Carbohydrate Storage In the Human Body
The chart below shows how fat burning drops off and carbohydrate
burning ramps up with increasing exercise intensity. We have already
discussed the effect of intensity on fat burning, and here we shall look
at carbohydrate burning.
Fat Isn't the Only Way Our Bodies Store Energy

Fat accounts for the vast majority of stored energy in the human body,
but not all of it. A small amount of energy is stored as a carbohydrate
called glycogen. Small amounts of glycogen are stored in muscle tissue,
but the body's main store is the liver. Glycogen is a starch-like
polysacharide composed of many individual glucose (blood sugar)
molecules that can be "broken off' as quick fuel. Indeed speed is the key
to glycogen and to understanding why we can only burn carbohydrate and
not fat when exercise becomes very intense.

Once exercise becomes so intense that VO2 exceeds about 65% of
maximum, fat becomes too slow to continue being a useful fuel and
carbohydrate, glycogen, takes over and functions as a very rapid-burning
fuel. Glycogen can even briefly supply energy when intensity actually
exceeds VO2max through "anaerobic" (oxygen-less) metabolism. In this
extreme case, absent oxygen to make carbon dioxide, glycogen becomes
glucose (blood sugar) which is then "burned" into lactic acid. Lactic acid
buildup from super-intense exercise is the cause of the muscle "burn" that
weight trainers feel when pushing all-out.

The main point here is that for weight loss, the goal with exercise is fat
burning which means keeping the intensity relatively low.

What Happens When We Run Out of Glycogen?

For most of us, this is rare. Glycogen is rapidly replenished when we eat
any carbohydrate so unless we exercise very hard for a very long time or
unless we eat a strict ultra-low carbohydrate diet for several days, we
should always have some glycogen to burn. This is important because
brain cells need glucose to survive normally. When glycogen stores are
totally depleted then there is no more "cheap" glucose to feed the brain
and at this point the body begins producing a type of brain-fuel called
"ketone bodies", and at great caloric expense, begins making its own
glucose from lactic acid in a process called gluconeogenesis.. Ketone
bodies are the brain's emergency backup fuel and during famines, they can
keep it going for a long time. If glycogen runs out for an athlete while
perform intense physical activity, the sudden drop in blood sugar causes a
wave of fatigue that is nearly impossible to overcome. This is why runners
practice what is called "carbohydrate loading" which is a technique for
increasing stored glycogen levels above normal in anticipation of a grueling
race.

Is Long-Term Fat Loss Slowed by Burning Carbohydrate In
Preference to Fat During High-Intensity Exercise?

Not necessarily.

Why?

Because depleted glycogen has to be replaced. This means that consumed
carbohydrates that MIGHT otherwise be converted to body fat are instead
shunted into glycogen. In a manner analogous to the basic priciples of
eating, it probably doesn't matter all that much what form the calories we
burn take, what matters is that we burn them.

Furthermore, regardless of where the calories originate, high-intensity
exercise burns calories faster than lower intensity.

Then is There Any Point to Trying to Stay in the Fat-Burning Intensity
Zone?

Not for athletes. Most of what is termed athletic "conditioning" occurs
during high-intensity exercise. But for a formerly sedentary middle-aged
person who is attemtping to use exercise to achieve fat loss, yes, staying
in the fat-burn "zone" makes sense because is it more comfortable, more
sustainable and safer (although NOT because of any magic). People
operating in this zone won't burn much carbohydrate during exercise so
they are less likely to feel hungry from it due to low blood sugar. Whether
the source of a burned calorie is germaine to the issue of weight loss or
not, many overweight people will enjoy exercise more if they stay in the
zone of fat-burning.
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