Reading a thread on metabolism and bicycle riding I wanted to know
a little about fat metabolism, so I looked for some articles on
the WWWeb. Some excerpts follow. The whole matter is vast. We
actually do mobilize and metabolize fat from adipose tissue during
exercise. One way to be sure of doing so is to deplete muscle
glycogen (bonk), then continue to exercise at a sustainable rate,
0.50 VO2. (Immediately after exercise eat carbohydrates so that
the muscles recover and have the fuel to increase their capacity
further.)
Another item that bears on the whole "weight loss program" is that
adipose cells are born, but do not die. Short of liposuction,
every fat cell you ever had you still have. Further, adipose
tissue has structure: collagen and the like. Even if one were to
burn every drop of fat stored in his adipose tissue, he would
still have the empty tank hanging about, not a "hard body".
<http://www.cptips.com/bscphys.htm>
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How can these facts help you in designing a program to maximize
your performance?? If one does not supplement glucose stores in
the body (snacking while riding), you will run out of carbohydrate
stored in your muscle and liver cells after 2 hours of aerobic
activity, and the bonk occurs. This term describes the fatigue
resulting from muscle glycogen depletion. Without adequate
carbohydrate to fuel continues high level muscle activity, it is
impossible to maintain a high level of energy output and one has
to slow to speeds of 50% VO2max where fat metabolism can provide
the needed Calories. The bonk can be delayed by using oral glucose
to supplement muscle glycogen stores. On a long ride, a rider that
snacks will have more glucose available to fuel that final sprint.
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<http://www.drlenkravitz.com/Articles/genderdiffer.html>
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Mobilization and Metabolism of Fat
The mobilization of fat refers to the process of releasing fat
from storage sites in the body, whereas, metabolism of fat is the
complete biological breakdown or oxidation (which means loss of
electrons) of fat into energy that can be used by the body. There
are two main enzymes that regulate the mobilization of FFA:
hormone sensitive lipase (HSL) and lipoprotein lipase (LPL). HSL
is located directly in the fat cell and is stimulated by the
hormone epinephrine. When HSL is stimulated, it acts to break
apart TG in the adipose tissue and release three FFA and glycerol
into the blood stream. This process is called lipolysis.
Epinephrine, which is released by the sympathetic nervous system
during exercise, is the primary stimulator of lipolysis (Rasmussen
& Wolfe, 1999). Epinephrine binds to specific receptors on the fat
cell, which in turn, activate HSL. An individual?~@~Ys
physiological state can affect the body?~@~Ys sensitivity to
epinephrine. For example, during aerobic exercise, HSL
responsiveness to epinephrine is enhanced due to an increase in
body temperature and a greater concentration of epinephrine in the
blood stream when compared to rest. In an endurance-trained
individual the HSL responsiveness to epinephrine is further
enhanced, such that HSL can be activated by a lower concentration
of epinephrine when compared to a non-endurance trained
individual. Therefore a metabolic training effect of aerobic
exercise is an enhanced receptiveness to mobilize and break apart
TG for energy use. In contrast, obesity blunts the HSL
responsiveness to epinephrine, meaning a higher concentration of
epinephrine is needed to activate HSL in obese individuals
(Rasmussen & Wolfe, 1999).
[...]
What Exercise Intensity Burns the Most Fat?
During low intensity exercise the majority of energy (kcals) comes
from fat. As exercise intensity increases, the percent of energy
derived from fat decreases. However, the absolute amount of energy
derived from fat is actually increased! As exercise intensity
increases, so does total energy expenditure (caloric expenditure).
Even though a smaller percentage of the energy expenditure is
coming from fat, more kcals of fat are burned because there is a
greater absolute energy expenditure. Therefore, expressing energy
derived from fat as a percentage of energy expenditure without
considering the total energy expenditure is misleading.Another
consideration is the effect that exercise has on energy
expenditure after exercise. Following a high intensity exercise
bout, the rate of metabolism is elevated for a slightly longer
period of time (when compared to a lower exercise bout), and more
energy is expended as your body returns to homeostasis (resting
conditions). With regular aerobic exercise, this post-exercise
energy expenditure will positively contribute to weight loss goals
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Michael Press