I want to check my understanding about fat-burning exercise:
After exercising for awhile, the body depletes its stores of
glycogen and shifts to burning fat for energy.  So the exercise
done after this point counts for a lot more.

1) Is the above correct?
2) How can one tell when the body shifts to fat-burning?  A
feeling of fatigue?
3) How long does it take for the body to produce more glycogen
and shift back out of fat-burning mode?  Can one stay in the
fat-burning mode even with extended rest breaks between exercises?
(I'm thinking primarily of running.)

Thanks in advance.

No, it is not.  You do not deplete your glycogen stores totally.  You only
burn enough glycogen to "prime" the fat oxidation cycle.  

The easiest way is by what most people call "the second wind."  This doesn't
mean that if you don't get your second wind you don't burn fat, that is how
I was taught to tell "out on the track."

It doesn't work this way  so this is not an applicable question for submax
exercise.

Interesting question.  Unfortunately I don't know an interesting answer
(at least not one that is true;).  My feeling is that once you stop the
fat oxidation cycle it won't come back up very easily.  The reason I
suggest this is because the fat oxidation cycle is very efficient when it
is running but it is very inefficient to start up.  Therefore,
I would suggest that once you take a "significant break" (maybe more than 30
secs-1 min???) that you have lost that fat oxidation efficiency and it then
becomes that much harder to get back because you are already depleted of energy.
BTW, lots of this type of discussion in misc.fitness.

Dan Kelaher
NCSU Ergonomics Lab

Not really.  The body does not alternate between either fat
burning or carb (glycogen burning) modes.  Fat to be burned can
come from two sources, 1) intracellular (which is the major
source of energy for resting muscle), and 2) fat stores,
adipose tissue (the stuff you want to lose as much as possible).

When you increase your workload (start exercising), your
muscles use glycogen because the energy is readily available.
At the same time (if the exercise proceeds long enough), the
body sends hormonal signals to release stored fat in adipose
tissue.  This takes a little time, the hormones have to be
released, travel to the fat cells, the triglyceride has to be
broken down to free fatty acids, exported into the {*filter*}, and
taken up by muscle cells.

All this time glycogen is being used for energy. As fat burning
increases due to mobilization of the adipose stores, the % of
energy derived from carb burning decreases and fat burning
increases (to perhpas a maximum of 50%).  Glycogen will be
limiting, so if you exercise long enough and intensely enough,
the glycogen will be exhausted, although you are likely to be
exhausted before this happens.

The above scenario is intensity dependent.  The more intense
you exercise, the greater the reliance on glycogen.  This is
because glycogen burning uses less oxygen than fat burning, so
if oxygen becomes limiting (you're breathing very hard) then
most of the energy has to come from glycogen.  At maximum
effort and pure glycogen burn (100% VO2max) you can only
perform for about 6 - 15 minutes.

Your body would like to burn just glycogen, but glycogen is
inefficiently stored (it takes alot of water and hence weight)
and is limited to only about 1800 calories for all muscles plus
the store in the liver.  Fat on the other hand is very
efficiently stored (but unsightly) and essentially
inexhaustible.  Even a svelte individual at 150 pound and 10%
body fat has over 40,000 calories of fat on themselves.

You can't since it's a continuium.  `Hitting the wall' after a
along time of exercise is likely to represent the point of
glycogen depletion.  A feeling of exhausting after a short time
is likely to represent exercising too intensely.  Either you
deplete glycogen because you went to hard and burned through
glycogen stores or your level of lactic acid has increased
beyond your threshold level.

Again, there is no fat burning mode, rather there is a pattern
of energy derivation depending on a number of factors
including, current nutritional status (fasted of fed), level of
training, intensity of exercise, duration of exercise.

You will have to eat to replenish glycogen stores.  Although
the body can manufacture sugar for glycogen by either breaking
down liver glycogen and exporting it into the {*filter*}, or
converting protein to sugar, the body needs to maintain a {*filter*}
sugar level for the brain and will convert muscle to other
energy utilization patterns since the liver's capacity to make
sugar is limited to supplying the brain.

The problem is not shifting out of fat burning mode, but
shifting into protein digesting mode.  As glycogen is depleted,
your body will catabolize muscle amino acids to provide for
direct energy as well as substrates for sugar synthesis by the
liver.

Michael K

Thanks for all the info.  It raises some more questions:
What triggers the body to release the hormonal signals which
release the stored fat?  Are the hormones released throughout
the exercising?  Once the fat cells begin to release the stored
fat, when do they stop releasing the fat?  What becomes of the
fat which is released at or around the end of the exercising
period?

I managed to drop 20 lbs in about as many weeks over the summer,
and I attribute this mainly to running 5 miles 3 times per week.
I'd like to have a better understanding of the process.  From the
info I've pieced together, it appears that running slow and steady
(i.e., finding a pace which doesn't cause hard breathing) for a
long time is the key, and that the first few miles are not the ones
which burn much fat.

By the way, how does one give the secret Caltech handshake over
the internet?

Re: fat burning...

Chinese qigong exercises, if done properly burn fat better than any
aerobic exercise.

--
W Paul Blakey -- Twin Eagles Educational & Healing Institute

I'm not sure I really know.  I assume that the increased energy
demands as well as incerases in heart rate, breathing,
increased {*filter*} flow.

Presumably.

In studies with enduranc exercise (low level for hours), the
increae in the {*filter*} level of free faty acids continues to rise
throughout the exercise period.

It gets stored back as fat.  I don't have data to back up my
assertion, but I've always felt like this aspect may have the
ability over a long time to redistribute the fat to more
accessible sites for future fat utilization and promote faster
mobilization (which is found with training).

The slow rate of weight loss (1# per week) suggests that the
majority of the weight was fat.

I never learned the Caltech secret handshake.  I was shown the
Gnome handshake, but since I haven't met another Gnome since
leaving Caltech, I have forgotten it.

Michael K