In this connection it may be well to state that blood is a tissue; thus meat and eggs build the blood, as well as muscle and sinew.
All nitrogenous foods contain considerable carbon—mostly in the form of fat in the meat elements—but the carbonaceous foods contain so little protein that the protein elements do not appreciably enter into the nutrition.
Carbon and nitrogen in the carbo-nitrogenous foods are about equal in proportion.
The nitrogenous or protein elements in the body constitute about one-fifth of its weight. They form the basis of blood, lymph, muscle, sinew, bone, skin, cartilage, and other tissues.
Worn-out body tissue is constantly being torn down and eliminated and the protein in the foods must daily furnish material for repair, as well as for building new tissue.
A young animal’s first need is for growth, as it has not learned to exercise sufficiently to use much latent energy. The first food it receives is an animal product—milk to babes and other mammals—while the young chicken or bird is nourished by the yolk of the egg contained within it. Sufficient yolk substance remains within the chick when it is hatched to sustain its life for the first day or two.
Nitrogenous foods are more concentrated and contain less waste; thus a smaller bulk is required than of vegetables and fruits. According to recent experiments, the average adult requires from two to four ounces of nitrogenous foods a day, to repair the waste. Happily, when more is consumed, the system has the power up to a certain limit (depending on the physical condition and the daily activity), to eliminate an excess.
It is needless to say that if the daily waste is not replaced, digestion and nutrition suffer. The system must have the two to four ounces necessary to supply the nitrogen daily excreted, or the tissues themselves will be consumed.
The proteins, of which meat is the principal member, are classified as:
Albuminoids: albumin (white of eggs), casein (curd of milk), myosin (the basis of lean meat and gluten of wheat);