An overview of fat and carbohydrate regulation
As at rest, the primary fuels during aerobic exercise are carbohydrate (muscle glycogen
and blood glucose) and FFA (from adipose tissue as well as intramuscular triglyceride) (13,14).
At low intensities, fat is the primary fuel source during exercise. As exercise intensity increases,
glycogen is used to a greater degree. This fact has been misinterpreted by some to suggest that
low-intensity activity is the best choice for fat loss. However, the absolute amount of fat used
during exercise is greater at higher exercise intensities. This topic is discussed in detail in chapter
22.
As exercise intensity increases, less fat and more glycogen is used as fuel. Eventually, as
exercise intensity increases, there is a crossover point where glycogen becomes the primary fuel
during exercise (15). This point corresponds roughly with the lactate threshold, described below.
The increase in glycogen utilization at higher intensities is related to a number of factors
including greater adrenaline release (1,15), decreased availability of FFA (16), and greater
recruitment of Type II muscle fibers (15,17,18). The ketogenic diet shifts the crossover (i.e.
lactate threshold) point to higher training intensities (15) as does regular endurance training
(1,5). As discussed further below, this means that endurance athletes are able to maintain
higher exercise intensities while relying on FFA for energy.
The interactions between fat and carbohydrate utilization during exercise has been studied
extensively. The determining factor of fat versus carbohydrate utilization appears to be related
to glucose availability rather than FFA availability (14,19). When carbohydrates are abundant,
they are the primary fuel for exercise.
As discussed in detail in chapter 3, depleting muscle glycogen and lowering glucose
availability in the muscle and bloodstream increases utilization of FFA while increasing muscle
glycogen stores increases carbohydrate use (and decreases fat utilization) during exercise (20).
Additionally, protein use increases with glycogen depletion.
The proposed reason that high glucose availability may impair fat burning is similar to the
processes which occur in the liver (see chapter 4). High levels of glucose and glycogen raise levels
of malonyl-Coa which inhibits enzymes necessary for the oxidation (burning) of fat for fuel. The
end result is an inhibition of fat oxidation when glucose availability is high and an increase in fat
oxidation when glucose is low (19).
Having discussed some of the general determinants for carbohydrate and fat metabolism
during exercise, we can examine the details of energy production from each fuel source.
Glycolysis
Glycolysis refers generally to the breakdown of carbohydrates for energy. Carbohydrates
are stored in the muscle and liver (in long chains called glycogen) and also circulate in the blood as
glucose. During exercise, glycogen or glucose is broken down to provide ATP as shown in figure 1.