One factor counteracting the low fat combustion at high exercise intensities is the effect of training. It has been convincingly shown that, at a certain exercise intensity, a trained individual uses more fat than an untrained individual. This effect is quite strong and occurs after relatively short periods of training. One group of subjects was studied after 5 and 31 days of training for 2 h daily at a moderately high exercise intensity (60% of the pretraining Vo2max) . Following 5 days of training, the total fat oxidation at this intensity had increased by i0% and after 3i days of training, the increase was as high as 70%. The oxidation of carbohydrates during the exercise bout showed the opposite pattern. It is therefore obvious that a good physical fitness level makes it much easier to maintain a high degree of fat combustion during intense exercise .
The source of the increased fat usage during exercise in endurance-trained subjects has been debated, however, since the plasma levels of free fatty acids during exercise are often lower than in untrained individuals . This is likely to be secondary to the lower sympathoadrenal activation after training [5i] which, unopposed, would lead to decreased lipolysis of not only adipose tissue, but also intramuscular triglycerides. When male subjects exercised at the same absolute intensity (64% of the pretraining Vo2max)
before and after a i2-week programme of endurance training, plasma free fatty acid and glycerol concentrations were found to be lower in the trained than in the untrained state . In spite of this, the respiratory exchange ratio was reduced after training, indicating a greater reliance on fat oxidation. Muscle triglyceride utilization was found to be twice as great and muscle glycogen utilization to be 40% lower after, as opposed to before, training. It was concluded that the greater utilization of fat in the trained than in the untrained state was fueled by increased lipolysis of intramuscular triglycerides. This conclusion was supported by a study showing a lower turnover of plasma free fatty acids in the trained state . In fact, Jansson and Kaijser  also concluded that the reduced reliance on carbohydrate metabolism in their trained, as compared to their untrained, individuals would have been covered by intramuscular triglycerides. They based this conclusion on the finding of no difference between trained and untrained individuals in the ratio of plasma free fatty acid extraction to O2 extraction by the working legs.
It is known that increased fat oxidation with training is a local effect since after one-leg training it occurs in the trained leg only [54,55]. Underlying this training response is an increased mitochondrial density and an increased content of mitochondrial enzymes in aerobi-cally trained muscle, accompanied by increases in the enzymes involved in activation, transfer into the mitochondria and b-oxidation of fatty acids [56-58]. Hol-loszy and coworkers have formulated a hypothetical biochemical mechanism whereby a large concentration of mitochondrial oxidative enzymes in trained muscle would lead to a greater reliance on fat metabolism, a lower rate of lactate formation and sparing of muscle glycogen during exercise . These adaptations in trained skeletal muscle would, at a given exercise intensity, permit the rate of fatty acid oxidation to be higher in the trained than in the untrained muscle, even in the presence of a lower intracellular fatty acid concentration in the trained state.
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