energy sources (dietary)

Last reviewed 01/2018

The dietary energy sources used in animals have in common the presence of carbon-carbon and carbon-hydrogen bonds which may be oxidized to carbon dioxide and water in an exothermic reaction:

  • glucose:
    • normally constitutes 50-95% of total energy expenditure
    • obligatory energy source for brain, erythrocytes, renal medulla and leukocytes
    • ingested glucose is phosphorylated in the liver to glucose-6-phosphate; it is then metabolised to glycogen, fatty acids or into glucose again depending upon metabolic demands
    • excessive carbohydrate intake - typically beyond 30kcal/kg/day - cannot be utilized for energy and instead is converted into fat. This conversion results in an increase in the respiratory quotient due to a rise in CO2 production. This increase in CO2 can precipitate respiratory failure in a susceptible patient and must be borne in mind in planning the diet of those with respiratory compromise e.g. ventilated patients.

  • fat:
    • normally constitute 10-40% of total energy expenditure
    • in healthy individuals, extensive stores are held throughout the body as triglycerides

  • protein: minimal contribution to energy output except in states of starvation and increased catabolism when increased gluconeogenesis may occur in the liver

Energy released by metabolic reactions is used to create energy storage compounds within cells e.g. ATP, glycogen.