Foods are substances which, when taken into the body, provide it with heat and other forms of energy, and furnish it with material for growth and repair. In the case of a grown person, foods supply the fuel necessary for various bodily activities - for walking, for mounting stairs, for lifting weights; and they keep the bodily machinery in repair. In the case of a growing person, they also supply materials for building up the bodily machinery.

In order to understand how food can serve the body in the ways mentioned, it is necessary to know something about the composition of the body and also about the composition of food. Both, however, are exceedingly complex, and even those who are devoting their lives to the study of foods in their relation to the body have still much to discover upon the subject. The science of nutrition is a growing science, about which we hope in time to know much more than we do at present.

yet discovered only about eighty elements and of these all compounds are made up.

In the body there are known to be fifteen elements, which are combined to form over one hundred compounds.

Elements Present In The Human Body (H. C. Sherman)

The body is constantly undergoing changes by which complex compounds are broken down, and new, simpler compounds are formed and are given off through the lungs and skin, from the kidneys, and otherwise. The result of this is that new material, containing the same elements as the compounds constantly excreted, must be supplied to the body in the form of food. A well-known chemist has referred to the material which the body can use for repair and building of tissues as its "building stones." These "building stones "must not only contain the elements of which the body is composed, but they must also be in a form in which the body can utilize them.

In order to prepare the "building stones" to repair the body and to furnish it with fuel, many of them must first be changed by heat in the process of cooking, and they are all acted upon by the digestive juices after being taken into the body.

All changes of matter are of two kinds, physical and chemical. A physical change is one that does not cause a change in the composition of a substance, as the dissolving of sugar in water, for the water being evaporated, the sugar will retain its physical properties.