>(deleted blah, blah, blah to make room for my own blah, blah, blah)
There are three major sources of fuel for the body's pathways in which
energy is stored as ATP:
1) Proteins (polymers of amino acids, and actually only a minor source of
2) Carbohydrates (polymers of sugars like glucose), and,
3) Fats (3 fatty acids + glycerol).
There are two pathways in which ATP is generated:
1) Anaerobic (producing ATP without oxygen, called substrate level
2) Aerobic (producing ATP in the presence of oxygen, called oxidative
The by-product of the anaerobic pathway is lactic acid, a compound that
causes muscle fatigue and pain. The by-products of the aerobic pathway are
carbon dioxide and water. The anaerobic pathway produces ATP much faster
than the aerobic pathway. However, the aerobic pathway produces more ATP
per cycle than the anaerobic pathway (~36 ATP as compared to 2 ATP). All
the fuel constituents mentioned above except fatty acids can be used to make
ATP in either pathway. Fatty acids can only make ATP in the aerobic
"A 100m sprint is powered by stored ATP, creatine phosphate, and anaerobic
glycolysis of muscle glycogen." (Stryer, 1995) These are the most immediate
sources of energy and thus produce the most rapid initial velocity.
However, this maximum pace cannot be maintained for much longer than 100m.
Virtually all the energy available from creatine phosphate is used in the
first few seconds. By 10 seconds, 1/3 of stored ATP is used and the
concentration of lactic acid in the *** is over 5 times higher than the
concentration at resting state. This not only causes muscle fatigue and
pain, it inhibits the regeneration of NAD+, a molecule necessary for
anaerobic ATP production.
Fortunately, we breathe when we run. When oxygen is present, ATP can be
generated by either the anaerobic or aerobic pathway. Additionally, NAD+ is
readily regenerated in the aerobic pathway, and the ATP produced gives the
liver time to lower the concentration of lactic acid in the ***.
The mechanism that determines whether the anaerobic or aerobic pathway is
favored is completely different from the mechanism that determines whether
fat burning or carbohydrate burning is favored. The degree to which either
pathway, anaerobic or aerobic, is preferred depends upon the availability of
oxygen, or in other words, by heart rate. The type of fuel, either fat or
carbohydrate, used to create ATP is determined by a feedback mechanism
concerned with concentrations of citrate, pyruvate, acetyl CoA, ADP, ATP, as
well as *** sugar level, but not heart rate.
So, when David F. said, "Without oxygen being supplied to the muscle you
burn nothing (not sugar, not fat)." he was not entirely correct. However, I
do agree with him when he wrote, "If you want to lose weight there is no
magic book, you must consume less calories than you expend, that's the big
secret." The statement might seem more positive if it ended " ... you must
expend more calories than you consume ..."
Overworked wrote, "Not true, I am "hung up" on the fact that if your heart
rate is above a certain level, your body switches from burning primarily fat
to burning sugar." Perhaps Overworked should not be so "hung up" on a fact
that isn't necessarily true. Remember, concentrations not rates regulate
the type of fuel burned. In Overworked's defense, reading a book should be
required before commenting on it.
David R. wrote about several situations of which I am not sure exist:
1) "Above 5K race pace (individual response will vary, of course), the body
uses only glycogen for fuel."
2) " ... your body is using some oxygen to produce energy even when your
only fuel source is glycogen."
3) In a 5K race, "The runner burns little (if any) fat during the race ..."
4) In a 10K race, "... while the ratio of glycogen (or "sugar") to fat
is approximately 90:10"
In regards to 1 and 2, except in starvation and bursts of activity lasting
less than 10-15 seconds, the body does not and cannot afford to use only one
source of fuel. As for 3 and 4, once again excepting very short sprints,
the type of fuel used by the body is not dependant on the runner's pace, or
heart rate. However, I'd like to read the studies where you got your
numbers on the very probable chance that I'm completely wrong.
After all, I am just an undergraduate at a state university. Anybody know
where to find a grain of salt big enough to take with this post? Then
again, all that salt might wreak havoc with one's *** pressure. :)
Now you see why I'll probably always be a jogger and not a runner. Happy