The Sources of Energy Affecting Physical Performance · muscles as required. If muscle and liver glycogen stores are full, excess glucose is converted to triglyceride (fat) and stored

The Sources of Energy Affecting Physical Performance

Essentials Workbook © Adelaide Tuition 2015. All rights reserved, copying of any pages is strictly prohibited by law. 5

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T O P I C

1 The Sources of Energy Affecting Physical Performance

TOPICS KEY IDEAS

1.1 Sources of Nutrients: Fats, Carbohydrates and Protein

• Characteristics of each nutrient• Available sources of fats (lipids), carbohydrates, and protein

1.2 Food Breakdown into Nutrients: Glucose, Glycogen, Triglycerides, Free fatty acids

• Breakdown process (digestion) and link of ‘Sources of macro nutrients’ to ‘Food breakdown into nutrients’ (e.g. protein into amino acids, carbohydrates into glucose)

• Function in the body (fuel use) of each nutrient during rest, sub-maximal activity, and maximal activity (e.g. carbohydrate – fat relationship, glycogen sparing)

• Storage and availability of nutrients in the body

1.3 Aerobic and Anaerobic Energy: ATP-CP System, Lactic Acid System, Oxygen System

• Key features of aerobic and anaerobic systems (e.g. fuel use at different intensities)

• Advantages and disadvantages of systems• Recovery and restoration of energy supplies

1.4 Contribution of Energy Systems in Specific Activities

• Understanding of the different energy system contributions based on the intensity and duration of activities

• Interpretation using a range of techniques (e.g. games, performance, graphs, tables) to explain how and why the different energy systems contribute to performance

• Understanding of the key processes involved during energy system use at different intensities (e.g. oxygen deficit, steady state, OBLA, lactate threshold)

• Exploration of activities where each system is the major (dominant) supplier of energy

• Exploration of how energy systems work together – the concept of interplay• Understanding of the energy systems during recovery (EPOC)

1.5 Acute Responses to Exercise: Responses in the Circulatory, Respiratory and Muscular Systems

• Understanding of the acute responses in the body• Examining/exploring key examples in:

• the circulatory system• the respiratory system• the muscular system

• Effect of the acute responses and systems to exercise (e.g. increased oxygen delivery to the muscles)

(Copyright SACE Stage 2 Physical Education Subject Outline 2015. Teachers and students are advised to check the current curriculum statement on the SACE website, <www.sace.sa.edu.au> for any changes).

Physical Education.indb 5 3/08/2015 9:11 pm

SACE 2 ESSENTIALS PHYSICAL EDUCATION WORKBOOK

Essentials Workbook © Adelaide Tuition 2015. All rights reserved, copying of any pages is strictly prohibited by law.6

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1.1 Sources of Macro Nutrients: Fats, Carbohydrates and Protein

All energy in the biological world originates from the sun in the form of light energy. It is the chemical process of photosynthesis in plants that converts this energy into stored chemical energy – in the form of three main nutrients: carbohydrates, fats and proteins. These nutrients are referred to as energy nutrients because they can be broken down in our cells to provide energy for the body. We obtain this energy directly by eating the plants that photosynthesise, or indirectly by eating the animals that feed on the plants.

When food is eaten, it travels through the mouth to the stomach and intestines where it is digested. The digestive system breaks down the nutrients contained in the food we eat and they are transported to various sites around the body.

FatsFats are found in food sources such as fatty meat (e.g. bacon), fast foods, butter, full cream dairy products and nuts. They are the secondary source of energy within the body. Once consumed, fats are converted into fatty acids for transport in the bloodstream and stored in the body as triglycerides in either skeletal muscle or adipose tissue.

CarbohydratesCarbohydrates are found in food sources such as pasta, fruit, breads and cereals. They are the primary energy source within the body. Once consumed, carbohydrate is broken down into glucose (the immediately usable form of carbohydrate) and transported via the blood to the muscles for energy release. If not required for immediate use, this glucose will be stored in the muscle as glycogen. Glycogen stored within skeletal muscle is ‘muscle specific’ – meaning it is locked in for use by that muscle only. If muscle glycogen stores are full, excess glucose is transported to the liver where it can be stored as liver glycogen. Glycogen stored in the liver can be converted back into glucose to maintain blood glucose levels, or it may be transported (via the blood) to various muscles as required. If muscle and liver glycogen stores are full, excess glucose is converted to triglyceride (fat) and stored in the body as adipose tissue (fat cells).

ProteinsProteins (amino acids) are found in food sources such as meat, fish, eggs and milk. They are the building blocks of the human body and used primarily for growth and repair. They are only used as a nutrient fuel in extreme circumstances (i.e. starvation – when the body is depleted of all carbohydrate and fat stores).

Energy StoredThe amount of energy stored in the two main energy nutrient fuels is as follows:

• Fats (1g) 37Kj (9Kcal)

• Carbohydrates (1g) 17Kj (4Kcal)

As can be seen from table 1.1, the total stores of carbohydrate in the body are limited to approximately 6375Kj of energy – this would be equivalent to the energy needed for 90 minutes of running (at an approximate speed of 16km/h).

Compared to carbohydrates, the amount of energy in total fat stores around the body is large and practically unlimited. Even on a lean individual with 15% Body Fat, fat stores can exceed 294500Kj of energy which could fuel over 238 hours of slow walking!

