Metabolism of free fatty acids (FFA) and intramuscular triglyceride (TG)
The body has two major stores of fats which can be used during exercise to provide energy:
adipose tissue and intramuscular triglycerides. One pound of fat contains 3,500 calories worth of
usable energy. A 154 lb (70kg) male with 12% bodyfat and 18 lbs (8.4 kg) of total fat has
approximately 70,000 calories stored in bodyfat and an additional 1,500 calories stored as
intramuscular triglyceride. Running one mile requires about 100 calories so this individual could
run 720 miles if he could use 100% fat for fuel.
Even the leanest athlete with only 5 lbs. of bodyfat (containing approximately 17,500
calories worth of usable energy) could run 17 miles if they were able to use just fat for fuel. This
has led most researchers to the conclusion that diets higher in fat are not necessary since the
body has more than enough stored (24). Others have suggested that adaptation to a higher fat
diet may be beneficial for the endurance athlete by sparing glycogen during exercise (20).
Why humans are unable to utilize 100% fat for fuel during activity is a question that many
researchers have asked and is a topic that is discussed in greater detail below (25).
Adipose tissue triglyceride metabolism
Bodily stores of adipose tissue may contain 70,000 calories or more of usable energy stored
in the form of triglyceride (TG). TG is composed of a glycerol backbone with three FFA attached
to it. While intramuscular TG are contained within the muscle and can be used directly, FFA
from adipose tissue must be carried through the bloodstream to the muscles to be used for
energy.
The process of burning adipose tissue TG involves four steps. First the TG must be
mobilized, which refers to the breakdown of TG to three FFAs and a glycerol molecule. Glycerol is
released into the bloodstream and regenerated into glucose in the liver (8). The breakdown of TG
occurs due to the enzyme hormone sensitive lipase (HSL) which is regulated by insulin and the
catecholamines, adrenaline and noradrenaline (8,26,27).
Adrenaline and noradrenaline (which increase during exercise) bind to beta-adrenergic
receptors in the fat cell stimulating HSL to release FFA into the bloodstream (8). Insulin (which
decreases during exercise but increases in response to increases in blood glucose) inhibits HSL
activity and blocks the release of FFA for energy production.
Once broken down within the fat cell, FFAs enter the bloodstream and travel to the muscle
or liver. Consequently, changes in blood flow during exercise affect FFA transport (8). FFA is
taken up into the muscle and transported into the mitochondria for burning via the enzyme
carnitine palmityl transferase 1 (CPT-1). FFA are also broken down in the liver and may be used
to make ketones if liver glycogen is depleted.
Finally, FFA is burned in the mitochondria to produce ATP and acetyl-CoA. The acetyl-
Coa is used to produce more energy in muscle. In the liver, excess acetyl-CoA is condensed into
ketones as discussed in chapter 4. Alternately, incoming FFA may be stored as intramuscular
triglyceride.
One molecule of FFA will yield 129 to 300 ATP or more depending on the length of the FFA