Chapter 5 Food fuels and the three energy systems

Key knowledge
Chapter preview
Food fuels
 CHO – carbohydrates are the body’s preferred fuel
source, particularly during exercise.
 Fats – concentrated fuel storage in the muscles and
adipose tissue, main source of fuel at rest and in
prolonged sub maximal exercise.
 Protein – make a small contribution during energy
production, mainly used for growth and repair.
Foods as energy sources
ATP = energy
 Adenosine triphosphate is the major source of energy
that keeps every cell in the body going, including
 ATP is a chemical fuel source. Energy is released when
one of the phosphates splits off, leaving ADP and an
inorganic phosphate Pi.
 Only a very small amount of ATP exists at the muscles,
around 2 seconds worth. ATP must therefore be
continually rebuilt or resynthesised so that energy can
be provided for longer periods.
ATP = energy
 To resynthesise ATP to create more energy in a form
muscles can use, energy from the breakdown of
phosphocreatine or nutrients(glucose, free fatty acids
and amino acids) is used to rejoin ADP and Pi.
Fuel sources for physical activity
 Glycogen is the body’s preferred source of energy for
 At rest however, the body has a clear preference for fats
as food fuel over CHO.
Carbohydrates, fats and protein
Fats (triglycerides)
Converted to
Carbohydrates (CHOs)
 It is important to have a carbohydrate rich diet in order
to increase glycogen stores.
 CHOs are preferred to fats as a source of energy during
exercise because they require less O2 to produce the
same amount of energy.
 CHO loading
 Fats are stored in the form of triglycerides in adipose
tissue and skeletal muscle.
 Triglycerides are broken down into free fatty acids
which are then broken down aerobically for energy.
 Fats become important during prolonged sub maximal
exercise, when glycogen levels are depleted.
 Protein is important in any diet as it provides the
building blocks of all tissue.
 Protein is not normally used as any energy source, it
requires large amount of oxygen to break down for
energy use.
 In extreme situations, when the body has depleted
glycogen and triglycerides, the body may resort to
using protein as an energy source.
Glycaemic Index
 An index which ranks foods on a scale of 1 to 100
according to how much they will raise blood glucose
over a two hour period.
For example:
- Low GI: apple 38
- Medium GI: white rice 64
- High GI: honey 83
Energy availability and oxygen cost
Food fuel
Maximum energy
(ATP per molecule)
Oxygen required
(litres per mole of ATP

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