However, because it takes substantially more O2 to metabolise fat than carbohydrate, the energy release from fat is usually too slow to meet all of the energy demands in intense muscular activity.




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Table 1.1: Approximate fuel and energy stores in the body (assuming the subject mass is 65kg with 15% Body Fat).

Nutrient Amount stored (g) Energy Stored (Kj)

Carbohydrate

Blood glucose 15g 17Kj × 15g = 255Kj

Muscle Glycogen 250g 17Kj × 250g = 4250Kj

Liver Glycogen 110g 17Kj × 110g = 1870Kj

Total Carbohydrate 375g 6375Kj

Fat

Adipose tissue 7800g 37Kj × 7800g = 288 600Kj

Intramuscular fat 160g 37Kj × 160g = 5920Kj

Total Fat 7960g 294529Kj

Focus Question 1.1

a. Explain the role of the following within the human body:

• Energy nutrients

• Carbohydrates

• Fats

• Protein




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b. Complete the table below, showing the location and type of fuel stores in the body.

Fuel Stored as Site

Carbohydrate a) Blood

Glycogen b)

Excess stored as fat c)

Fat d) Blood

Triglycerides e)

Adipose Tissue f)

Protein g) Around body

c. Explain why protein is generally not used as a nutrient fuel by the body?




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1.2 Food Breakdown into Nutrients: Glucose, Glycogen, Triglycerides, Free fatty acids

The energy stored in foods is not used directly by the body for biological work. Instead this energy is released to rebuild a chemical compound called adenosine triphosphate (ATP). ATP is the ‘energy currency’ of the human body. ATP is an energy rich molecule that consists of an adenosine molecule and three phosphate molecules chemically joined together. The breaking away of one of the ‘high energy’ phosphate bonds forms adenosine diphosphate (ADP) and releases the energy required for all forms of biological work. This is a reversible reaction, meaning ADP + P can be resynthesised to reform ATP (but it requires energy).

Diagram 1.1:

+

Adenosine triphosphate

Adenosine diphosphate Phosphate

M S L N RACTION I E I N

NERV R N MISSION

ENERGY RE EASED

CARBOHYDRATES FATS PROTEIN

A

A

P P P

P P P

A

A

Adenosine triphosphate

Adenosine diphosphate phosphate

Food Energy

CarbohydratesFats

Energy Released

Muscular ContractionDigestion

Nerve Transmission

ATP broken down to ADP releasing energy. Food energy is used to resynthesise ATP.

Carbohydrates and fats are the main nutrient fuels used to supply the energy needed for the resynthesis of ATP, but the relative contribution of carbohydrate and fat as a fuel will depend to a large degree on the exercise duration and intensity.

Fats can only be broken down to resynthesise ATP in a process requiring oxygen called aerobic lipolysis. Carbohydrates can be broken down aerobically in a process called aerobic glycolysis, but they may also be broken down without oxygen (anaerobic) in a process called anaerobic glycolysis.




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Diagram 1.2:

100

80

60

40

20

00 40

% VO2 max80

% Energyfrom Fat and Carbohydrate

Carbohydrate

Fat

The ‘Crossover’ Concept

As exercise intensity increases, there is a progressive increase in carbohydrate as a fuel.

Source: Adapted from Howley & Powers (date unknown), Exercise Metabolism.

Diagram 1.2 and 1.3 represent the “crossover” concept, which is a theoretical means by which we can describe the effects of exercise intensity and exercise duration on the contribution of carbohydrate and fat metabolism during exercise.

Diagram 1.2 illustrates that at rest, up to 75% of energy may come from fat and only 25% from carbohydrate; but as exercise intensity increases (measured as %VO2 max), the reliance on glycogen as a fuel source increases.

This is because fats are larger molecules and require more oxygen to break down. Therefore fats tend to be more dominant at low intensity exercise when oxygen delivery is not a limiting factor.

Because carbohydrates require considerably less oxygen to break down, glycogen usage tends to dominate with more intense exercise. For this reason, carbohydrate energy is more accessible during exercise than fat, and therefore carbohydrates are considered the body’s principal exercise fuel.

Diagram 1.3:

70

60

50

40

30

10 50Exercise time (min)

90

% Energyfrom Fat and Carbohydrate

Carbohydrate

Fat

The ‘Crossover’ Concept

As exercise duration increases, there is a progressive increase in fat as a fuel.

Source: Adapted from Howley & Powers (date unknown), Exercise Metabolism.

Diagram 1.3 illustrates that there is shift in fuel use from carbohydrate to fat as the duration of exercise increases.

Although the preferred fuel for high level performance in a marathon or other long duration events would generally be carbohydrates, the body has a limited supply of glycogen stores (refer table 1.1). As a result, when an athlete continues to exercise for an extended period of time, their muscle and liver glycogen stores will slowly become depleted. After approximately 90 minutes of intense exercise, glycogen stores will be depleted and the body must begin to rely on more fat as the major fuel source.

However, because fat requires more oxygen to break down than carbohydrate, the body cannot cope with the increasing oxygen demands of fat breakdown and the oxygen demands of exercise, so the exercising muscles must significantly reduce their work output (exercise intensity) to cope.


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The Sources of Energy Affecting Physical Performance · muscles as required. If muscle and liver glycogen stores are full, excess glucose is converted to triglyceride (fat) and stored