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Energy is required for the maintenance of life against the tendency to disorder. It is derived primarily from the oxidation of food substrates after absorption.

The energy liberated from catabolism is passed to cellular compounds for storage; this usually occurs by the formation of new bonds. The breakdown of these new bonds can power alternative metabolic pathways.

The energy requirements of the individual vary with numerous factors such as age, physical activity and disease states. An inherited deficiency of the enzymes used in energy-dependent reactions can manifest themselves as disease e.g. the glycogen storage diseases.

Energy is measured in S.I. units of joules.

Last reviewed 08/2018