SOME FACTS…
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There are 3 energy nutrients that our systems digest: Proteins, Fats, and Carbohydrates.
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Bioenergetic Conversion is the process in which our bodies convert these nutrients into a source
that allows us to function and carry out physical activities.
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Carbohydrates yield 4.1 calories per gram
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Of these 3 nutrients, carbohydrates are our most important source of energy.
o
Proteins yield 4.3 calories per gram
o
Fats yield 9.3 calories per gram
Metabolism is the process by which energy is supplied throughout the body and energy-rich
material are assimilated by the body for the purpose of energy renewal.
THE ROLE OF CARBOHYDRATES

They are the most abundant organic substances in nature – they are essential for human and
animal life.
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They mostly originate from plants, such as vegetables, fruits, and grain-based foods, such as
breads and pasta.

Carbohydrates are formed by green plants from carbon dioxide, water, and photosynthesis.

During photosynthesis, carbon dioxide from the air combines with water in the presence of
sunlight to yield carbohydrates. Oxygen is released as a by-product of this reaction.
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Humans (and other animals) use the process of cellular respiration to extract energy from these
organic compounds and release carbon dioxide.
CARBOHYDRATES AND HUMAN ENERGY NEEDS

Glucose is the form of carbohydrates that humans use.

Glucose is stored within either skeletal muscle, or the liver, as Glycogen.

Glucose stored in skeletal muscles and the liver can be broken down under conditions of stress, or
the demands of muscular activity – it is carried through the body by the blood.
ATP – THE COMMON ENERGY CURRENCY

In order for any nutrient to be usable, these nutrients need to be reconstructed into a universal
form of energy – a “free energy” that can then be used for muscle contractions and other
physiological processes that go on in our bodies.

The final form of this free energy is called ATP (Adenosine Triphosphate) – the common
energy molecule for all living things.

ATP captures the chemical energy resulting from the breakdown of food. ATP consists of 3
phosphates attached by high-energy bonds to adenosine.

Since ATP is in such high demand in our bodies, it is used up very quickly.
TWO ENERGY SYSTEMS
A. ANAEROBIC SYSTEM:

Occurs relatively quickly in the muscle fibre, and creates energy for powerful and
short-lived physical actions.
B. AEROBIC SYSTEM:

Occurs in the mitochondria (the cell’s power station) and leads to the complete
breakdown of glucose, as well as fats and proteins.

