Another purpose of food is to provide the organism with energy resources. The building of new molecules is in itself a process which calls for a certain amount of energy. Then there are the muscles and all the other organs of our body, most of which never stop functioning, not even for a single minute. Even when one is asleep the heart goes on working, as do the respiratory muscles, the liver, kidneys, alimentary tract, and endocrine glands. Even the brain goes on expending energy on quite a large scale, although this is the one we notice least.
Energy losses can be quite easily restored. Used as ‘fuel’ are fats, carbohydrates and, to a certain extent, proteins which ‘burn’ in the- organism to form carbon dioxide and water. The organism actually consumes only one kind of fuel — glucose. Fats and proteins are first converted to glucose, before they become an energy-supplying material.
It is easier to provide the organism with fuel than to supply it with all the necessary building materials. Human body consists mainly of carbon, nitrogen, oxygen and hydrogen, with small, sometimes negligible, amounts of other chemical elements.
Gabriel Bertrand, a French biochemist, calculated that the body of a man weighing about 100 kilograms contains:
Fluorine, bromine, manganese and copper are present in even lesser amounts. It is quite possible that all the other elements, even those which are not very active chemically, such as gold, can be found in the organism, but we still do not understand the part they play.