Trained individuals prefer using fat from muscle than from adipocyte
The ability of our body shift from using carbohydrate into fat defines our adaptation to exercise, especially endurance training. One of the most debatable properties of trained individuals is their ability to produce less cathecolamine even in the same workload with untrained individuals. There is a study shown that plasma cathecolamine of trained individual dropped into 70% compared to untrained individuals. This effect, accompanied by the incresing of insulin level, and reduction of fatty acid/ FA from adipocyte in the plasma.
Interestingly, in trained individuals there is indication that fat oxidation increased although plasma FA reduced. This paradox happends because muscle tissue of trained individual prefers to use TG from fat depots around the tissue than using FA in the plasma. In contrast with trained individuals, untrained individuals use more FA from adipose tissue as the source of energy and this process requires action of cathecolamines.
This issue is important to address if we want to study the role of exercise on beta adrenergic sensitivity. It seems that beta adrenergic receptor sensitivity reduce after prolong stimulation of cathecolamine. I assume that trained individuals have better sensitivity even before the exercise started because cathecolamine rarely used to induce lipolysis.
Ranallo RE, Rhodes EC. Lipid metabolism during exercise. Sport Med, 1998, 26: 29-42