DESIGN OF THE OXYGEN AND SUBSTRATE PATHWAYS .2. DEFINING THE UPPER LIMITS OF CARBOHYDRATE AND FAT OXIDATION

This paper quantifies maximal flows of carbohydrates and lipids throug h the pathways supplying the mitochondria, Maximal flow rates are the main functional parameter used in testing the principle of symmorphosi s, 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 obtai n large differences in absolute rates, We exercised the animals for lo ng enough to reach steady-state O-2 and CO2 exchange rates at intensit ies eliciting 40 %, 60 % and 85 % of the maximal rate of oxygen consum ption (M(O2max)), We then calculated rates of fat and carbohydrate oxi dation from the ratio of CO2 produced to O-2 consumed (the respiratory exchange ratio), The dog's M(O2max) was more than twice that of the g oat (6517 versus 3026 mu mol O(2)kg(-1)min(-1)). We found the same pat tern of fuel selection as a function of exercise intensity in both spe cies, and it appears to be general to mammals, Maximal rates of fat ox idation were reached at 40 % exercise intensity, where 77 % of the ene rgy was supplied by fat, As exercise intensity increased, all addition al energy was supplied by carbohydrates. We conclude that the partitio ning of fuel supply to the fat and carbohydrate pathways follows the s ame pattern in both dogs and goats.