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During aerobic exercise, levels of adrenaline increase, raising heart rate and mobilizing fuel
for energy. However, regular aerobic training decreases the amount of adrenaline released during
exercise (7). This is accompanied by greater oxidation of FFA and a larger proportion of the FFA
oxidized comes from intramuscular triglycerides (3,7). Thus, despite lower levels of adrenaline,
there are higher rates of fat breakdown indicating an increase in tissue sensitivity to adrenaline
(8). These adaptations in adrenaline sensitivity are completed after three weeks of regular
training (3) and last as long as training is performed at least once every four days (9). Individuals
performing aerobic exercise should exercise at least once every four days or these adaptations
begin to disappear. Older individuals typically show a decrease in tissue sensitivity to adrenaline
which may be partially corrected with regular aerobic training.

Finally, chronic aerobic training causes a shift in muscle fiber type from Type IIb towards
Type IIa and I characteristics (10). That is, Type IIb and IIa fibers (which are typically strength
and power oriented), take on the characteristics of Type I fibers (endurance oriented) (2).

Regular aerobic exercise also causes a decrease in fiber size and loss of muscle (6,11).
Individuals in strength/power sports (powerlifting, etc) as well as bodybuilders should incorporate
aerobic training sparingly to avoid a loss of muscle size and strength.

Section 2: Aerobic Fuel Metabolism

Aerobic exercise can rely on multiple fuel sources for energy. These include glycogen (a
storage form of carbohydrate) in the muscle and liver, free fatty acids (FFA) from adipose tissue,
intramuscular triglyceride (droplets of fat stored within the muscle fibers), ketones and protein.
The use of protein during aerobic exercise has implications for protein requirements, as discussed
in chapter 9. The body’s total stores of each appear in table 1 (reprinted from page 19).

Table 1: Comparison of bodily fuels in a 150 lb man with 22% bodyfat

Tissue Average weight (lbs) Caloric worth (kcal)
Adipose tissue triglyceride ~33 135,000
Carbohydrate stores
Muscle glycogen (normal) ~ .25 480
Liver glycogen ~ .5 280
Blood glucose 0.04 80
Total carbohydrate stores 0.8 840
Intramuscular TG 0.35 1465
Ketones (a) Varies Varies
Muscle protein (b) ~13 24,000

(a) Ketones rarely provide more than 7-8% of total energy yield even in highly ketotic individuals

(b) Protein only provides 5-10% of total energy yield (up to 13 grams of protein per hour).

Source: “Textbook of Biochemistry with Clinical Correlations 4th ed.” Ed. Thomas M. Devlin.
Wiley-Liss, 1997.