Food fuels and three energy systems
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Food Fuels and Three Energy Systems. -Characteristics and interplay of the 3 energy systems (ATP-CP, anaerobic glycolysis and aerobic system) -Fuels required for resynthesis of ATP during physical activity and utilisation of food for energy

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Food Fuels and Three Energy Systems

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Food Fuels and Three Energy Systems

-Characteristics and interplay of the 3 energy systems (ATP-CP, anaerobic glycolysis and aerobic system)

-Fuels required for resynthesis of ATP during physical activity and utilisation of food for energy

-Relative contribution of the energy systems and fuels used to produce ATP in relation to the exercise intensity, duration and type.


  • We get energy from 3 food sources Carbohydrates, Fats and Proteins (alcohol does not count)

  • Once consumed they are changed into chemical fuel Glycogen. Triglyerides and Amino Acids

  • For our body to use any available chemical fuel it must breakdown the molecule to help synthesis and resythesis our energy currency ATP

  • Lastly we have 3 energy systems Aerobic System (with oxygen) the Anaerobic Glycolsis System/Lactic Acid System (no oxygen) and ATP-PC (no oxygen)

  • These systems can operate simultaneously, with one being the biggest contributor towards ATP resynthesis (interplay)

Energy Systems

ATP=ADP+P1+ energy

ATP-PC System

PC= C +P1 + energy

Anaerobic Systems

Glycogen from carbohydrates

(anearobic Glycolysis

Anaerobic Glycolysis


Energy Systems

Energy for movement

Glycogen from carbohydrates (aerobic glycolysis)

Aerobic Systems


FFAs from fats

(aerobic lipolysis)

Amino Acids from Proteins

Food Fuels- What can you recall?


  • ATP is our energy currency! The energy released from the breakdown (metabolism) of food is not able to be transferred directly to cell. Therefore it is critical that we can capture this energy in a form that can be used for biological work. The ATP molecule offers an effective storage solution, however ATP can only be stored with in the body in limited amounts (approximately 2 seconds of maximal performance).

  • This is why ATP must be continually replenished or resynthesised.

  • This is achieved through the breakdown of fuel sources by three different systems

All food energies are broken down; either used immediately, excreted or stored as chemical energy.




Fatty Acids (FFA) + Glycerol

Glucose+ Glycogen

Amino Acids



Aerobic oxidation




Lactic Acid

Acetyl CoA





CO2 + H20

Energy release from the breakdown of ATP

  • Use the diagram to explain the energy release from ATP Logo demonstration









To rebuild ATP and create more energy in a form that muscles can use, energy from he breakdown of phosphocreatine (PC) or nutrients (glucose, FFA and AA) is used to re-join ADP and inorganic phosphate (Pi)


Energy for Movement

ATP= Energy

  • ATP is resynthesised almost as quickly as it is broken down

  • ATP is an adenine nucleotide bound to three phosphates

  • When cells need energy, it breaks the bond between the second and third phosphate groups, which releases a large amount of energy

  • When the cells has excess energy (from the breakdown of PC or nutrients), it resynthesises ATP and ADP and Pi

Fuel Sources for Physical Activity

  • Glycogen is the preferred energy source

  • Glycolysis: the breakdown of glycogen either aerobically or anaerobically.

  • During Glycolysis, the glycogen is broken into two pyruvic Acid molecules and energy is released to form ATP.

Under aerobic conditions with sufficient oxygen, the pyruvic acid enters the mitochondria and breaks down the glycogen to produce

more ATP and CO2 and H2O

Under anerobic conditions with insufficient oxygen, the pyruvic acid transforms into

lactic acid and hydrogen ions

Mitochondria…On the most basic level, mitochondria are our aerobicengines

  • We can only live for a few minutes without the constant energy supply derived from aerobic respiration

  • Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. C6H12O6 (aq) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l)

  • Aerobic respiration requires oxygen in order to generate energies (ATP). Although carbohydrates, fats, and proteins can all be processed and consumed as reactant, it is the preferred method of pyruvate breakdown in glycolysis and requires that pyruvate enter the mitochondrion in order to be fully oxidized by the Krebs cycle.

