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for perhaps 5% of the total energy yield in men and less in women (see section on gender
differences below). With glycogen depletion, this may increase to 10% of the total energy yield,
amounting to the oxidation of about 10-13 grams of protein per hour of continuous exercise (38).
The primary type of AAs oxidized are the branch-chain amino acids (BCAAs): valine, leucine, and
isoleucine.

BCAA oxidation has been studied extensively in individuals with a metabolic defect called
McArdle’s disease. Due to an enzyme insufficiency, these patients are unable to utilize glycogen
for fuel during exercise. This makes them a good (although extreme) group to study in terms of
glycogen depletion as would be seen during prolonged exercise or a ketogenic diet (39).

During prolonged exercise, McArdle’s patients show increased levels of ammonia which
comes from the breakdown of ATP shown in figure 3.

Figure 3: Breakdown of ATP to ammonia when glycogen is unavailable

ATP ADP AMP IMP + ammonia

To buffer the increased ammonia, glutamate is converted to glutamine, which carries the
ammonia to the liver to be excreted as urea. The BCAA’s (especially leucine) are used to
generate glutamine which is then released into the bloodstream (39). The increased ammonia
load seen in this situation may be one cause of fatigue during exercise. During prolonged exercise,
the increase in ammonia is caused by AA oxidation due to muscle glycogen depletion (39).

The protein oxidized in this fashion appears to comes from the intramuscular AA pool and
not from actual contractile tissue. However several studies have shown an increase in protein
requirements for endurance athletes in heavy training (40,41) indicating that depletion of the
intramuscular AA pool is ultimately damaging to the body. Therefore, excessive aerobic activity
should be avoided to prevent muscle loss. Specific recommendations for aerobic exercise appear
in chapter 24.

As the contribution of protein and ketone bodies to energy generation during exercise is
generally small, they will both be ignored in further discussion of exercise metabolism.

Summary of IM TG, ketone and protein oxidation

As an alternative source of fat derived fuel, the body has a store of triglyceride within the
muscle which can be used to provide energy during higher intensity aerobic exercise. As well,
ketones and protein can provide small amounts of energy during aerobic exercise. In general
ketones do not provide significant amounts of energy during aerobic exercise and protein will only
be broken down to a great degree when glycogen is depleted.