic substrate oxidation are correlated with changes in aerobic capacity (e.g., Refs.…”

Exploring key issues of aerobic scope interpretation in ectotherms: absolute versus factorial

“…The concept has since been applied to a great range of aquatic and terrestrial species (e.g. Auer et al 2015;Bishop 1999;Moberly 1968;Overgaard et al 2012;Schippers et al 2014;Sokolova and Pörtner 2003). On the surface, aerobic scope seems to be a relatively straightforward variable.…”