The main sources of dietary energy are the carbohydrates, fats and proteins. These sources of energy, on complete metabolism, yield different amounts of energy (table). An adequate and constant amount of energy must be made available to the body for cell survival and maintaining the following vital functions at optimal levels.
Activity of the brain and the nervous system and transmission of nerve impulses.
Pumping of heart and distribution of blood to supply oxygen and nutrients to various tissues, part of it against gravity.
Liver function for the catabolism and synthesis of nutrients and other essential bio-compounds and detoxification of toxic chemicals ingested through food and air.
Excretory function of the kidney.
Table: Energy yields from various sources of Energy.