making the nitrogen
factor about 5.68 instead of 6.25. If wheat contains 2 per cent of
nitrogen, it is equivalent to 12.5 per cent of crude protein, using the
factor 6.25; or to 11.4, using the factor 5.7. The nitrogen content of
foods is absolute; the protein content is only relative.[9]

21. Food Value of Protein.--Because of its complexity in composition,
protein is capable of being used by the body in a greater variety of
ways than starch, sugar, or fat. In addition to producing heat and
energy, protein serves the unique function of furnishing material for
the construction of new muscular tissue and the repair of that which is
worn out. It is distinctly a tissue-building nutrient. It also enters
into the composition of all the vital fluids of the body, as the blood,
chyme, chyle, and the various digestive fluids. Hence it is that protein
is required as a nutrient by the animal body, and it cannot be produced
from non-nitrogenous compounds. In vegetable bodies, the protein can be
produced synthetically from amids, which in turn are formed from
ammonium compounds. While protein is necessary in the ration, an
excessive amount should be avoided. When there is more than is needed
for functional purposes, it is used for heat and energy, and as foods
rich in protein are usually the most expensive, an excess adds
unnecessarily to the cost of the ration. Excess of protein in the ration
may also result in a diseased condition, due to imperfect elimination of
the protein residual products from the body.[10]

22. Albuminoids differ from proteids in general composition and, to
some extent, in nutritive value. They are found in animal bodies mainly
in the connective tissue and in the skin, hair, and nails. Some of the
albuminoids, as nuclein, are equal in food value to protein, while
others have a lower food value. In general, albuminoids are capable of
conserving the protein of the body, and hence are called "protein
sparers," but they cannot in every way enter into the composition of the
body, as do the true proteins.

23. Amids and Amines.--These are nitrogenous compounds of simpler
structure than the proteins and albuminoids. They are sometimes called
compound ammonia in that they are derived from ammonia by the
replacement of one of the hydrogen atoms with an organic radical. In
plants, amids are intermediate compounds in the production of the
proteids, and in some vegetables a large portion of the nitrogen is
amids. In a