This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
In the last chapter we have considered foods merely as sources of energy, but they are also required to build up the substance of the body. From birth to manhood we increase in bulk and weight, and that not merely by accumulating water and such substances, but by forming more bone, more muscle, more brain, and so on, from the things which we eat. Even after full growth, when the body ceases to gain weight, the same constructive processes go on; the living tissues are steadily oxidized and broken down as they work, and as constantly reconstructed.
Foods are therefore needed, not only to supply the body with work-power by their oxidation, but to supply material from which new living tissue, can be constructed.
Most foods serve for both purposes, energy supply and tissue formation; they are built up by the living cells into new tissue before they are oxidized to set energy free. Our food must, therefore, contain such substances as the body can utilize for tissue formation.* The living tissues when analyzed are found to consist mainly of carbon, hydrogen, nitrogen, and oxygen, and we might at first suppose that these chemical elements in their uncombined form would serve to nourish us. Experience, however, teaches that this is not the case. Four fifths of the air is nitrogen, but we cannot feed on it; hydrogen gas is of no use as a food; and a lump of charcoal (carbon) might fill the stomach, but would not keep a man from starving. Oxygen can be utilized when taken by the lungs from the air; but all other elements to be of use as food must be taken, not in their separate state, but in the form of complex compounds, in which they are chemically combined with other things; as, for example, in starch, and sugar, and fat, and oil, and albuminous substances.
What use have foods besides supplying energy to the body? Illustrate from the growth of a child. Why are foods needed for construction after growth has ceased?
What purposes do most foods serve? Are they usually oxidized before making tissue? What sort of substances must our food contain?
* Whether any food is ever oxidized in the body before being built up into a tissue, as coal is burnt in an engine without ever forming part of the engine, must still be regarded as an open question in physiology. The old doctrine that some foods, as starch and sugar, were useful only to set free heat, and others, as albumen and flesh, alone built tissue, must be given up. It seems certain that under some conditions sugar and starch may be used in building tissue, though they cannot do it alone; but whether they are under any circumstances ever burnt before making part of a tissue is not certain. On the other hand, there is some reason to suspect that albuminous substances may, when eaten la excess, be oxidized in the body without ever forming part of a living cell.