Every atom composing the human body is constructed from food. The
number and the proportion of the various chemical elements composing
the body are well known, and were it not for the fact that the body is
constantly casting out old cells and waste-products, the problem of
nutrition would resolve itself into the simple process of supplying the
body with the materials needed for growth.
[Sidenote: Formation of new tissue and destruction of old]
We could analyze an adult man and a new-born infant, and know that
the infant, in order to reach maturity, would need to add to its body
so many pounds of oxygen, carbon, sulfur, iron, etc. The problem of
nutrition, however, is more complex. Not only must we consider the
formation of new tissue, but we must also allow for the rebuilding of
the old, and for all those processes of vital activity that involve the
consumption of food material and the destruction of body-tissue. Nor can
this allowance be accurately proportioned from the analysis of the body,
because the various elements composing it do not change with equal
rapidity. Thus, a man in a harvest field might pass through his blood in
one day ten or fifteen pounds of oxygen (in the form of water and carbon
dioxid), which would amount to ten per cent of the oxygen contained in
his body, but if he should take calcium or fluorin to the extent of
ten per cent of that contained in the body, death from poisoning would
speedily ensue.
We can better understand the use of foods and the process they undergo
in building the body by considering separately each class of food
material from the time it is absorbed from the alimentary tract until
it is excreted from the bowels, or from the lungs and the kidneys, or
deposited in the body as bone, fat, or tissue.