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Timing is Everything: Why the Duration and Order of Your
Exercise Matters
Life doesn’t always seem
fair. There are some people who lift weights for 30 minutes and then run for
another 30 minutes only twice a week and still somehow manage to burn more
fat than someone who runs half an hour a day, five days a week. While there
are many variables to be considered like type of diet and intensity of
exercise, in exercise, like many other endeavors, timing can be everything. Learning the scientific
basis of exercise metabolism can help the athlete become fit the smart way
instead of the hard way. The order and duration of your exercises are
important variables in influencing macromolecular metabolism. Simply put,
working out for half an hour four times per week is not the same as working
out for one hour twice a week, even though they add up to two hours total. Without a doubt, aerobic
exercise with the goal of decreasing body fat should be done for longer than
half an hour. It must be done for longer than half an hour because there is a
sequence of macromolecules that must be metabolized first before fat is
predominately m etabolized. The first of these energy supplies depleted in
exercise is creatine phosphate and glycogen1. These provide quick
energy for short term, high-intensity exercise, or fuel for the very
beginning of moderate intensity exercise. The type of exercise and how long
it is performed will determine the primary macromolecule that is metabolized.
High intensity exercise is
by definition anaerobic. The bulk of ATP for anaerobic exercise comes from
glycolysis. The glycolytic process requires that muscle cells breakdown
glycogen to glucose via the glycogen phosphorylase pathway2. However, in high
intensity anaerobic exercise, the body initially uses up all of the glycogen
in the skeletal muscle and the liver through the glycolysis pathway, creating
buildup of lactic acid. There are two mechanisms of muscle fatigue that limit
action in high intensity training: fatigue due to depletion of
phosphocreatine (see article on creatine basics) and fatigue due to lactic
acid induced muscle acidification3. Lactic acid fatigue only
occurs during high intensity training. The acid will lower the pH of the
muscle from a resting value of approximately 7.1 to as low as 6.4. At a pH of
6.9 the function of the glycolytic enzyme phosphofructokinase is inhibited,
slowing glycolytic ATP production. At pH of 6.4, all glycogen breakdown is
inhibited. Moreover, H+ from lactic acid interferes with calcium binding to
Troponin C and the subsequent shift of tropomyosin, preventing the
actin-myosin crossbridges and thus decreasing contractile force3. More on
fatigue will follow in a later article. Meanwhile, to prevent
muscle shut down, the lactate, acid, and pyruvate are transported out of the
cells passively. But, the transfer and conversion is not fast enough to
prevent the lactic acid buildup. Lactic acid shuts down the muscle in
approximately 30 seconds of maximum intensity exercise, while the time scale
of reestablishing pH takes place over approximately 15 minutes when not doing
high intensity training. Consequently the duration of anaerobic exercise is
short. Because the fast lactic acid buildup prevents the body from exercising
longer, the body cannot exercise past its allotment of glycogen and then
proceed to fatty acid metabolism. It is during a long duration of moderate
intensity exercise, where lactic acid is not building up, that athletes get
fat-burning benefits. Exercising for longer than
30 minutes shifts the primary macromolecules that are metabolized from
glucose to fatty acids. Shifting from glucose and glycogen supplies allows
the body to efficiently mobilize and utilize free fatty acids (FFAs) derived
from lipids in adipose tissue, which resides mainly under the skin. The body
preferentially metabolizes FFAs in order for the glucose being produced in
gluconeogenesis to be used by the brain. The brain only can metabolize
glucose mainly and ketone bodies to a lesser extent; the brain cannot
metabolize fatty acids. Physiologically, lipolysis
to utilize fatty acids is controlled by the pancreatic hormones insulin and
glucagon and also by catecholamine hormonal regulation. Mechanistically,
insulin will activate cAMP phosphodiesterase and reduce levels of cAMP in the
body, deactivating lipolysis. This pathway is responsible for the storage of
fat in obtaining energy. Glucagon mainly opposes insulin’s function to
increase glycogen breakdown and increase gluconeogenesis 2. After the first 30 minutes
of exercise, the body runs out of its glycogen storage and then turns mainly
to what is left of the glucose in the blood and then finally to fat and amino
acids derived from muscle protein. Supporting evidence of fatty acid release
comes from physiologic research where human gluteal fat cells isolated after
30 minutes of biking showed that cathecholamine induced lipolysis had
increased between 35-50% 4. If exercise does not last until 30 minutes then
fat burning is never achieved because all of the glycogen is not used u p. So
while one may be able to prevent adding fat to the body, one is not metabolizing
fat from the adipose tissues during the exercise. In short, exercises
aerobically for less than 30 minutes, one is just maintaining the adipose
tissue status quo and decreasing muscle mass. Most importantly when
concerning exercise duration, after 40 minutes of moderate intensity exercise
the body burns primarily fatty acids for its energy. This is the time period
of exercise that is most beneficial to health and a healthy appearance.
Burning the fatty acids reduces the fat on the body, providing the rationale
behind having a fat burning workout that lasts longer than 30 minutes. In
addition, since the majority of the body fuel is FFAs, the muscle protein is
used at a lesser rate. Weightlifting (or any type
of high intensity training) fits into a smarter schedule of exercise when it
is the first component of the workout. As mentioned before, high intensity
training is anaerobic and uses creatine phosphate and glucose as its fuel
sources. Creatine phosphate is always the first source of energy in any type
of exercise that is used up, as it replenishes ATP after the conversion to
ADP (see article on Creatine Basics). Additionally, glycolysis extracts
energy quickly from glucose that is derived from blood glucose or glucose
extracted from glycogen phosphorylation. When the body uses all of the
glycogen-derived glucose anaerobically, it must then rely on liver breakdown
of proteins and lipolysis for the body’s energy. The transition to
moderate-intensity exercise also allows the skeletal muscles to transfer the
lactic acid and pyruvate to the liver so that the pH can return to normal so
that the skeletal muscles can return to function. If the body does not have
creatine phosphate or muscle glycogen to burn quickly because aerobic
exercise was performed first, then glycolysis for weightlifting is
sub-optimized. If one does aerobic exercise first, then one will use their
creatine and glycogen reserves without burning much fat. Then when one turns
to anaerobic exercise afterwards he/she will be without his/her reserves of
energy needed for glycolysis. When one does aerobics first and then high
intensity training the workout is sub-optimized in all aspects. Thus, MedFitness recommends
that high intensity training is completed prior to aerobic exercise for
individuals who are trying to maximize fat burning. This ensures that the
energy needed for glycolysis is used in weightlifting. Then after 30 minutes
of weightlifting, the glycogen and blood glucose have been used up and the
body uses primarily fatty acids for its fuel. Switching to moderate intensity
exercise lets the body consume FFA for metabolic energy and also gives the
body time to remove the lactic acid so that aerobic exercise can be
performed. Thus lifting weights first for 30 minutes and then doing aerobic
exercises is the best way to maximize energy used in high intensity training
while selectively burning fat efficiently References: 1. Widmaier, E., Raff, H.,
Strang, K. Vander, Sherman, & Luciano’s Human Physiology 9th ed.,
McGraw-Hill, 2004. 2. Nelson, D. L., Cox, M. Lehninger
Principles of Biochemistry 3rd edition. Worth Publishing, 2000. 3. Wilmore, J., Costil,
D.L., Physiology of Sport and Exercise, Human Kinetics Publishing,
2004. 4. Hargreaves, M. ed. Exercise
Metabolism, Human Kinetics Publishing, 1995. |
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