This is because proteins alone contain sufficient amounts of the great element called _nitrogen_, which forms a large part of every portion of our bodies.

This is why they are called proteins, meaning "first foods," or most necessary foods.

Whatever we may live on in later life, we all began on a diet of liquid meat (milk), and could have survived and grown up on nothing else.
Composition of Proteins.

Nearly all our meats are the muscle of different sorts of animals, made of a soft, reddish, animal pulp called _myosin_; the other principal proteins being white of egg, curd of milk, and a gummy, whitish-gray substance called _gluten_, found in wheat flour.

This gluten is the stuff that makes the paste and dough of wheat flour sticky, so that you can paste things together with it; while that made from corn meal or oatmeal will fall to pieces when you take it up.
The jelly-like or pulp-like myosin in meat is held together by strings or threads of tough, fibrous stuff; and the more there is of this fibrous material in a particular piece or "cut," of meat, the tougher and less juicy it is.