By Yuri Elkaim
The type of substrate (fuel) and the rate at which it is
utilized during exercise is largely dependent on the intensity
and duration of the exercise. During strenuous exercise there is
an obligatory demand for carbohydrate (CHO) oxidation that must
be met; fat oxidation cannot substitute. In contrast, there is
an increase in fat oxidation during prolonged moderate intensity
exercise as carbohydrate fuels are depleted. Fats are also more
dominant in long duration exercise since fat oxidation requires
more oxygen than does carbohydrate oxidation.
Fuel Selection During Exercise
Again, fats are the primary source of fuel for muscle during
low-intensity exercise (70% of VO2max). Proteins contribute less
than 2% of the substrate used during exercise of less than one
hour’s duration but may increase up to 5-15% during the final
minutes of exercise lasting 3-5 hours.
At about 40% of VO2max there is a “crossover point” where
carbohydrate becomes more dominant than fat in supplying fuel to
the muscle. This shift to CHO metabolism is caused by two
factors: the recruitment of fast twitch fibers (which are better
equipped to metabolize CHOs) and increasing levels of
epinephrine in the blood (which contribute to glycogen
breakdown). This shift can also be seen when examining the
contribution of fat oxidation at various exercise intensities.
At 20%, 50%, 80%, and 100% of VO2max, fat oxidation yields 175
kcal/min, 250 kcal/min, 200 kcal/min, and 25 kcal/min,
respectively. As such, someone looking to burn more fat during
exercise would want to hover in the 50% to 80% VO2max range.
As the duration of exercise increases, there is a greater
contribution from fat and less from carbohydrate. This is the
result of increased levels of blood lipase, an enzyme that helps
break down fat, during low-intensity longer duration exercise.
Muscle Glycogen Utilization
At the onset of most types of exercise, and for the entire
duration of very strenuous exercise, muscle glycogen is the
primary carbohydrate fuel for muscular work. The intensity of
exercise determines the rate at which muscle glycogen is used as
a fuel. The heavier the exercise, the faster glycogen is broken
down. Furthermore, studies have shown that glycogen is depleted
faster from fast-twitch fibers, especially during interval type
Historically, it has been thought that increased epinephrine
levels in the blood were responsible for the initiation of
glycogenolysis (glycogen breakdown). However, other research has
shown that the breakdown is also triggered within the muscle
itself as a result of increased Calcium ions .
Maintenance of Normal Blood Glucose Levels During Exercise
As glycogen, and thus glucose, is the primary fuel sources
during most exercise, it is needs to be remembered that glucose
plays a primary role in supplying our vital organs with energy.
As such, the body has many systems in place to maintain adequate
levels of glucose in the blood during times of inadequate
carbohydrate intake (starvation/fasting) and accelerated glucose
removal from the blood (exercise). Blood glucose concentration
is maintained through four different processes:
• Mobilization of glucose from liver glycogen stores
• Mobilization of fats to spare blood glucose
• Synthesis of new glucose in the liver from amino acids,
lactic acid, and glycerol
• Blocking of glucose entry into the cell to force the
substitution of fat as a fuel.
These processes are controlled by several “slow” and “fasting”
acting hormones such as thyroxine, cortisol, growth hormone,
epinephrine/norepinephrine and insulin/glucagon. Maintaining
normal blood glucose concentrations is a major task when you
consider that the liver may only have 80g of glucose before
exercise begins, and the rate of blood glucose oxidation
approaches 1g/min in heavy exercise or in prolonged (>3 hours)
moderate intensity exercise.
Carbohydrate Replenishment and Diets
This is one of the main reasons why it is recommended to
replenish glucose and glycogen levels during activities such as
soccer games, marathons, triathlons, 3-5 set tennis match, and
so forth. Each of these sporting activities require moderate to
high levels of exertion for extended periods of time and thus
fuel replenish, mainly via sports drinks, is essential in
maintaining physical performance and skill execution.
The effect of carbohydrate intake and diet composition on
performance has been heavily studied. The vast majority of
studies show that a high carbohydrate diet is essential for
those who engage in intense or long duration exercise. For
instance, one study had trained subjects run a 30 km race twice,
once following a high carbohydrate (CHO) diet and the other time
after a mixed diet. The initial muscle glycogen level was
3.5g/100g of muscle following the CHO diet and 1.7g/100g of
muscle during following the mixed diet. The best performance of
all subjects occurred during the high CHO diet. Although the
starting pace was not faster, the additional CHO allowed them to
maintain the pace for a longer period of time .
Bear in mind that substrates and hormones can interact and
alter certain processes. This is commonly seen when a high
glycemix index CHO food is ingested just prior to exercise. In
this case, the resulting spike in blood glucose elicits a rise
in insulin levels. As a result, fat mobilization is reduced (due
to insulin’s anabolic effect on fats) forcing the muscle to use
additional muscle glycogen. This is not desired if you are
looking to sustain a given exercise intensity for prolonged
periods of time.
Putting It All Together
The common misconception is that to lose weight you must stay
in your “fat burning zone”. Eventhough there is some truth to
this statement, it is inaccurate in the sense that losing weight
is about burning as many calories as possible. As such,
exercising at a higher intensity for moderate durations would be
most effective in burning calories. It is important to remember
that a calorie is a calorie regardless of its source and since 1
lb of fat is equivalent to 3500 calories, calorie-reducing
exercise regimens are most suitable for those looking to lose
Such exercise programs can take the form of interval work,
where a bout of high-intensity exercise is combined with a bout
of low-intensity. This type of training of has been shown to
elevate the body’s metabolism for up to 16 hours post-exercise.
It also fosters the maintenance of lean body mass while
stimulating the release of lipase – a good thing since
maintaining muscle and losing fat should be the goal. Having
said this, I would also recommend implementing longer duration
low-intensity exercise as well for it’s aforementioned benefits
on fat oxidation. However, ensure to keep the intensity in the
50-80% VO2max range for best results!
Essen et al. (1978). Glycogen depletion of different fiber
types in human skeletal muscle during intermittent and
continuous exercise. Acta Physiologica Scandinavia, 103: 446-55.
Hultman et al. (1967). Physiological role of muscle glycogen in
man with special reference to exercise. In Circulation Research
XX and XXI, ed. C.B. Chapman, 1-99 and 1-114. New York: The
American Heart Association.
Karlsson, J. & Saltin, B. (1971). Diet, muscle glycogen and
endurance performance. Journal of Applied Phsyiology, 31:
About the Author: Yuri is the owner of Total Wellness
Consulting and creator of the only 12 week body shaping fitness
program designed for Ipod/Mp3 players. Please visit
http://www.myFitterU.com for more info