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Exercise Intensity & Energy Source

Energy is primarily supplied from two sources:

  • Carbohydrates - in the form of glycogen stored in the muscles
  • Fat - stored around the body

During exercise, we use a combination of these energy sources. At a high intensity the main source of energy is carbohydrate and at a low intensity fat is the predominate source. As there is a limit to the amount of carbohydrate that can be stored in the muscles, high intensity work can only be sustained for short periods. We have large stores of fat so low intensity work can be maintained for long periods.

Intensity and Energy Source

The following table, adapted from O'Neil (2001)[1], shows the relationship between exercise intensity (% of your Maximum Heart Rate) and the energy source (carbohydrate and fat).

Intensity % HRmax % Carbohydrate % Fat
65 to 70 40 60
70 to 75 50 50
75 to 80 65 35
80 to 85 80 20
85 to 90 90 10
90 to 95 95 5
100 100 0

Respiratory Exchange Ratio (RER)

Carbohydrates, fat and protein all play a part in energy metabolism and for a certain volume of oxygen the energy released will depend upon the energy source. It is possible to estimate which particular fuel (carbohydrate, fat or protein) is being oxidised by calculating the Respiratory Exchange Ratio (RER). RER is the ratio of carbon dioxide (CO 2 ) produced to oxygen (O 2 ) consumed and is known as the Respiratory Quotient (RQ).

If carbohydrate is completely oxidised to CO 2 and water (H 2 O) then the relationships is as follows:

  • 6O 2 + C 6 H 12 O 6 » 6CO 2 + 6H 2 O + 38ATP (Adenosine Triphosphate)
  • RER = 6CO 2 ÷ 6O 2 = 1

If fat is completely oxidised to CO 2 and H 2 O then the relationships is as follows:

  • C 16 H 32 O 2 + 23O 2 » 16CO 2 + 16H 2 O + 129ATP
  • RER = 16CO 2 ÷ 23O 2 = 0.7

The RER for protein is approx. 0.8 but as it plays a very small part in energy metabolism. A value between 0.7 and 1.0 indicates a mixture of fat and carbohydrate as the energy source. A value greater than 1.0 indicates anaerobic respiration due to more CO 2 being produced than O 2 consumed.


  1. O'NEIL, T. et al. (2001) Indoor Rowing Training Guide. Concept 2 Ltd, p. 27

Related References

The following references provide additional information on this topic:

  • FOX, E. L. et al. (1969) Metabolic energy sources during continuous and interval running. J Appl Physiol, 27 (2), p. 174-178
  • WELLS, G. D. et al. (2009) Bioenergetic provision of energy for muscular activity. Paediatric respiratory reviews, 10 (3), p. 83-90
  • GOEDECKE, J. H. et al. (2000) Determinants of the variability in respiratory exchange ratio at rest and during exercise in trained athletes. American Journal of Physiology-Endocrinology And Metabolism, 279 (6), p. E1325-E1334

Page Reference

If you quote information from this page in your work then the reference for this page is:

  • MACKENZIE, B. (2001) Exercise Intensity and Energy Source [WWW] Available from: [Accessed

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