Endurance trained athletes possess increased insulin sensitivity and fat oxidative capacity over untrained individuals. They also have increased levels of intramuscular fat known as intramyocellular lipid. It appears that aerobic exercise training increases levels of this fat while improving obesity indices.
A 16 week moderate aerobic exercise training program produced a 21% increase in intramyocellular lipid in overweight individuals, while other obesity indices improved. The increase in insulin sensitivity to aerobic training may be the cause of the improved fat partitioning.
Aerobic training also improved fat oxidative capacity, glycogen content, capillary density and muscle fiber type.