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Figure 1: Pathway of glucose/glycogen breakdown

glycolytic enzymes (Aerobic)

Glucose/Glycogen ATP + Pyruvate Krebs cycle, liver, other

pathways

(Anaerobic)

Lactate/lactic acid

During exercise below the lactate threshold (discussed below), glycolysis results primarily
in the production of pyruvate which can be reused in the mitochondria to produce more ATP.
Alternately pyruvate is released into the bloodstream where it travels to the liver and is
converted to glucose (to be released back into the bloodstream) through a process called
gluconeogenesis. Regardless of the ultimate fate of pyruvate, it is reused to produce ATP. The
overall energy yield of aerobic glycolysis is 36-39 ATP molecules per molecule of glycogen or
glucose broken down (5). This is sometimes called slow glycolysis.

At higher exercise intensities, pyruvate is converted to lactic acid, which lowers pH inside
the muscle and causes a burning sensation. Lactic acid separates in the muscle to lactate (a
salt) and H+ (a proton). The accumulation of H+ lowers the pH of muscle, inhibiting further
energy production. High levels of lactic acid/lactate are likely involved in fatigue during high-
intensity aerobic exercise. The production of lactate from the breakdown of glycogen or glucose is
referred to as fast glycolysis (5).

It should be noted that lactate is always being produced by the muscle to a small degree.
Lactate levels are low at rest and increase gradually as exercise intensity increases (21). In the
past, lactate was thought of as only a waste product of glycolysis that caused fatigue. It is now
recognized that lactate is another useful fuel during and after exercise. Lactate can be used for
energy by slow twitch muscle fibers (Type I) as well as by the heart. Alternately, lactate can
diffuse into the bloodstream, travel to the liver, and be converted to glucose or glycogen through
the process of gluconeogenesis.

Following exercise, lactate can be regenerated to muscle glycogen which may have
implications for individuals following a standard ketogenic diet as glycogen availability is the
limiting factor in many types of exercise. Post-workout glycogen resynthesis from lactate is
discussed in chapter 10.

Exercise and the lactate threshold

The body has a limited capacity to buffer the lactate produced during glycolysis. As
exercise intensity increases, the body’s ability to buffer and/or reuse lactate is surpassed and
lactate accumulates in the bloodstream. Although there is still debate in the literature over
proper terminology, this is generally referred to as the lactate threshold (LT). (21)

The LT represents the maximum exercise intensity which can be sustained for long periods
of time. Above this level lactate levels increase quickly (see figure 3) causing fatigue. In