The food substances which contain nitrogen are commonly called proteids, or, if these compounds are considered together, the name protein may be given the group. Protein is not a single compound, but includes all substances which contain the element nitrogen in such combinations as are available for assimilation in the human body.

Only proteid foods contain nitrogen

Protein is the most important group of nutrients in the animal body. The proteid substances in the body must be formed from proteids taken in the form of food, because only proteid foods contain the element nitrogen. All proteids contain nitrogen, but all nitrogen does not contain protein. All proteids, therefore, are nitrogenous compounds.

Formation of organic nitrogen

The animal body does not possess the power of combining elementary nitrogen with other elements. Bacteria have the power to utilize the nitrogen of the air to form mineral salts or nitrates. Plants have the power to unite the nitrogen derived from these nitrates with carbon, oxygen, and hydrogen. In this way organic nitrogen, or proteids, are formed. The animal body may digest these proteids, however, and transform them into other proteid compounds. All proteids contain carbon, hydrogen, oxygen and nitrogen; most of them contain sulfur, and a few contain phosphorus, iron, copper, and bromid.

The percentage by weight of the various elements which form proteid matter is about as follows:

Carbon ... ...52%
Hydrogen ... ...7%
Oxygen ... ...22%
Nitrogen ... ...16%
Sulfur ... ...2%
Phosphorus ... ...1%

The following table gives three groups of proteid substances:

Simple ProteidsCompound ProteidsAlbuminoids
AlbuminsRespiratory pigments Collagen
GlobulinsGluco ProteidsGelatin
Nucleo albuminsNucleinsElastin
AlbuminatesNucleo proteidsReticulin
Coagulated proteidsLecith albuminsKeratin
Proteoses (Albumoses)
Peptones