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Old July 6th 03, 12:08 PM
Van Bagnol
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Default How long before aerobics "kicks in"??

In article ,
"Peter Webb" wrote:

SNIP

To burn more calories, you have to be more active than that. So, you
increase your activity by exercise -- the more intense and/or longer the
duration, the more calories burned.

But when you exercise more intensely, not all of the energy production
will be able to come from fatty acids that your fat stores provide. A
certain percentage of muscle tissue will face shortfalls of the oxygen
it needs, so glucose from the body's glycogen stores is necessary to
"keep the flame going", so to speak, and provide energy to muscle tissue
momentarily starved for oxygen. You can get by with existing body
glycogen for a little while, but you will eventually have to replenish
it -- with carbs.


Absolute crap. Why can't "all of your energy come from fatty acids that your
fat stores provide"? More to the point, people on zero carb diets obviously
can and do manage to exercise intensely.


Not _absolute_ crap, but perhaps I oversimplified it for you. Energy
(ATP) production is _always_ a mixture of several different processes,
some of which cannot come from fat sources.

Not all ATP production is aerobic. A certain percentage is anaerobically
derived even if you are exercising at an "aerobic" level: a modest 230w
power output can still produce ~2 mmol/L of blood lactate -- perhaps not
enough to feel a "burn" but present nonetheless -- indicating anaerobic
glycolytic activity. Furthermore, energy from fatty acids is _only_
aerobic. It has to enter the Krebs/NADH2 cycle directly as acetyl co-A
to produce ATP via oxidative phosphorylation, which requires oxygen.

As exercise intensity increases, and without enough oxygen and enzymatic
machinery for oxidation, the rest of the energy demands must be met more
by the anaerobic processes such as phosphocreatine (which is depleted so
rapidly it really isn't considered in duration exercise) and, of course,
glycolysis. Glycolysis requires glucose, no surprise.

That's why the "percentage of fat" utilization for energy declines with
extreme intensity exercise. It's not that you burn less fat (in fact
it's the opposite), you burn more -- a lot more -- carbohydrate. It
takes 18 times as many glucose molecules 'burned' anaerobically to
provide the same energy as one glucose molecule burned aerobically.

More to the point, I'm not saying people on zero-carb diets can't
exercise intensely. I _am_ saying that they have to temporarily deplete
muscle glycogen to do it, which will have to be replenished before it
runs out. That's why (no surprise either) people bonk.

BTW, glycogen can also be replenished by metabolising protein through
gluconeogenesis. People on zero carb diets do not have zero glucose or zero
glycogen levels.


True, I'd forgotten to mention that certain (C3) amino acids can be
converted to pyruvic acid which is interconvertible to glucose, but I
was addressing the OP's question about 'needing carbs to burn fat' by
explaining that the answer's not quite that simple. I was showing the
manner in which the statement is true, not seeking circumstances where
the statement is false.

Certainly, the most immediate way to replenish carbs is to eat carbs.
And it is possible for the body to synthesize carbs from protein, but
nonetheless, gluconeogenesis is about turning protein into carb, which
was still my point.

The OP was contemplating a low carb diet, but worried that this would make
him stop his exercise regime. It most definitely would not. Low carb diets
are doubly effective if combined with plenty of aerobic exercise.


Actually the OP was already on a low carb diet, and was wondering when
he would start seeing improvements from exercise because he gained 8 lb
since hitting the gym. After you suggested an even _lower_ carb diet, he
was uncertain of your advice because of a differing opinion by another
poster.

Van

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Van Bagnol / v a n at wco dot com / c r l at bagnol dot com
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