New insights into the interaction of carbohydrate and fat metabolism during exercise

Abstract

Fat and carbohydrate are important fuels for aerobic exercise and there can be reciprocal shifts in the proportions of carbohydrate and fat that are oxidized. The interaction between carbohydrate and fatty acid oxidation is dependent on the intracellular and extracellular metabolic environments. The availability of substrate, both from inside and outside of the muscle, and exercise intensity and duration will affect these environments. The ability of increasing fat provision to downregulate carbohydrate metabolism in the heart, diaphragm and peripheral skeletal muscle has been well studied. However, the regulation of fat metabolism in human skeletal muscle during exercise in the face of increasing carbohydrate availability and exercise intensity has not been well studied until recently. Research in the past 10 years has demonstrated that the regulation of fat metabolism is complex and involves many sites of control, including the transport of fat into the muscle cell, the binding and transport of fat in the cytoplasm, the regulation of intramuscular triacylglycerol synthesis and breakdown, and the transport of fat into the mitochondria. The discovery of proteins that assist in transporting fat across the plasma and mitochondrial membranes, the ability of these proteins to translocate to the membranes during exercise, and the new roles of adipose triglyceride lipase and hormone-sensitive lipase in regulating skeletal muscle lipolysis are examples of recent discoveries. This information has led to the proposal of mechanisms to explain the downregulation of fat metabolism that occurs in the face of increasing carbohydrate availability and when moving from moderate to intense aerobic exercise.

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 (P_i) 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

Fig. 2

A schematic representation of the potential effects of carbohydrate ingestion before dynamic exercise in decreasing the plasma free fatty acid (FFA) concentration and downregulating fat metabolism in skeletal muscle. Ingested glucose increases the release of insulin, which inhibits adipose tissue lipolysis and reduces the plasma [FFA]. Increased insulin may also inhibit FFA transport across the plasma membrane (PM) and the mitochondrial membrane (MM) and decrease intramuscular triacylglcerol (IMTG) breakdown in skeletal muscle. Carbohydrate oxidation, from plasma glucose and/or muscle glycogen, is increased. ALB albumin, CoA coenzyme A, FABP fatty acid binding protein, G-1-P glucose-1-phosphate, G-6-P glucose-6-phosphate, HK hexokinase, PDH pyruvate dehydrogenase, PFK phosphofructokinase, PHOS glycogen phosphorylase