Food fuels and the three energy systems

Key knowledge

Chapter preview

Food fuelsCHO – 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 = energyAdenosine triphosphate is the major source of

energy that keeps every cell in the body going, including muscles.

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 = energyTo 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 activityGlycogen is the body’s preferred source of

energy for exercise.At rest however, the body has a clear

preference for fats as food fuel over CHO.

Carbohydrates, fats and proteinFuel Source Converted to Stored

Carbohydrates

Fats (triglycerides)

Proteins

Phosphocreatine

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

FatsFats 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.

ProteinProtein 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 IndexAn 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 costFood fuel Maximum energy

(ATP per molecule)Oxygen required(litres per mole of ATP produced)

Carbohydrates

Fats

Protein