That's right Robert. You bring up several interesting observations about
relative utilization of energy stores while exercising. One thing that I have
been taught is that at 30%VO2(max) you burn the most amount of fat to glycogen.
If you are a well trained athlete you tend to spare the glycogen even more. The
reason for this as I see it is because at 30%, you still have unutilized O2
capacity in the {*filter*} so you can afford to use fatty acids and spare glycogen.
We know that when that glycogen is depleted you hit the wall. I'm not sure I
buy the time dependant aspect of 30 min. you mention above, I'll have to look
into it more. Protien utilization is another issue.  
If you really want to burn fat. You need to train regularly and at an intensity
high enough to reach 30%. (i.e. brisk walking, slow swimming, biking). People
should be able to do that for extended peoriods without fatigue.

Thanks Robert.
-=-SP

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Uucp: ...{gatech,ames,rutgers}!ncar!asuvax!stjhmc!207!106!STEVE.POSSON

Ok, the way I see it we have two primary fuel sources while we
exercise, fat and carbohydrate. Protien is of little importance in the
grand scheme of exercise metabolism. The average man has about 1,880
calories as carbohydrate (80 {*filter*} glucose, 350 liver glycogen and
1450 muscle glycogen), but about 142,000 calories stored as fat. So
carbohydrate is quite a bit smaller value than fat. An athlete has
relatively less fat and is able to store more glycogen in the muscle
than the "average" man, still the fat stores are relatively unlimited
when compared to glycogen. If we used our glycogen exclusively, a
marathon would not be an event in the olympics.
        With that out of the way let's look at the questions at hand.
There are three factors that determine which fuel will be used for ATP
production during an exercise bout.
1. Intensity and duration. Sprinting uses mainly glycogen, it is an
anaerobic exercise and glycogen is the only energy source that can be
used anaerobically. As the intensity decreases and duration increases,
fat is the preferred energy source. Glycogen is still utilized and to
a greater degree during the early period of the bout. You will always
have some glycogen metabolism going on during exercise.
2. Fitness. As aerobic fitness improves your ability to utilize fat as
an energy source improves therby sparing glycogen. This enables you to
utilize the reserve energy in the vast fat stores while sparing
glycogen. As you know, when glycogen is depleted, you hit 'the wall.'
3. Diet. When a high carbo. diet is consumed prior to a bout,
relatively more carbohydrate is used at any given workload, than fat.
        So you see, it is not as simple as the functions you suggest
i.e. FAT+GLYCOGEN, FAT/GLYCOGEN. There will always be a ratio involved
but it will not be a direct relationship as there is always some
glycogen being used even in sitting around and always some fatty acids
being mobilized, even in a 100m dash. Since more than one factor will
determine the ratio and the determining variables change during the
exercise, a function would be impossible to generate using a
human model. There are several metabolic pathways working in
conjunction and the efficiency of the cardiovascular/pulmonary system
to be considered as well. Not to mention the actual perfusion of the
muscle in question as it contracts.
        Yes there is a large variation among individuals, but the same
principles hold true.
        To answer your question about rates of increase; at
<30%VO2(max), you use mainly fat; at 40-60% you use glycogen and fat
evenly; and at 75% mainly CHO; but >80% you use only Glycogen.
        To read more, here is a reference; Chasiotis D. Role of
Cyclic AMP and Inorganic Phosphate in the Regulation of Muscle
Glycogenolysis During Exercise. Med Sci Sports Exerc 20:545-550,
1988,. Another good one is, Gollnick PD. Metabolism of Substrates:
Energy Substrate Metabolism During Exercise As Modified by Training.
Fed Proc 44:353-357, 1985. The latter one is probably more along the
lines of what you are looking for.

-=-SP

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Uucp: ...{gatech,ames,rutgers}!ncar!asuvax!stjhmc!207!106!Steve.Posson