This paper quantifies maximal flows of carbohydrates and lipids through the pathways supplying the mitochondria. Maximal flow rates are the main functional parameter used in testing the principle of symmorphosis, which states that structural capacities are quantitatively matched to functional demand. Only under rate-limiting conditions will all of the structural capacity be used. Dogs and goats were compared to obtain large differences in absolute rates. We exercised the animals for long enough to reach steady-state O2 and CO2 exchange rates at intensities eliciting 40%, 60% and 85% of the maximal rate of oxygen consumption (MO2max). We then calculated rates of fat and carbohydrate oxidation from the ratio of CO2 produced to O2 consumed (the respiratory exchange ratio). The dog's Mo2max was more than twice that of the goat (6517 versus 3026 mumol O2 kg-1 min-1). We found the same pattern of fuel selection as a function of exercise intensity in both species, and it appears to be general to mammals. Maximal rates of fat oxidation were reached at 40% exercise intensity, where 77% of the energy was supplied by fat. As exercise intensity increased, all additional energy was supplied by carbohydrates. We conclude that the partitioning of fuel supply to the fat and carbohydrate pathways follows the same pattern in both dogs and goats.

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