The contribution of carbohydrates, fats and proteins (p. 120-122)

  • 1. Explain the process of Carbohydrate loading?

  • 2. Compare and contrast the use of carbohydrates compared to fats as a primary fuel source.

  • Why are fats an important fuel source?

  • Why is Protein vitally important in the diet?

  • Explain the concept of Glycaemic Index? For an athlete why is it important to understand GI foods?

Fuel Sources for Physical ActivityAt Rest…

ATP + Lactic Acid


ATP + Co2 + H2O

Maximal Activity…

What do you notice in food energy use when exercise intensities vary?


ATP + Lactic Acid


ATP + Co2 + H2O


ATP + Lactic Acid




ATP + Co2 + H2O



  • Energy for muscular contractions stems first from the muscle glycogen and then the liver glycogen.

  • The level of carbohydrate intake varies on the nature of the activity.

  • Prolonged endurance activities intermittent in nature or continuous lasting around 90minutes needs well-filled glycogen stores

  • While events lasting 2hours or more need ‘super-filled’ glycogen stores obtained from carbohydrate loading.

  • Carbohydrate loading is the practice of increasing glycogen stores within the muscles and the body by increasing carbohydrate intake and tapering training time leading up to major events (up to 10 days). The diet can consist of as much as 80% carbohydrates.

  • This is important in order to increase glycogen stores, which facilitate high-intensity efforts via anaerobic glycolysis as well as aiding endurance performance (aerobic energy system)

  • It is important to note the crucial role that carbohydrates play in:

    • The anearobic (high intensity/short duration) events, as the primary fuel source once PC has been depleted.

    • The aerobic glycolysis (moderate intensity, longer duration) as carbohydrates require less oxygen that than the same amount of energy converted from fats.

Fats- Read p.121

  • Explain what role fats in our diet play?

  • What form are they stored?

  • Explain the oxygen cost of lipolysis.

  • What is the total energy contribution of fats?

Protein- p.121-122

  • Explain the role that protein plays in our diet?

  • What function do enzymes serve?

  • What is the recommended contribution of protein in the diet? Explain why athletes may need to consume more?

  • Explain why there may be disadvantages associated with exceeding protein intake by more than 10-15 %.

Glycaemic Index

  • Define glycaemic index.

  • Describe and list food that are high and low in GI, offer an explanation as to why they belong to either group.

  • Identifying when it is appropriate for people to consume either high or low GI foods.

The carbohydrate-fat fuel during prolonged exercise p.122-123

  • Explain the process of ‘fuel mixing’ during a marathon event.

  • Describe and define hyperglycaemia and what effect does this have on an endurance athlete.

  • How can an athlete develop glycogen sparing and describe how it can be beneficial to endurance performance?


Interesting things toknow…

  • An 80kg person can store only 100g of glycogen in the liver and approximately 400g in the muscles (15g per kilo of muscle). This is enough for a 25km race.

  • Approximately 50% of the bodies everyday energy needs is supplied from fats

  • One glucose molecule yeilds 36-38 ATP in the presence of oxygen, but only 2-3 ATP without.

  • Complete oxidation of a glucose molecule results in 38 ATP, while oxidation of a triglyceride (fat) molecule yeilds 450 ATP.

  • However, six oxygen molecules are needed to metabolise one 6 carbon glucose molecule = 36 ATP’s per oxygen molecule. While 26 oxygen molecules are required to produce 147 ATP’s from one 18 carbon free fatty acid= 5.7 ATP’s per oxygen molecule.

The Energy Systems

Foods as Energy Sources

  • Our energy systems are like a car engine. They differ in both power/intensity and capacity/duration to sustain.

