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|[[File:Absolute substrate usage against intensity.jpg|none|thumb|400px|The contribution of different energy sources changes<ref name="romijn"/> with exercise intensity. These values were taken after 30 min. of exercise. Note that the total calories available from the blood (free fatty acid and glucose) remains about the same regardless of exercise intensity.]]
|[[File:Glycogen usage and intensity.jpg|thumb|400px|Percentage of energy from glycogen plotted against exercise intensity as percentage of [[VO2max|VĖO<sub>2</sub>max]].]]
|[[File:muscle fuel utilisation.jpg|right|thumb|400px|Another study also looked at the utilization of different energy sources during exercise and produced similar results<ref name="LoonGreenhaff2001"/>.]]
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At low exercise intensity the majority of the energy comes from free fatty acids in the blood, with a little bit of blood glucose and a little bit of muscle triglyceride. As the exercise intensity increases the contribution of free fatty acids drops. The contribution of blood glucose increases with exercise intensity, but not as dramatically as the contribution of muscle glycogen. At higher intensity muscle glycogen is the major energy source and is critical for performance.
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=References=
<references>
<ref name="Schutz-1980"> Y. Schutz, E. Ravussin, Respiratory quotients lower than 0.70 in ketogenic diets., Am J Clin Nutr, volume 33, issue 6, pages 1317-9, Jun 1980, PMID [http://www.ncbi.nlm.nih.gov/pubmed/7386422 7386422]</ref>
<ref name="plowman-2007"> Sharon A. Plowman, Denise L. Smith, Exercise physiology for health, fitness, and performanc, date 2007, publisher Lippincott Williams Wilkins, location Baltimore, MD, isbn 0-7817-8406-9</ref>
<ref name="LoonGreenhaff2001">Luc J. C. van Loon, Paul L. Greenhaff, D. Constantin-Teodosiu, Wim H. M. Saris, Anton J. M. Wagenmakers, The effects of increasing exercise intensity on muscle fuel utilisation in humans, The Journal of Physiology, volume 536, issue 1, 2001, pages 295â304, ISSN [http://www.worldcat.org/issn/0022-3751 0022-3751], doi [http://dx.doi.org/10.1111/j.1469-7793.2001.00295.x 10.1111/j.1469-7793.2001.00295.x]</ref>
</references>