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untrained individuals, LT may occur at exercise intensities as low as 50% of maximum heart rate
but this can be raised to 85-90% of maximum heart rate in elite athletes. Exercise above LT is
generally considered anaerobic and is discussed in the following chapter. With training at and
above LT, there is a shift to the right in LT. This allows higher intensities of exercise to be
performed before fatigue sets in. The changes which occur in LT are shown in figure 3

Figure 3: Change in lactate threshold with training

Lactic Lactate Threshold (LT)

Acid Untrained

(mmol)

4 Trained

2

0

Exercise intensity

Note: Circles represent LT.

Liver glycogen

In addition to muscle glycogen and freely circulating blood glucose, liver glycogen plays a
role in energy production during exercise. The liver stores about 110 grams of glycogen under
normal conditions and this can be almost doubled to approximately 200 grams with a high
carbohydrate diet. A ketogenic diet will reduce liver glycogen to approximately 13 grams (22).

Liver glycogen breakdown accelerates in response to the increase in adrenaline and
noradrenaline during exercise (see section 3 of this chapter). Glycogen depletion will occur 15-24
hours after carbohydrates have been removed from the diet depending on initial liver glycogen
levels (23).

Approximately 2 hours of low-intensity aerobics are necessary to totally deplete liver
glycogen following an overnight fast (22). High-intensity exercise will cause greater liver
glycogen output (1) although it is difficult to estimate exactly how much exercise would be needed
to totally deplete liver glycogen. As discussed in detail in chapter 4, the depletion of liver glycogen
is critical for the rapid establishment of ketosis, especially for individuals on a CKD. The effects
of exercise on the establishment of ketosis is discussed in chapter 22.

Additionally, glucagon and cortisol levels (which increase with exercise) further influence
liver glycogen release into the bloodstream. When liver glycogen is depleted, blood glucose drops
and the resulting hypoglycemia (low blood sugar) may be one cause of fatigue during aerobic
exercise.

It should be noted that total bodily glycogen and glucose stores can only provide
approximately 1500 calories of energy (this can be doubled with carbohydrate loading), enough to
run approximately 15 miles. As this is still fairly limiting energy wise, the body has several other
sources of fuel that it can utilize during exercise. The other major fuel for energy during aerobic
exercise is fat, in the form of free fatty acids or intramuscular triglyceride.