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. [Sidenote: 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. [Sidenote: 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 Proteids_ Albumins Globulins Nucleo albumins Albuminates Coagulated proteids Proteoses (Albumoses) Peptones _Compound Proteids_ Respiratory pigments Gluco Proteids Nucleins Nucleo proteids Lecith albumins _Albuminoids_ Collagen Gelatin Elastin Reticulin Keratin [Sidenote: Amido compounds] Besides these real proteids there are a few substances known as amido compounds which exist in small quantities in vegetables, and a number of nitrogenous substances which exist in meat and meat extracts, which are not true proteids, as they have little or no nutritive value, but act as stimulants or irritants in the body. [Sidenote: Ptomains--how formed] Ptomains are another class of substances which are often found in food products. They are formed by the growth of bacteria, and are in reality the nitrogenous waste-products of bacterial life. Ptomains develop in meats and dairy products held in cold storage, and are sometimes the cause of serious poisoning. Nitrogenous waste-products will be further discussed in Lesson VI, under "Metabolism of Proteids." (See p. 209.) [Sidenote: Sources, coagulation and solubility of albumin]