  • Energy System: chemical pathways in our body that resynthesise ATP for everyday activities.

  • Power: the rate of AP resynthesis, related to intensity.

  • Capacity: the yield of ATP resynthesis related to the exercise duration


  • A situation in which all three energy systems contribute to ATP production with one system being the major ATP producer.

    Provide 2 written examples, that highlight interplay in physical performance.

Which energy system???

  • Name the energy system in predominant use and briefly dot point why you think this is so.

  • 100m in 9.58 seconds

  • A 100 m sprint took Usain 9.68 seconds to complete.

  • The ATP-PC Energy system was in predominant use.- the ATP-PC energy system exhausts after around 10-15 seconds (ie PC stores are depleted after around 10-15 seconds of max activity)- the ATP-PC system takes 3 minutes of passive rest to fully recover PC stores- after 30 seconds of passive rest, around 50% of PC stores have recovered


  • Production of energy throught the breakdown of PC (phosphocreatine) to resynthesis ATP

  • These reactions do not require the presence of oxygen

  • All activities are carried out above 100% VO2max (the maximum amount of oxygen that can be taken up, transported and utilised per minute).

  • Stored ATP= 2seconds of exercise, therefore regeneration is needed.

  • Copy fig 5.6 Page 125

  • PC is stored in the muscles and contains phosphate bonds which when broken apart provide energy that results in linked resynthesis of ATP.

ATP-PC System… continued

  • ADP+Pi is reformed using the energy released from the breakdown of PC.

  • ATP-PC system is exhausted quickly, after 6-10 seconds of intense muscular activity.

  • PC is replenished within 3 minutes of the activity ceasing and a passive recovery time.

  • Once PC is depleted at the muscle, ATP must be resynthesised from another substance- typically this is glycogen via anaerobic glycolysis.

Which energy system



  • At competition level rowing a race of 2000m takes between 5.5 and 8.0 minutes requiring maintenance of high power for the duration

  • Rowingplaces great demands on both the aerobic and anaerobic energy systems and requires great power and strength.

  • Aerobic energy is thought to account for 70-75% of the energy and anaerobic energy for the remaining 20 - 25%.

The anaerobic system or lactic acid system

  • Energy provided through the incomplete breakdown of glucose when oxygen is not available

  • Glycogen stores in the muscles can only sustain maximum effort for about 20 seconds.

  • All pyruvic acid produced is converted into lactic acid, which lead to the by-production of Hydrogen ions (H+)

  • These ions are responsible for impeding maximal muscle contractions, due to the inhibition of certain enzymes.

  • Reduced muscle contractility is a safety mechanism employed to stop the destruction of cells in acidic conditions.

  • The hydrogen ions then combine with pyruvate to form lactate, which is then converted to glycogen and made available to release further energy.

  • Around 80% of lactic acid is diffused from the muscles and circulated through the liver for reconversion into glucose, although some H+ ions accumulate in muscle tissue, making muscle contraction painful and cause fatigue.

  • Copy figure 5.7

Lactate Inflection Point (LIP)

  • The exercise intensity beyond which lactate production exceeds removal in the blood.

  • This can be referred to as anaerobic threshold, lactate threshold, onset of blood lactate accumulation (OBLA), maximal lactate steady state or LIP

  • The LIP reflects the last point where lactate entry into and removal from the blood are balanced. It is identified as the final exercise intensity or oxygen uptake value at which blood lactate concentration is relatively stable during maximal aerobic exercise. A maximal intensity at which blood lactate is stable.

  • Lactate removal in the cells cytoplasm occurs via different mechanisms:

  • Lactate to be reconverted to pyruvate for immediate oxidation in mitochondria

  • Lactate to be transported out to the blood, which is then oxidised by other muscles (heart, slow twitch fibres) and some is converted into glucose in the liver.

  • Lactate production in cells increases in direct proportion to increased work-rate. However, lactate concentrations remain relatively stable during sub-maximal rates, as the body is able to remove it at a similar rate to its appearance in the blood.


