This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
There are 3 main fuels used during exercise by the contracting muscle: 1)
Protein; 2) Carbohydrate; 3) Fat.
PROTEIN: A majority of text books written will not acknowledge
protein as a major fuel, and it likely is not. It should be pointed out
that protein requirements of someone who is running/exercising on a
regular basis are GREATER than those of a sedentary population. Is this
something to worry about? Most "North American" diets contain more protein
than is needed. So the bottom
line is you get more than you need so don't worry. Vegetarians? Again the
answer is likely yes, they also get enough protein. Even when consuming a
pure protein diet there is enough protein to more than cover the needs of
a person who regularly runs/exercises. Moreover, most vegetarians are
aware of what they eat and plan their diets very well.
FUELS: Fats and carbohydrates (CHOs are then the major fuel sources
for the exercising person. The balance of the use of these fuels is
dependant upon exercise intensity and duration (the two are inversly
related). The general rule is that the lower the intensity the greater the
energy cost of exercise can be covered by fat. Hence, the greater the
exercise duration the more fat will be burned, usually because the
intensity of one's workout will decrease - FATIGUE! The flip side then, is
that during higher intensity exercise (>70% of max), one relies heavily on
SOURCES: Fats are stored as adipose, body fat, and muscle fat
(triglycerides). CHOs are stored as muscle and liver glycogen (long chains
of glucose) and blood glucose.
During a workout the early phases are characterized by a reliance
on CHOs, both muscle glycogen and blood glucose. The blood glucose comes
from the breakdown of liver glycogen. Again this is dependent upon
intensity (see above). However, the muscle can also use fat as a fuel, The
sources of this are from the inside of the muscle or from the outside -
i.e. from adipose tissue. The problem is that levels of fats from adipose
take a while to reach high enough levels for their use to become
significant. Their concentration in the blood only reaches very high
levels when the intensity of the exercise is low (i.e. 50% of max or less)
and if the duration is sufficient (1 hour or more). However, when the
concentration of fats from outside of the muscle is high enough the muscle
can use these instead of glycogen and delay the use of glycogen, this is
critical at times since muscle glycogen is a "rate-limiting" fuel for
muscle. That is when muscle glycogen runs out, or gets very low, then you
feel terrible - you've BONKED or HIT THE WALL (see below).
BONKING/HITTING THE WALL: Lots of people talk about the phenomenon
of bonking. It hits some people harder than others, I don't know why and
have never seen any good information why? However, bonking is a
combination of two processes. The first is a lack of muscle glycogen (see
above). The second is low blood glucose. When muscle glycogen is low the
muscle runs into a fuel crisis. It cannot burn fats at a rate high enough
to sustain the muscle's maximal output. The consequence is that your
muscle switches to burning more fats and so you have to slow down. The
crappy feeling that you experience at the same time, often characterized
by nausea and disorientation, is likely a consequence of low blood
The trick then is to alleviate/delay the onset of these symptoms by
consuming sugar solutions, or simply by becoming so well trained that you
don't have to worry (see TRAINING below). Why is low blood sugar bad?
Because your brain, eye tissue, and others are able to burn only glucose.
That is when the levels of glucose are low your brain runs out of fuel, so
you feel awful. Your vision might become impaired also.
FATS vs. CHOs: However, as I've said above your muscle can burn
fats and if given the chance your muscle will burn whatever fuel it has in
the greatest abundance, even lactate! So, if supplied with enough fat
muscle can burn fat and hence, "spare" muscle glycogen. This is the idea
behind many runners drinking caffeine/coffee before a race. The caffeine
has effects that cause release of fats from adipose tissue and the level
of fats in the blood increases. The end result is that for the early
phases of the race the runner's muscle's can use fat and delay the use of
muscle glycogen, hence, sparing that glycogen for later use.
One should be cautioned, however, that this mechanism for increasing fat
usage has only been shown with some very high doses of caffeine that are
not achievable without taking caffeine pills. It also critically dependent
upon the person's habitual caffeine intake ("big" coffee drinker appear
not to derive as great of a benefit as non-habitual users). There are
other ways to maximize the use of muscle glycogen, however.
CHO LOADING: CHO loading is a practice that many athletes use
before a longer duration event to "supercompensate" their muscles with
glycogen, delay it's running out (see above). The practice is of little
use when the duration of the event is less than 60 minutes, since muscle
glycogen will usually be able to meet the demands of such a duration.
However, it should be noted that repeated bouts of high intensity exercise
will also deplete one's muscles of glycogen (for example wrestling 3-4
bouts in one day).
There are two basic protocols for CHO loading, one is just as good as the
other. However, they involve an initial bout of exercise to deplete the
muscle's glycogen (under normal dietary conditions), followed by a period
of high CHO diet (i.e. 70% or more of one's total calories from CHO). This
period should be the 4-5 days prior to the event and should be a time when
the athlete tapers their training, so as not to deplete muscle glycogen
too much. The result is an overload of glycogen in one's muscles.
Two notes: 1) This procedure will result, if done correctly, in most
people gaining 2-5 pounds. Why? Because muscle and liver glycogen is
stored with water and increasing glycogen will increase water content -
i.e. increased weight is water. 2) Preliminary evidence indicates that
this procedure is less effective in women. That is to say that if a female
runner were to increase her CHOs to 70% (or >) of her caloric intake she
may not have an increase in muscle glycogen. Why? It may relate to a
gender difference in the ability to store muscle glycogen or in the amount
of CHOs that 70% of the female athlete's diet represents (i.e. 70% of a
2000 calorie diet would be 1400 Cal from CHO, eating this may not be
enough to increase muscle glycogen content). Stay tuned for more info here!
TRAINING: When one trains or conditions by completing endurance
exercise changes occur at many levels, including the muscle. The changes
that occur at the level of the muscle include an increased ability to
utilize fats. Not surprisingly then one's endurance is increased. How? An
increased utilization of fats means less reliance on glycogen, less
reliance on glycogen means you don't run out of the fuel that allows you
to maintain a high rate of muscle contraction, and hence a high rate of
running/exercising. Another adaptation that occurs is that your muscle
uses less glucose, this is important for tissues such as brain (see