that is burned. Compared to aerobic glycolysis (which produces 36-39 ATP per molecule of
glucose), fats provide far more energy. However, more oxygen is required to burn one molecule of
FFA compared to burning one molecule of carbohydrate. This means that the body has to work
harder to oxidize fats than glycogen during exercise. Although FFA produces more energy per
molecule, carbohydrate is still a more efficient fuel.
Questions about fat metabolism
One question that arises is why fats cannot be used as the sole source of energy during
exercise, especially considering their abundance compared to carbohydrate stores. (25) The
limiting factor in fat oxidation is related to the muscle’s oxidative capacity (i.e. mitochondrial
density, enzyme activity). Recall that the major adaptation to aerobic training is an increase in
the amount and activity of mitochondria and the enzymes needed for fat oxidation. At high
exercise intensities the inhibition of FFA release may also limit fat oxidation.
The rate of FFA oxidation during exercise is generally related to its concentration in the
bloodstream (28). During low-intensity exercise (below 65% of maximum heart rate), fats can
provide nearly 100% of the energy required (14,28). The rest comes from blood glucose.
As exercise intensity increases to about 75% of maximum heart rate, the rate of FFA
appearance into the bloodstream decreases, but the rates of fat oxidation increase (16). This
indicates an increased reliance on intramuscular TG use at higher intensities.
However, at this intensity, higher levels of FFA do not further increase fat burning
indicating that the muscle is not able to use fat quickly enough (16,29). The limiting factor
appears to be the muscle’s oxidative capacity (28). During high-intensity activity, more fast
twitch muscle fibers are called into play but the ability of fast twitch muscle fibers to derive
energy from fat is low (13). Recall that the primary adaptation to regular aerobic training is an
increased capacity to use fat for fuel at any intensity.
As exercise intensity increases to 85% of maximum heart rate (or roughly the lactate
threshold), blood FFA levels do not increase during exercise and FFA utilization decreases (28).
The decrease in FFA release during high-intensity exercise may be related to one of several
factors. The first is a decrease in blood flow through adipose tissue at high exercise intensities.
Additionally, FFA appears to become trapped in the adipose cells due to high levels of lactic acid
(13,16,25).
This ‘trapping’ effect of lactic acid is indirectly supported by a large post-exercise FFA
release following exercise at 85% VO2 max. (16). Also, when the blood is made alkaline (with
sodium bicarbonate), higher rates of FFA release are seen during exercise further supporting the
effects of blood lactic acid levels and pH on FFA release (30).
Summary of adipose tissue metabolism
The amount of fat utilized during exercise depends on the intensity and duration of exercise.
At low intensities, there is abundant FFA in the bloodstream and the rate of oxidation appears to
be limited by the muscle’s capacity to oxidize them for energy. As exercise intensity increases to