Can you identify LIP on the graph?

Describe what you see in the data. Offer an explanation for this.

Describe how lactate accumulates beyond LIP?

Analogy to LIP-The leaky boat…

  • Imagine a person in a boat that has sustained a leak. Whilst the leak flows slowly the person can easily use a bucket to scoop the incoming water out of the boat. As the hole gets bigger and the water flows more rapidly the person will still be able to empty the water at a rate faster than it is coming in and things are OK. BUT, as soon as the water comes in at a faster rate than it can be removed the person is in trouble because the water will increasingly accumulate in the boat until it eventually sinks. The point at which the entry of water exceeds the rate at which it can be removed is where LIP has been exceeded. Think about the associated workloads and students quickly picture the rate of entry vs. bucketing and at maximal levels a steady state is reached but this cannot be sustained for long before the water starts to rise and fill the boat – this also allows us to discuss lactate shuttling (moving the water from the boat to it’s surroundings)

  • Wheelie Bin activity with bean bags

Which energy system

Aerobic Energy System

  • The complete breakdown of glucose when plenty of oxygen is available.

  • Produces energy through glycolysis (glycogen-preferentially during exercise) and lipolysis (free fatty acids-during rest)

  • Aerobic energy production takes the most amount of chemical reactions, with APT production at the slowest rate but with the greatest yield and supply for the longest duration.

  • Steady state is when sufficient oxygen is supplied to meet demand.

  • Using the aerobic system, any lactate production has the capability to be oxidised/removed or converted back into glycogen.

  • Again, there is a higher O2 cost when breaking down fats, which is why performers are forced to slow down when fats are used during high-intensity activities or of extended durations

  • Study and copy table 5.4 and figure 5.9 on page 129

Dominant energy systems at various exercise intensities…

Fill in the remaining two columns.

The interplay and operation of the 3 systems depend on …

  • .

  • .

  • .

  • .

The energy system interplay or continuum

  • Review Figure 5.11 (p. 137). Explain how the three systems overlap and vary their contribution to energy/ATP production depending on the duration of the activity.

  • Review Figure 5.12. What is oxygen deficit? Explain how the contributions from the aerobic and anaerobic system as the length of an event increases.

Approximate reserve values and capacity from various fuels

  • Review Table 5.8. which highlights the total possible energy (yield) and the ‘running time’ or rate of depletion for various fuels when working at 75% of max HR.

  • Review Table 5.9. Draw in books. This confirms that when the aerobic system is activated and becomes the major energy contributor, it provides 50 times more energy (moles of ATP) than the ATP-PC and Lactic acid system combined.

  • In early stages of exercise (low oxygen and transport) the two anaerobic systems supply APT

  • Notice the trade off between rate and yield for each of the energy systems. As the rate of ATP resynthesis increases, the yield (amount of ATP being produced) decreases.

Training the energy systems

  • What is specificity and how does it apply to improving performance?

  • Outline 2 ways to improve the :

  • ATP-PC system

  • The lactic Acid System

  • The aerobic system

    Copy Table 5.11 (p.141) into books and interpret.

Comparing the Three Energy Systems

Energy available and oxygen cost

  • Study table 5.2. Copy into books and discuss

2nd Analogy…

  • When we take a hot shower the exhaust fan can handle the build up of steam and as long as the fan sucks out the steam faster than it is being built up things are OK – the mirror doesn’t fog up. But, if we crank up the hot water the steam will occur more rapidly and in greater volumes –the exhaust fan might still be able to handle this and as long as the mirror doesn’t fog up we haven’t triggered the “MFP” (mirror fogging point). This goes on until a maximal steady steam state (MSSS) is achieved and the largest amount of steam can barely be removed by the fan. Any more steam beyond this point will cause the mirror to steam up because rate of steam production exceeds rate of steam removal (by the fan).

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