high levels, FFA release from the adipose cell is inhibited by lactic acid and the decrease in fat
oxidation (and subsequent increase in glycogen utilization) is related primarily to decreased
availability.
Additionally, during high-intensity activity, even with sufficient FFA present, fat oxidation
is still impaired. This indicates that other factors, such as oxygen availability, also play a role.
Oxygen availability during exercise is determined both by the amount of blood being pumped (by
the heart) as well as the muscle’s capacity to use oxygen in the bloodstream.
Intramuscular triglyceride (TG) metabolism
As an additional source of energy, there are droplets of fat, called intramuscular
triglyceride (IM TG), stored within the muscle fiber (1,5,31). IM TG is oxidized in the same
manner as blood borne FFA and play a large role in energy production (32). As this type of TG is
stored within the muscle, they are thought to be a more rapidly accessible form of fat energy
during exercise. The utilization of IM TG is highest in Type I fibers due to higher levels of
oxidative enzymes (3,31). Type II muscle fibers do not readily use IM TG for fuel, relying almost
solely on glycogen for energy (31).
Utilization of IM TG ranges from 10% during low-intensity exercise to 50% at high
intensities. (3,31) This is related to changes in hormone levels and blood flow (32). As adrenaline
levels go up at higher intensities, IM TG use is stimulated. Well trained individuals use more IM
TG during exercise than untrained (14, 31). Following exercise which depletes IM TG, there is a
rapid uptake of FFA into the muscle to replenish the TG stores (31).
Ketones
The oxidation of ketones for fuel is similar to that of FFA and intramuscular triglyceride.
Under normal (non-ketotic) conditions, ketones may provide 1% of the total energy yield during
exercise (33). During the initial stage of a ketogenic diet, ketones may provide up to 20% of the
total energy yield during exercise (34). After adaptation, even under conditions of heavy ketosis,
ketones rarely provide more than 7-8% of the total energy produced, a relatively insignificant
amount (35-37).
The reason that more ketones are not used during exercise is to ensure that the brain has
adequate amounts of ketones. Therefore, during aerobic exercise, the muscle will primarily use
FFA and glucose for fuel and ketones can generally be ignored as a fuel source. As we will see in a
later section, blood glucose availability does not appear to limit aerobic exercise on a ketogenic
diet.
Protein
In each muscle there is a pool of amino acids (AAs), that can be used to provide energy
under specific circumstances. Generally, the oxidation of AAs during exercise is small, accounting