Summary
The overall hormonal response to aerobic exercise is geared towards fuel mobilization and
the maintenance of blood glucose. The primary storage hormone of the body, insulin, decreases
while the major fuel mobilization hormones (adrenaline, noradrenaline, cortisol, glucagon, and GH)
all increase. This mimics the hormonal response to a ketogenic diet and aerobic exercise is
inherently ketogenic in nature.
Section 4: Fatigue during aerobic exercise
The cause of fatigue during aerobic exercise depends on the intensity and duration of
activity. This has implications for the effects of a ketogenic diet and each intensity of exercise is
discussed.
During low-intensity exercise (65% maximum heart rate and below), only Type I muscle
fibers are recruited (51). Type I fibers have a high oxidative capacity and use mainly fat for fuel
(13). Additionally, Type I fibers do not generate much lactic acid. The majority of energy during
exercise at this intensity comes from the oxidation of FFA with a small contribution from blood
glucose. As there is essentially an unlimited amount of bodyfat to provide energy, fatigue during
this type of exercise is caused by dehydration, boredom and hypoglycemia (22,52). A ketogenic
diet would not be expected to affect exercise of this intensity.
As exercise intensity increases towards the lactate threshold, more Type II muscle fibers
are recruited (13). Recall that Type II fibers rely more heavily on glycogen for fuel and there is a
greater reliance on stored muscle glycogen as intensity of exercise increases. Fatigue at this
intensity generally correlates with muscle glycogen depletion (22,35). Increasing glycogen levels
with diet invariably improves performance time (52,53).
Interestingly, the exact reason that glycogen depletion causes fatigue is not known (54). It
does not appear to be related to a lack of ATP so it is not simply a matter of a lack of energy. It
may be that some glycogen breakdown is necessary to provide Krebs cycle intermediates for
FFA breakdown (54). Alternately, changes in potassium levels or an impairment of muscle
contraction may occur with glycogen depletion (54).
Another possible source of fatigue during exercise of this type of the buildup of ammonia
in the bloodstream (55). As shown in figure 2 above, ammonia is generated from the breakdown
of ATP and tends to occur when carbohydrates are unavailable. Ammonia production can also
occur from the oxidation of amino acids (56,57). Studies of the ketogenic diet have shown no
change in ammonia levels at rest (57) but inreased levels during aerobic exercise. (56,57).
A ketogenic diet will have a negative impact on performance during moderate- intensity
aerobic exercise (between 75% of maximum heart rate up to the lactate threshold) as this type of
exercise due to the lack of dietary carbohydrates.