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.
All the active tissues of the body are found to yield on chemical analysis large quantities of albumens. (See p. 21.) As the tissues work this proteid is broken down, and its nitrogen carried off in the form of a peculiar ammoniacal substance, urea; to repair the wasted living tissue new proteids must be laid down in it. So far as we know at present the human body (like that of most animals) is unable to make proteids out of other things; given one variety of them it can turn it into other varieties, but it cannot make proteids from things which are not proteids. Hence these albuminous or proteid substances are an essential article of diet.
What elements do the tissues yield on analysis ? Can we feed on these elements in their uncombined state 1 Name one which is absorbed in a free state and used. Whence is it derived ? What organs receive it? Name substances containing the necessary elements in combination and used as food.
What do we find in all the active tissues? What becomes of the nitrogen of working tissues? Explain why proteids are an essential article of diet.
From what has been said above it is clear that our bodies are, on the whole, destructive rather than constructive in relation to the outer world. They require for their nutrition very complex chemical compounds (starch, sugar, fat, proteids), build these up into living tissues, and then oxidize the tissues and return the carbon, hydrogen, and nitrogen, which were received from outside in the form of complicated chemical molecules, to the outer world in the form of much simpler chemical compounds, namely, carbon dioxide, water, and urea. None of these latter substances is capable of nourishing an animal; it cannot from them alone build up its tissues or set free energy.