It is the key to endurance events.
THREE METABOLIC PATHWAYS
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Within these 2 systems (anaerobic and aerobic) there are 3 main “metabolic pathways” by which
ATP energy reserves are restored:
a) ATP-PC Pathway (Anaerobic Alactic)
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It depends on PC (Phosphocreatine) which is a compound that is normally stored in muscles and is
readily accessible. PC is a high-energy molecule where the phosphate can be broken off easily and
used to convert ADP back into ATP.
o
o
Examples: 50 and 100 meter sprint, high jump, 1 rep maximum in weight lifting.
o
o
PC + ADP → ATP + Creatine
These are activities that only last a few seconds and require a large burst of energy.
ATP-PC is important for these events as it provides the highest rate of ATP synthesis. However,
muscles do not have large supplies of PC. After 10-15 seconds, the system has depleted.
o
This system is known as the anaerobic alactic system since it produces energy without oxygen
and does not yield lactic acid as a by-product.
Glycolysis Pathway (Anaerobic Lactic)
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Glycolysis is the 2nd energy pathway.
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C6H12O6 + 2ADP + 2P → 2C3H6O3 + 2ATP + 2H2O
(Glucose)
o
(Lactate)
The ATP energy produced during this process will allow an athlete to engage in a high level of
performance for an additional 1-3 minutes.
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Examples: 400 and 800 meter track events, a typical shift in a hockey game.
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Like ATP-PC, Glycolysis is capable of producing ATP rapidly and without the need of oxygen.
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The main product of glycolysis is pyruvate (pyruvic acid). Pyruvic acid is then converted into
lactic acid, and exhaustion and painful muscle agony begins to set in. This is why the glycolysis
pathway is called the “anaerobic lactic system”
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The buildup of lactic acid hampers the breakdown of glucose and decreases the ability of the
muscle fibres to contract. Lactic acid removal requires 30-60 minutes of exercise recovery or 12 hours of rest recovery.
b) Cellular Respiration (Aerobic)
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In order for an athlete to sustain intense activity longer than 90 seconds, a 3rd energy system
must come into place. It is called cellular respiration, and it occurs in the mitochondria of the
cells.
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Fats and proteins can be used as energy sources at this stage. Fats are the predominant source
of energy for exercise that lasts longer than 20 minutes.
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Our bodies use this system for any endurance activities, such as running a marathon, long
distance swimming, or soccer matches.
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This aerobic pathway results in the complete breakdown of glucose.
o
The chemical equation for cellular respiration of glucose is:
o
o
C6H12O6 + 36ADP + 36P → 6CO2 + 36 ATP = 6H2O
The ATP produced by the aerobic method far exceeds the other 2 pathways. 36 molecules of
ATP are produced for every molecule of glucose (20x more than what is produced by the
anaerobic systems).
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Cellular respiration actually involves 3 separate sub-pathways:
o Glycolysis:
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The 1st stage of cellular respiration.
o
It is the same as the glycolysis in the anaerobic pathways except that in the
presence of oxygen, pyruvic acid is converted into CoA (Acetyl CoA) instead of
lactic acid.
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Acetyl CoA then enters the Krebs Cycle, which is the central pathway for the
metabolism of fats and proteins.
o Krebs Cycle:
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Through a series of 8 reactions, 2 ATP molecules are produced at this stage.
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From here, high-energy electrons that are produced in the Krebs Cycle are sent to
a process within the mitochondria, known as the electron transport chain.
o Electron Transport Chain:
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The final state of aerobic respiration, large amounts of ATP are produced, with
carbon dioxide and water as the only by-products.
LACTIC ACID
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The term Onset of Blood Lactate Accumulation (OBLA) refers to the point at which blood
lactate levels begin to accumulate very rapidly.
o
Generally, untrained individuals have a low anaerobic threshold, whereas elite endurance
athletes have a high threshold. Untrained individuals typically reach their lactate
threshold at about 50-60% of their VO2 max, whereas trained athletes may not reach
their lactate threshold until 70-80% of their VO2 max.
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In order to build your lactate threshold, you must improve both anaerobic (power) training and
aerobic style (aerobic) exercises. Your cardiorespiratory capacity will improve since there will be
an increase in the concentration of mitochondria and oxygen-carrying molecule myoglobin in
muscle fibres. This will then improve the efficiency of oxygen transfer at the cellular level.
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Lactate is transported by the blood to the liver where most of it is converted back to glucose.
o The Cori Cycle is the process by which lactic acid is converted into pyruvate for future
conversion to glucose and glycogen.
ENERGY FROM FATS AND PROTEINS
A) FATS:
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Fats are an ideal source of fuel since they contain large quantities of stored energy. Fats
have more than double the amount of energy of either proteins or carbohydrates.
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Fatty acids are the primary type of fats found in muscle cells or adipose tissue. They are
stored in the body as triglycerides.
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Through a process called Lipolysis (the release of lipids or fats) triglycerides are
broken down, and the resulting fatty acids become available as an energy source.
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However, before fatty acids can be used, they must first be converted into acetyl-CoA.
This is achieved through a process called Beta Oxidation which occurs within the
mitochondria of cells. The Acetyl-CoA then enters the Krebs cycle, and becomes the
primary energy source for the production of ATP within the electron transport chain.
B) PROTEINS:
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Protein contains the same amount of energy as carbohydrates. However, unlike
carbohydrates and fats, there are no “protein reserves” in the body.
o
Protein in the body is composed of 20 different amino acids (which are used by the body
to form various body tissues). Nine of these (called essential amino acids) cannot be
synthesized by the body and must be consumed as food.
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In order to be used as an energy source, protein must be broken down into these separate
amino acids. The amino acid, Alanine, is converted by the liver into glycogen, which is then
transported as glucose through the bloodstream to working muscles.
o
Contrary to popular belief, eating a high amount of high-protein foods (eg. Lean meats) or
taking protein supplements, does not result in an increase of muscle mass. Indeed,
excessive intakes of protein may put excessive strain on the liver and kidneys.
MUSCLE FIBRE TYPES
A) SLOW-TWITCH MUSCLE FIBRES:
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Are red, or dark, in colour. They generate and relax tension relatively slowly.
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They are able to maintain a lower level of tension for long durations.
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They have low levels of an enzyme called myosin ATPase (which the body uses to provide
instant energy for muscle contractions). These fibres also contain low levels of glycolytic
enzymes (which permits the release of glycogen within muscles), and contain high levels of
oxidative enzymes.
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Because of these traits, slow-twitch muscle fibres are most active during endurance
activities such as long distance running, swimming or cycling.
B) FAST-TWITCH MUSCLE FIBRES:
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Are pale in colour. They have the ability to tense and relax quickly, and generate large
amounts of tension with relatively low endurance levels.
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They have high levels of myosin ATPase and contain high levels of glycolytic enzymes.
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They can activate at a rate of 2-3x faster than slow-twitch muscle fibres, making them
ideal for fast, powerful muscle contractions needed for activities such as short sprints,
powerlifting, and explosive jumps.
*** The difference in muscle fibre types are mainly due to how much a particular muscle relies on oxygen
for the production of energy. The protein myoglobin is the oxygen storage unit that delivers oxygen to
working muscle. The more a muscle utilizes aerobic processes for energy production, the more it is able
to sustain longer-term activity. Slow-twitch muscle fibres are high in myoglobin and are ideal for
endurance activities. Fast-twitch muscle fibres have low myoglobin concentrations and are ideal for
shorter bursts of effort.***