Fig. 1.
A contemporary view of the reciprocal relationship between carbohydrate and fat oxidation during exercise at power outputs of 40 %, 65 %, and approximately 80 % maximal oxygen uptake (V o 2max). Increasing the availability of plasma free fatty acids (FFAs) had no effect on acetyl-coenzyme A (CoA) and glucose-6-phosphate (G-6-P) contents (X = no effect) at any power output and increased citrate content only at 40 and 65 % V o 2max. Reduced FFA availability did reduce pyruvate dehydrogenase (PDH) activity at 40 and 65 % V o 2max and the flux through glycogen phosphorylase (PHOS) at all power outputs. The effect on phosphorylase flux was dominant at approximately 80 % V o 2max and was less important at 40 and 65 % V o 2max. The accumulation of free adenine diphosphate (ADP), adenine monophosphate (AMP) and inorganic phosphate (Pi) was reduced during exercise (as indicated by dashes) in the presence of increased FFA availability. Mitochondrial nicotinamide adenine dinucleotide (NADH) may be more abundant with high fat provision at the onset of exercise, increasing the aerobic production of adenosine triphosphate (ATP) and reducing the mismatch between ATP demand and supply and accounting for the reduced accumulation of ADP, AMP, and inorganic phosphate. ALB albumin, FABP fatty acid binding protein, G-1-P glucose-1-phosphate, HK hexokinase, IMTG intramuscular triacylglycerol, MM mitochondrial membrane, PFK phosphofructokinase, PM plasma membrane